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The Mean Along-Isobath Heat and Salt Balances over the Middle Atlantic Bight Continental Shelf
Abstract
The mean heat and salt balances over the Middle Atlantic Bight continental shelf are investigated by testing the hypothesis that surface fluxes of heat or freshwater are balanced by along-isobath fluxes resulting from the mean, depth-averaged, along-isobath flow acting on the mean, depth-averaged, along-isobath temperature or salinity gradient. This hypothesized balance is equivalent in a Lagrangian frame to a column of water, for example, warming because of surface heating as it is advected southward along isobath by the mean flow.
Mean depth-averaged temperatures increase from north to south along isobath at a rate of 2°C (1000 km)−1 at midshelf, which is consistent with the hypothesized balance and mean surface heat flux estimates from the 50-yr NCEP Reanalysis. However, mean surface heat flux estimates from the higher-resolution 20-yr Objectively Analyzed Air–Sea Fluxes (OAFlux) reanalysis are too small to balance the along-isobath heat flux divergence implying a cross-shelf heat flux convergence. It is unclear which surface heat flux estimate, NCEP or OAFlux, is more accurate. The cross-shelf heat flux convergence resulting from the mean cross-shelf circulation is too small to balance the along-isobath heat flux divergence.
Mean depth-averaged salinities increase from north to south along isobath at a rate of 1 (psu) (1000 km)−1 at midshelf. Mean precipitation and evaporation rates nearly balance so that the net freshwater flux is too small by more than an order of magnitude to account for the observed along-isobath increase in salinity. The cross-shelf salt flux divergence resulting from the mean cross-shelf circulation has the wrong sign to balance the divergence in the along-isobath salt flux. These results imply there must be an onshore “eddy” salt flux resulting from the time-dependent current and salinity variability.
The along-isobath temperature and salinity gradients compensate for each other so that the mean, depth-averaged, along-isobath density gradient is approximately zero. This suggests that there may be a feedback between the along-isobath density gradient and the onshore salt and heat fluxes that maintains the density gradient near zero.
... The shelf and slope waters are separated at the shelf break by a persistent front with strong gradients in physical and biogeochemical properties (e.g., Falkowski et al., 1988;Linder and Gawarkiewicz, 1998;Vaillancourt et al., 2005;Hales et al., 2009). Exchange of water masses between the MAB shelf and open ocean can significantly affect shelf water properties (Lentz, 2010;Zhang and Gawarkiewicz, 2015), offshore dispersal of shelf materials (e.g., Walsh et al., 1988;Biscaye et al., 1994), and cross-shelf transport of marine biota (e.g., Rypina et al., 2014). ...
... Assuming the offshore transport in the shelf-water streamer was volumetrically balanced by onshore transport of warmer and saltier slope water, Joyce et al. (1992) estimated the onshore heat and salt fluxes associated with the streamer were 1.8 × 10 13 W and 0.94 × 10 6 kg s − 1 , respectively. The former is greater than the maximum monthly mean ocean-to-air heat flux over the entire MAB shelf (~100 × 800 km 2 ); the latter is 1.5 times the onshore salt flux at the shelf break required to balance the MAB shelf salt budget (Lentz, 2010). In situ measurements also showed that the same shelf-water streamer resulted in offshore fluxes of suspended particulate matter (SPM) and dissolved oxygen (O 2 ) of 190 kg s − 1 and 2.3 × 10 5 mol s − 1 , respectively. ...
... The volume of the MAB shelf is conserved. Because properties of the upstream shelf water are presumably similar to those on the MAB shelf (e. g., Lentz, 2010;Wang et al., 2013), in this EM1 case, the aforementioned estimates of volume, heat, salt, carbon and O 2 fluxes in the streamer are net shelf-to-slope transport induced by the streamer, and represent an upper bound of the net transport. On the other end (designated as EM2), the offshore volume transport of the shelf water in the streamer could be entirely balanced by onshore intrusion of surface ring water with no net volume transport across the shelf break. ...
Significant exchanges between the Mid-Atlantic Bight (MAB) continental shelf and the neighboring open ocean can be induced by shelf water streamers, submesoscale filaments of shelf water entrained into the open ocean by Gulf Stream warm-core rings (WCRs) impinging onto the MAB continental shelf. Shelf water streamers have distinctive surface temperature and chlorophyll signals, and are thus visible from space. Satellite-measured sea surface height, temperature and chlorophyll show the evolution of a WCR over its 6-month lifespan in February-August 2019 and the persistent shelf water streamer it generated on its outskirt. In situ measurements from a two-week cruise in July 2019 were analyzed to investigate the physical, biological and biogeochemical characteristics of the shelf water streamer below the surface, and to quantify the associated cross-shelf transport of volume, heat, salt, carbon and oxygen. The analyses demonstrated that offshore transport of shelf water by the streamer, which was presumably balanced by either onshore intrusion of ring water or enhanced transport of shelf water from upstream, represented a major form of exchange between the MAB continental shelf and the open ocean. The streamer caused significant net onshore transport of heat and salt, and a significant net offshore transport of organic carbon and oxygen. Primary productivity in the streamer was higher than the surrounding slope and ring waters on the surface, which likely resulted from subsurface nutrients in the offshore-flowing shelf water being gradually consumed as the overlying water became clearer. WCR-induced shelf water streamers thus enhanced surface biological productivity in the slope sea.
Plain Language Summary: Waters of the shallow Mid-Atlantic Bight continental shelf and the neighboring deep slope sea have distinctly different physical, biological and chemical properties. Mixing between them can affect the shelf ecosystem and the dispersal of coastal materials into the deep ocean. One type of cross-shelf-edge mixing process results from strong clockwise-rotating vortices – so-called warm-core rings – formed from meanders of the Gulf Stream. As a warm-core ring intrudes onto the shelf edge, it often draws shelf water offshore, forming a thin filament in the slope sea. This filament is called a shelf-water streamer and has distinctive surface temperature and chlorophyll signals that are visible from space. Warm-core rings can also push offshore water onto the shallow shelf. This study examines the evolution of a warm-core ring over its 6-month lifespan in 2019 and the shelf-water streamer the ring induced in 5 of the 6 months. Interdisciplinary measurements from a field expedition in July 2019 were examined to assess the subsurface patterns of the streamer and to quantify the induced cross-shelf fluxes of heat, salt, organic carbon and oxygen. The analysis showed that the streamer represented a major form of cross-shelf mixing and caused a substantial onshore transport of heat and salt, as well as a substantial offshore transport of organic carbon and oxygen.
... However, years 2001 and 2010 were excluded from this analysis because of data availability or quality control issues. Temperatures were recorded at SHN and SHS beginning in 2003through 2017, respectively, and at sites SR and YB from 2002to 2009and 2000to 2009 At each site, temperature was measured at four depths including at 1 m below a surface-mounted float and from sensors at 1, 6, and 11 m above a bottom-mounted mooring ( Figure 1b). From 1998 to 2007, StowAway XTI and Tidbit V2 temperature sensors from Onset Computer Corp. were used, which were replaced with Onset HOBO U22 Water Temp Pro V2 sensors between 2008 and 2009. ...
... However, years 2001 and 2010 were excluded from this analysis because of data availability or quality control issues. Temperatures were recorded at SHN and SHS beginning in 2003through 2017, respectively, and at sites SR and YB from 2002to 2009and 2000to 2009 At each site, temperature was measured at four depths including at 1 m below a surface-mounted float and from sensors at 1, 6, and 11 m above a bottom-mounted mooring ( Figure 1b). From 1998 to 2007, StowAway XTI and Tidbit V2 temperature sensors from Onset Computer Corp. were used, which were replaced with Onset HOBO U22 Water Temp Pro V2 sensors between 2008 and 2009. ...
... Surface heat flux is both a critical component of the heat budget and requires observations not typically collected during monitoring efforts such as the PISCO program. Additionally, small uncertainties in this term can lead to large imbalances in the budget at seasonal timescales (Lentz, 2009). As there were no local measurements of surface heat flux over the record period, fields from the nearest ERA5 grid location were used to estimate the net surface flux at SH15. ...
Summer temperature and velocity measurements from 14 years in 15 m of water over the inner shelf off Oregon were used to investigate interannual temperature variability and the capacity of the across‐shelf heat flux to buffer net surface warming. There was no observable trend in summer mean temperatures, and the standard deviation of interannual variability (0.5°C) was less than the standard deviation in daily temperatures each summer (1.6°C, on average). Yet net surface heat flux provided a nearly constant source of heat each year, with a standard deviation less than 15 % of the interannual mean. The summer mean across‐shelf upwelling circulation advected warmer water offshore near the surface, cooling the inner shelf and buffering the surface warming. In most years (11 out of 14), this two‐dimensional heat budget roughly closed with a residual less than 20 % of the leading term. Even in years when the heat budget did not balance, the observed temperature change was negligible, indicating that an additional source of cooling was needed to close the budget. A comparison of the residual to the interannual variability in fields such as along‐shelf wind stress, stratification, and along‐shelf currents found no significant correlation, and further investigation into the intraseasonal dynamics is recommended to explain the results. An improved understanding of the processes that contribute to warming or cooling of the coastal ocean has the potential to improve predictions of the impact of year‐to‐year changes in local winds and circulation, such as from marine heat waves or climate change, on coastal temperatures.
... Ocean temperature is a fundamental environmental variable that profoundly influences marine ecosystems. Thus, understanding of the ocean's heat balance and its temperature variability is a long-standing research topic (Beardsley et al., 1985;Lentz et al., 2003a;Brink et al., 2009;Lentz, 2010;Fewings and Lentz, 2011;Connolly and Lentz, 2014). Growing evidence reveals long-term warming of Northwest Atlantic coastal waters Forsyth et al., 2015;Pershing et al., 2015) and extreme warming events such as the ocean heat wave of 2012 (Chen et al., 2014a(Chen et al., , 2015. ...
... However, the relative importance of these two processes in driving temperature changes and the dependence on timescales need clarification. For example, using historical temperature profiles and atmospheric reanalysis products, Lentz (2010) tested the hypothesis that the climatological mean heat balance on the Middle Atlantic Bight continental shelf is between the surface heat flux and the along-shelf advective heat flux resulting from the mean equatorward along-shelf flow. However, he noted that the validity of this simple balance is sensitive to the choice of the meteorological heat flux product. ...
... Cross-shelf exchange processes are also important to the heat budget, particularly on an individual event timescale of days to weeks. Note that the mean crossshelf heat flux in the Middle Atlantic Bight is small (e.g., Lentz, 2010), but the eddy flux (deviation from the mean) over an event timescale can be large. Gulf Stream warm core rings can induce significant cross-shelf transport of shelf water when they impinge upon the shelf break (e.g., Cenedese et al., 2013). ...
Knowledge of heat balance and associated temperature variability in the Northwest Atlantic coastal ocean is important for understanding impacts of climate change such as how ocean warming will affect the management of fisheries. Heat balances are particularly complicated near the edge of the continental shelf, where the cross-shelf temperature gradients within the shelf-break front complicate the competing influences of air-sea flux anomalies versus ocean advection. We review the atmospheric and oceanic processes associated with heat balance over the Northwest Atlantic continental shelf and slope, with an emphasis on the scale-dependent nature of the heat balance. We then use data from the Ocean Observatories Initiative (OOI) Pioneer Array to demonstrate heat balance scale dependence at the southern New England shelf break, and the capability of the array to capture multiscale ocean processes. Comparison of the cumulative effects of air-sea heat fluxes measured at the OOI Pioneer Array from May 2015 to April 2016 with the actual temperature change shows the importance of advective processes in overall heat balance near the shelf break.
... Additional data were collected using Onset TempPros so that vertical resolution is ∼1 m for temperature. [2004] and Shearman and Lentz [2010] address the topic of long-term trends in the MAB. ...
... Along-shelf advection can therefore have opposite The along-shelf temperature gradient observed during winter to the west of MVCO (∼4×10 −5 • C m −1 ) is an order of magnitude larger than estimates of the mean along-shelf gradient throughout the entire MAB. Lentz [2010] estimates that the mean along-shelf temperature gradient has magnitude 2×10 −6 • C m −1 at the 70-m isobath and 4×10 −6 ...
... Similarly, the winter temperature anomalies at MVCO are significantly correlated with those at the closest measurement site at buoy 44008 (R 2 = 0.56 , not shown), but not as strongly as with those at other inner shelf locations. Therefore, in addition to alongshore separation, bottom depth is a major factor determining the correlation of interannual temperature anomalies at different locations in the MAB.Winter anomalies in the Shearman andLentz [2010] SST data, using the 31 stations north of Cape Hatteras with at least 20 years of data, show a similar relationship when compared with the longest available time series at Woods Hole(Fig. 4a). ...
The shallow depth of the inner continental shelf allows for rapid adjustment of the ocean to air-sea exchange of heat and momentum compared with offshore locations. Observations during 2001-2013 are used to evaluate the contributions of air-sea heat flux and oceanic advection to interannual variability of inner-shelf temperature in the Middle Atlantic Bight. Wintertime processes are important for interpreting regional interannual variability at nearshore locations since winter anomalies account for 69–77% of the variance of the annual anomalies and are correlated over broad alongshelf scales, from New England to North Carolina. At the Martha's Vineyard Coastal Observatory on the 12-m isobath, a heat budget is used to test the hypothesis that interannual differences in winter temperatures are due solely to air-sea heat flux. Bimonthly averages of air-sea heat flux are correlated with temporal changes in temperature, but overestimate the observed wintertime cooling. Velocity and satellite-derived temperature data show that interannual variability in wintertime surface cooling is partially compensated for by alongshore advection of warmer water from the west at this particular location. It is also shown that surface heat flux is a strong function of air-sea temperature difference. Because of this coupling between ocean and air temperatures in shallow water, along-shelf advection can significantly modify the surface heat flux at seasonal and interannual time scales. While alongshelf advection at relatively small (˜100-km) scales can be an important component of the heat budget over the inner shelf, interannual temperature variability is still largely determined by adjustment to large-scale air-temperature anomalies.
... Bisagni, 1983;Gawarkiewicz et al., 2001;Cenedese et al., 2013), frontal instabilities (Garvine et al., 1988;Gawarkiewicz, 1991;Lozier et al., 2002;Gawarkiewicz et al., 2004;Zhang and Gawarkiewicz, 2015a), and wind forcing Lentz, 2003;Castelao et al., 2008;Siedlecki et al., 2011). A net annual salt flux was estimated by Lentz (2010) by balancing surface and lateral (coastal) inputs with a necessary onshore salt flux to match the observed alongshelf evolution of the salinity field along the 70 m isobath. The number obtained, 0.7 kg/m 2 s 2 , is very similar to the value directly observed by Gawarkiewicz et al. (2004). ...
... The Pioneer Array data will be useful in providing more quantitative observations pertaining to the relative breakdown of air-sea flux versus ocean advective contributions to seasonal and inter-annual anomalies. There is a paucity of quality air-sea flux measurements over the outer continental shelf and upper continental slope, and the typical air-sea flux products show a high degree of uncertainty in this geographic region (see Lentz, 2010 for a thorough discussion of this issue). Similarly, the gliders provide information in shifting temperatures of inflowing waters from the Gulf of Maine and Georges Bank into the Middle Atlantic Bight, as well as onshore flows from slope and Gulf Stream waters. ...
The Ocean Observatories Initiative (OOI) of the National Science Foundation in the USA includes a coastal observatory called the OOI Pioneer Array, which is focused on understanding shelf/slope exchange processes. The OOI Pioneer Array has been designed and constructed and is currently in operation. In order to fully understand the design principles and constraints, we first describe the basic exchange processes and review prior experiments in the region. Emphasis is placed on the space and time scales of important exchange processes such as frontal meandering and warm core ring interactions with the Shelfbreak Front, the dominant sources of variability in the region. The three major components of the Pioneer Array are then described, including preliminary data from the underwater gliders and Autonomous Underwater Vehicle (AUV) deployments. The relevance of the Pioneer Array to important recent scientific issues in the area, including enhanced warming of the continental shelf and increasing frequency and spatial extent of Gulf Stream interactions with the continental shelf is discussed. Finally, similar observatories in Asia are briefly described, and general conclusions regarding principles that should guide the design of shelfbreak observatories in other geographic regions are presented.
... At the shelf break, these eddies export shelf water offshore (e.g., Fig. 1; Joyce et al. 1992) while also transporting eddy and slope waters on to the shelf (e.g., Lee and Brink 2010;Zhang and Gawarkiewicz 2015). Eddyforced fluxes are one of many time-dependent mechanisms that could accomplish the shelf-water export and deep-water import required to satisfy volume, heat, and salt budgets for the Mid-Atlantic Bight (Brink 1998;Lentz 2010). The relative importance of such eddyforced fluxes in satisfying these shelf budgets is still unconstrained (Cherian and Brink 2016). ...
... Third, the onshore flux of eddy and slope waters has yet to be parameterized. Such a scaling is critical for assessing how important eddy-driven cross-shelf exchange is to closing the salt budget of Lentz (2010). Fourth, the apparent insensitivity of the supply jet's cross-shelf scale when bottom friction is varied significantly must be examined in more detail. ...
Isolated monopolar eddies in the ocean tend to move westward. Those shed by western boundary currents may then interact with the continental margin. This simple picture is complicated by the presence of other flow features, but satellite observations show that many western boundary continental shelves experience cross-shelfbreak exchange flows forced by mesoscale eddies translating near the shelf break. Here we extend our previous study of eddy interaction with a flat shelf to that with a sloping shelf. Using a set of primitive equation numerical simulations, we address the vertical structure of the onshore and offshore flows forced by the eddy, the origin of the exported shelf water, and the extent to which eddy water can penetrate onto the shelf. The simulations reveal an asymmetry in the vertical structure of cross-shelfbreak flows: the offshore flow is generally barotropic, whereas the onshore flow is always baroclinic. The exported shelf water is sourced from downstream of the eddy in the coastal-trapped wave direction and is supplied by a barotropic alongshore jet. This "supply jet" has a Rhines-like cross-shore length scale proportional to (eddy velocity scale/shelf topographic beta)1/2 measured from the shelf break. Eddy water is forced onto the shelf and is present up to a distance of one internal Rossby deformation radius, defined using shelf properties, from the shelf break. We rationalize these horizontal and vertical scales, connect them to existing observations, and extend our previous parameterization of eddy-forced offshore shelf-water flux to account for nonzero shelf slopes.
... Studies suggested that the roles of the surface heat flux and ocean heat transport, which directly relate to the heat content, may vary on different time scales [Kushnir, 1994;Weisberg, 2002, 2003;Gulev et al., 2013] and are subjected to regional differences [Halliwell, 1998;Dong and Kelly, 2004]. In the Middle Atlantic Bight (MAB), the sea surface heat flux is balanced by the heat advection especially the along-isobath heat advection in the sense of the long-term mean [Lentz, 2009]. On time scales from days to weeks, the heat advection drives the temperature variability over the New England shelf [Lentz et al., 2010]. ...
... The observed spatial viabilities of advection components are determined by the distributions of both the temperature gradient and the circulations over the shelf, which may further relate to various ocean dynamic processes subjected to regional differences. One example is the existence of the warm slope water and the Gulf Stream (and thus the increased temperature gradient) [Lentz, 2009] in the southern MAB, which may partially explain the southward intensification of the heat advection all through these advection components. Also, the occurring of the shelf break fronts [Linder and Gawarkiewicz, 1998;Flagg et al., 2006] are probably related to the stronger heat advection along the shelf break region (Figure 4a). ...
The upper ocean horizontal heat advection over the middle and outer shelf of the Middle Atlantic Bight (MAB) is investigated using satellite and in-situ observations. In the upper mixed layer, the heat advection is mostly positive indicating that it decreases the shelf heat content by bringing cold water from upstream. The heat advection driven by the barotropic geostrophic current is one order larger relative to the density-driven geostrophic shear and the wind-driven current. The barotropic geostrophic advection components in the alongshore and offshore direction are of the same order. To investigate the temporal properties of the heat advection, the temperature and currents are decomposed into different time scales using Fast Multidimensional Ensemble Empirical Decomposition (FMEEMD). The cross-spectral interactions within the advection are quantitatively evaluated with major components identified. Due to the cross-spectral interaction, energy within the heat advection are found to be redistributed through different time scales, with at least 46.5% variation retained within the original band for the barotropic geostrophic advection. Our results help to better understand the temporal variability of the heat advection, provide a baseline of the non-linear energy transfer framework within different time scales in the heat advection, and imply the possibility of interplays between short and long term oceanic phenomena.
... The cross-shelf density gradient is large at the shelfbreak front, driving an equatorward geostrophic shelfbreak jet [Houghton et al., 2006]. Onshore intrusions of the offshore waters affect shelf circulation and biogeochemistry [Gawarkiewicz et al., 1996;Lentz, 2003]; the onshore salt flux induced by shelfbreak exchange is a key for maintaining along-shelf density homogeneity [Lentz, 2010]. Shelfbreak exchange processes include cross-frontal eddy transport [Gawarkiewicz et al., 2004], midcolumn saline intrusions [Lentz, 2003], wind-driven nearsurface flows [Gawarkiewicz et al., 1996], and topographically influenced bottom flows [Allen and Durrieu de Madron, 2009]. ...
... Sv in the 2006 shelf water streamer [Chen et al., 2014]. Applying a temperature difference between the shelf and ring waters of ΔT = 10°C, the mean onshore heat transport of the 2014 intrusion is Q t = ρ 0 C p Q v ΔT ≈ 9.4 × 10 12 W, >50% of the maximum monthly mean surface upward heat transport over the entire MAB shelf (~100 × 800 km 2 ) [Lentz, 2010]. Here C p = 4190 J(kg°C) À1 is seawater heat capacity. ...
Onshore intrusions of offshore waters onto the Mid-Atlantic Bight shelf can greatly affect shelf circulation, biogeochemistry and fisheries. Previous studies have concentrated on onshore intrusions of slope water. Here we present a direct intrusion of Gulf Stream warm-core ring water onto the shelf representing a previously unknown exchange process at the shelfbreak. Impingement of warm-core rings at the shelfbreak generates along-isobath intrusions that grow like Pinocchio's nose, extending hundreds of kilometers to the southwest. By combining satellite and Ocean Observatory Initiative Pioneer Array data and idealized numerical simulations, we discover that the intrusion results from topographically induced vorticity variation of the ring water, rather than from entrainment of the shelfbreak frontal jet. This intrusion of the Gulf Stream ring water has important biogeochemical implications and could facilitate migration of marine species across the shelfbreak barrier and transport low-nutrient surface Gulf Stream ring water to the otherwise productive shelfbreak region.
... Mean southward transport over the Middle Atlantic Bight (MAB) shelf and slope transports relatively fresh cold water towards Cape Hatteras, NC, throughout the year (Chapman and Beardsley, 1989;Lentz, 2008). Shelf edge exchanges between the outer shelf and the open ocean occur within and across the MAB shelf break front (SBF) all along the MAB, and have important ramifications for MAB heat, salt, nutrient and organic matter budgets (Lentz, 2010;Houghton et al., 1988). Shelf edge processes contributing to that exchange have been investigated extensively, both to understand the dynamics of the SBF (Fratantoni et al., 2001) and to quantify those exchanges (Houghton et al., 1988). ...
... In addition to studies focusing on the SBF carried portion, other studies have examined the middle and inner shelf components of mean southward transport in the MAB; for example, the recent momentum, heat and salt budgets of Lentz (2010). Cold inner and middle shelf water transits the entire MAB, arriving at Cape Hatteras more slowly than the SBF-carried component, with more time for surface heat flux alteration along the way. ...
Studies of Middle Atlantic Bight (MAB) shelf water export to the open ocean at Cape Hatteras have produced fairly consistent estimates of ∼0.25 Sv, with just under half of the total carried within the Shelfbreak Front (SBF), and the remainder carried southward over the MAB shelf inshore of the shelf break and the SBF. Mean northward transport along the northern South Atlantic Bight (SAB) delivers SAB shelf water to the Cape Hatteras, which must also be exported to the open ocean. In the following, the magnitude of year-round SAB shelf water export at Cape Hatteras is estimated for the first time, and is found to be as large as the exported MAB shelf water volumes, with large seasonal variability. In summer, SAB export volume is much larger than the volume of exported MAB shelf water, while in late winter, MAB export volume exceeds that of SAB shelf water. Several aspects of circulation fields at Cape Hatteras that affect export pathways and residence times of both MAB and SAB shelf water are examined. The importance of these large and variable export volumes to carbon budgets in the southern MAB and northern SAB are considered.
... The increasing frequency of these intrusions likely has important ramifications for the salt balance on the continental shelf in the Middle Atlantic Bight and certainly calls for an update to the salt budget estimated by Lentz (2010). However, given the uncertainty over both estimating the duration and along-shelf scales of the intrusions, which are not attainable from the data sets examined herein, we hesitate to provide even an order of magnitude estimate for the increased salt flux to the continental shelf. ...
Shelfbreak exchange processes have been studied extensively in the Middle Atlantic Bight. An important process occurring during stratified conditions is the Salinity Maximum Intrusion. These features are commonly observed at the depth of the seasonal pycnocline, and less frequently at the surface and bottom. Data collected from NOAA's National Marine Fisheries Service Ecosystem Monitoring program as well as data collected from the fishing industry in Rhode Island show that the middepth intrusions are now occurring much more frequently than was reported in a previous climatology of the intrusions (Lentz, 2003, https://doi.org/10.1029/2003JC001859). The intrusions have a greater salinity difference from ambient water and penetrate large distances shoreward of the shelf break relative to the earlier climatology. The longer term data from the Ecosystem Monitoring program indicates that the increase in frequency occurred in 2000, and thus may be linked to a recent regime shift in the annual formation rate of Warm Core Rings by the Gulf Stream. Given the increased frequency of these salty intrusions, it will be necessary to properly resolve this process in numerical simulations in order to account for salt budgets for the continental shelf and slope.
... In a study by Kirincich and Lentz (2017), the effects of O(1-5 km) eddies in the inner shelf resulted in exchange equivalent to half the wind-driven, depth-dependent exchange and significant across-shelf heat flux. Over the shelf as a whole, both model (Wilkin, 2006) and observational (Lentz, 2010) results suggest that lateral, or "eddy" advection has a cooling effect on the shelf, removing as much as half of the seasonal temperature increase that would be produced by the surface fluxes alone. Thus, improved in situ estimates of the eddy fluxes driven by the energy cascade (i.e., Capet et al., 2008b) are necessary to further constrain budgets of basic water properties over the shelf, particularly as the data availability from SST imagery is not sufficient to understand the time dependence of the fluxes. ...
This study examines the spatial and temporal variability of eddy kinetic energy over the Northeast Shelf using observations of surface currents from a unique array of six high frequency radar systems. Collected during summer and winter conditions over three consecutive years, the horizontal scales present were examined in the context of local wind and hydrographic variability, which were sampled concurrently from moorings and autonomous surface vehicles. While area‐averaged mean kinetic energy at the surface was tightly coupled to wind forcing, eddy kinetic energy was not, and was lower in magnitude in winter than summer in all areas. Kinetic energy wavenumber spectral slopes were generally near k−5/3, but varied seasonally, spatially, and between years. In contrast, wavenumber spectra of surface temperature and salinity along repeat transect lines had sharp k⁻³ spectral slopes with little seasonal or inter‐annual variability. Radar‐based estimates of spectral kinetic energy fluxes revealed a mean transition scale of energy near 18 km during stratified months, but suggested much longer scales during winter. Overall, eddy kinetic energy was unrelated to local winds, but the up‐ or down‐scale flux of kinetic energy was tied to wind events and, more weakly, to local density gradients.
... In comparison to the middle/outer shelf profiles, the temperature and salinity during November 2016 to February 2017 were both above the long-term climatology. While high temperature can be expected in the context of long-term warming of the Northwest Atlantic coastal ocean , the large salinity anomalies at such shallow locations would almost certainly be caused by the intrusion of offshore waters as freshwater flux at the air-sea interface, that is, evaporation-minus-precipitation, is negligible in the regional salt budget (Lentz, 2010). Although only two profiles were collected in November 2016 and only one profile was collected in January 2017 at this inner shelf location, the salinity anomaly is clearly visible from these three profiles ( Figure 5). ...
Observations and high‐resolution numerical modeling are used to investigate the dynamical processes related to the initiation of an advective Marine Heatwave in the Middle Atlantic Bight of the Northwest Atlantic continental shelf. Both the observations and the model identify two significant cross‐shelf intrusions in November 2016 and January 2017, with the latter inducing large‐magnitude water mass anomalies across the shelf. Model prognostic fields reveal the importance of the combination of cyclonic eddies or ringlets and upwelling‐favorable winds in producing the large‐distance cross‐shelf penetration and temperature/salinity anomalies. The cyclonic eddies in close proximity to the shelfbreak set up local along‐isobath pressure gradients and provide favorable conditions for the intensification of the shelfbreak front, both processes driving cross‐isobath intrusions of warm, salty offshore water onto the outer continental shelf. Subsequently, strong and persistent upwelling‐favorable winds drive a rapid, bottom intensified cross‐shelf penetration in January 2017 composed of the anomalous water mass off the shelfbreak. The along‐shelf settings including realistic representation of bathymetric features are essential in the characteristics of the cross‐shelf penetration. The results highlight the importance of smaller scale cyclonic eddies and the intricacy of the interplay between multiple processes to drive significant cross‐shelf events.
... An example is the current system that transports water from east of Greenland, around the Labrador Sea to the Gulf of Maine and Middle Atlantic Bight, before turning offshore at Cape Hatteras (Chapman and Beardsley, 1989;Fratantoni and Pickart, 2007). The quasi-continuous shelf flow and shelfbreak jet system is an important conduit of cold low salinity water from high to mid latitudes in the western North Atlantic (Lentz, 2010) and transports anomalies in both heat and salt (Shearman and Lentz, 2010;Feng et al., 2016). Similar shelf current systems exist in other ocean basins. ...
Ocean surface winds, currents, and waves play a crucial role in exchanges of momentum, energy, heat, freshwater, gases, and other tracers between the ocean, atmosphere, and ice. Despite surface waves being strongly coupled to the upper ocean circulation and the overlying atmosphere, efforts to improve ocean, atmospheric, and wave observations and models have evolved somewhat independently. From an observational point of view, community efforts to bridge this gap have led to proposals for satellite Doppler oceanography mission concepts, which could provide unprecedented measurements of absolute surface velocity and directional wave spectrum at global scales. This paper reviews the present state of observations of surface winds, currents, and waves, and it outlines observational gaps that limit our current understanding of coupled processes that happen at the air-sea-ice interface. A significant challenge for the coming decade of wind, current, and wave observations will come in combining and interpreting measurements from (a) wave-buoys and high-frequency radars in coastal regions, (b) surface drifters and wave-enabled drifters in the open-ocean, marginal ice zones, and wave-current interaction “hot-spots,” and (c) simultaneous measurements of absolute surface currents, ocean surface wind vector, and directional wave spectrum from Doppler satellite sensors.
... [WSW]) meet and produce the Gulf's water properties (Townsend, Rebuck, Thomas, Karp-Boss, & Gettings, 2010; Townsend et al., 2015). While water temperatures in the GoM are responsive to atmospheric heat flux (Mountain, Strout, & Beardsley, 1996), the relative advection of these three water masses into the GoM has the greatest influence on bottom ocean temperature (Bigelow, 1933;Chen et al., 2014;Lentz, 2010). The degree that each water mass influences these temperatures is subject to the latitudinal variability of the boundary between WSW and LSW on an interannual time scale (Chapman & Beardsley, 1989;Csanady & Hamilton, 1988;Townsend et al., 2010;. ...
This study uses the Maine Department of Marine Resources Lobster Sea Sampling data (2000–2016) and logistic models to develop the first time series for the timing and suddenness of onset of the initial intra‐annual molt of American lobster in the Gulf of Maine (GoM), an annual fishery recruitment event crucial to fishermen. Data from three GoM regions (eastern, central, and western coastal Maine) were further divided by sex and estimated maturity of sampled lobsters for analysis. We found differences in the patterns of initial molt timing and suddenness between the regions, sexes, and stages of maturity. Using the Northeast Coastal Ocean Forecasting System hindcast temperatures, seasonal accumulated degrees above 5°C were used to describe the thermal history for each region at ocean depths of about 5 and 110 m. These temperature metrics were used in generalized linear models to investigate the potential effects of seasonal temperatures on the initial molt season. Results showed that initial intra‐annual molting of lobsters was variable from 2000 to 2016, with periods of both earlier and more sudden molts and later and more protracted molts. Warmer temperatures, specifically inshore temperatures, were generally associated with an earlier molt, but without complete uniformity in the direction and magnitude across seasons, regions, and lobster demographics. We also discuss why developing molt time series and quantifying the connection to the bottom temperatures are necessary and emphasized why existing monitoring programs and the applied quantification techniques herein make this relationship difficult to quantify.
... In the upper 100 m, shelfbreak frontal water was estimated to contribute about 75% of the volume of the eddy. In his analysis of the salt budget of the MAB, Lentz (2010) concluded that the southward increase in the depth-averaged salinity of 1 psu (1000 km) 21 at midshelf must be balanced by a substantial onshore flux of salt. ...
The Lagrangian motion in the eddy field produced from an unstable retrograde jet along the shelf break is studied from idealized numerical experiments with a primitive equation model. The jet is initially in thermal wind balance with a cross-isobath density gradient and is not subjected to any atmospheric forcing. Over the course of the model integration, the jet becomes unstable and produces a quasi-stationary eddy field over a 2-month period. During this period, the cross-slope flow at the shelf break is characterized by along-slope correlation scales ofO(10) km and temporal correlation scales of a few days. The relative dispersion of parcels across isobaths is found to increase with time as t b , where 1 < b < 2. This mixed diffusive-ballistic regime appears to reflect the combined effects of (i) the short length scales of velocity correlation at the shelf break and (ii) the seaward excursion of monopolar and dipolar vortices. Cross-slope dispersion is greater offshore of the front than inshore of the front, as offshore parcels are both subducted onshore below density surfaces and translated offshore with eddies. Nonetheless, the exchange of parcels across the jet remains very limited on the monthly time scale. Particles originating from the bottom experience upward displacements of a few tens of meters and seaward displacements of O(100) km, suggesting that the eddy activity engendered by an unstable along-slope jet provides another mechanism for bottom boundary layer detachment near the shelf edge.
... Water temperatures in the GoM are also responsive to atmospheric heat flux (Mountain et al. 1996), but advection of different water masses mixing into the GoM are thought to have the greatest influence (Bigelow, 1933;Chen et al., 2014;Lentz, 2010). ...
We investigated the phenological and fisheries dynamics surrounding the spring molt of American lobster (Homarus americanus, Milne Edwards 1847) in the Gulf of Maine. We created a time series from Maine Department of Marine Resources Lobster Sea Sampling data using logistic models to estimate the timing and duration of the spring molt for eastern, central, and western regions of the Maine coast. These estimates revealed substantial inter-annual variability in the timing of the spring molt for all regions and that 2012 was indeed anomalously early relative to other years. Each region experienced significantly different molt timing for any given year, indicating that there are spatially-distinct molting phenologies along the Maine coastline. Generalized Linear Models were constructed using the molting time series and hindcasted bottom temperatures from the Northeast Coastal Ocean Forecasting System using the Finite Volume Composite Ocean Model to analyze how nearshore and offshore bottom ocean temperatures might shape molting trends and differences. This analysis revealed that the influence of nearshore temperatures was significant in the eastern region only and the relationship between nearshore temperatures and the timing of the spring molt weakened from east to west. Logistic models were also applied to Maine Department of Marine Resources Landings Program data to estimate and evaluate multiple landings-based proxies for the timing of the spring molt via the fishing fleet’s ability to synchronize with the lobster molting phenology. Newshell landings, as a percent of the annual weekly maximum, were identified as the best proxy, given relative difference from the annual in-situ estimates of spring molt timing and lower standard error values. The fleet’s ability to synchronize with variable spring molting phenology was assessed using a correlation analysis. This analysis revealed that both eastern and western fleets followed the same temporal patterns as the lobster molt timing in their region and the western fleet showed a poorer, more variable ability to absolutely synchronize their timing when compared to the eastern fleet. Maine lobstermen were interviewed to investigate how they achieve an optimal synchrony, revealing the utilization of several environmental and non-environmental variables. General temperature, lunar and tidal phases, and Penobscot River discharge were fishermen-nominated variables tested using correlation analysis. These analyses showed that fishermen methodology and its association with spring molt timing were spatially variable. General temperatures displayed the same weakening association with spring molt from east to west; tidal phase was significant in the east only; and river discharge was significantly associated in the eastern and central regions. River discharge association with molting was also temporally variable, showing strongest significant positive relationships during April. We discuss these investigations into the temporal and spatial dynamics of the spring lobster molt along the Maine coast and the fishery’s response to inter-annual variation, creating a baseline of information about the spring molt for Maine. We also discuss the degree to which the fleet is able to approximate and adapt to inter-annual variation in this phenology and some of the methods they have been using to accomplish this synchrony.
... Regardless of the southward reduction, the horizontal transport remains a dominant factor governing interannual salinity further south even in estuaries like the Long Island Sound where anomalous coastal freshwater discharge explains only 25% of the variance of the salinity anomaly [Lee and Lwiza, 2005]. Expecting that along shelf reduction in the southwestward freshwater transport is accompanied by the associated reduction in cold water transport, one may note that even in the MAB, the horizontal heat transport remains the major term balancing the positive 10 Wm 22 net surface heat flux into the ocean [Beardsley and Boicourt, 1981; Journal of Geophysical Research: Oceans 10.1002/2016JC012580 Lentz, 2010]. In comparison with the mixed layer heat budget, the horizontal advection is expected to play even a greater role for the mixed layer salt budget [e.g., Foltz et al., 2004]. ...
Sea surface salinity (SSS) from the Aquarius and SMOS satellite missions displays a steady increase of ∼1psu over the entire northwestern Atlantic shelf south of Nova Scotia during the 2011-2015. Put in the context of longer ocean profile data the results suggest that mixed layer salinity and temperature north of the Gulf Stream experience positively correlated shelf-wide interannual oscillations (1psu/2degC). Salty and warm events occur coincident with anomalous easterly-southeasterly winds and Ekman transport counteracting the mean southwestward shelf currents. They are coincident with the weakening of both branches of the Scotian Shelf Current (SSC), but only moderately correlate with shifts of the Gulf Stream North Wall. This suggests that salt advection by anomalous SSC acting on the mean salinity gradient is the primary driver regulating the transport of fresh/cold water from high latitudes. The advection mechanism imposes a connectedness of the larger-scale interannual variability in this region and its tie to atmospheric oscillations. In the second part, an analysis of 15-year long numerical simulations is presented which show 8 interannual salinity oscillations (positive and negative). Six of these are driven by the horizontal advection by slow varying currents (>2 months), while 2 events are driven by the horizontal eddy advection (<2 months). In line with observations, salt/warm model events correspond to anomalously weak SSC, correlate with southeasterly wind anomaly, and confirm that interannual horizontal salt advection drives the interannual salinity. Indeed, vertical exchanges provide a negative feedback, while interannual horizontal diffusion and the net surface salt flux anomalies are small. This article is protected by copyright. All rights reserved.
... In more typical coastal oceans, the alongshore variation in temperature is controlled in part by alongshore currents. This correlation is illustrated by a simple model of alongshore temperature variation which has proven successful in the Mid-Atlantic Bight, and which should be informative along many coasts in the absence of intense upwelling or tidal mixing (Lentz 2010). The model assumes that the temperature evolution of a parcel of water at the surface is simply the surface heat flux into the parcel, and the alongshore variation of temperature is given by the integral of the surface heat flux along the path of surface water. ...
Dispersal of many coastal marine species is mediated by flows with strong directionality; bathymetric and topographic effects lead to strong alongshore variability in this transport. Using a simple model of the population dynamics of competing benthic species in a coastal ocean, we found that alongshore variability in dispersal can lead to clustering of species range boundaries for species whose dispersal is dominated by coastal currents. Furthermore, species can be absent from areas where they would have a relative competitive advantage because the presence or absence of a species is determined not only by local conditions but also by propagule supply, which is often affected by larval transport from far upstream. Our model demonstrates the quantitative linkages between alongshore variation in coastal currents, spatial gradients in competitive strength, and the geographic extent of a species. We show that the predictions of the model are consistent with observed species distributions in the Gulf of Maine and Mid-Atlantic Bight, USA. A mechanism for extensive coexistence of competing species where range boundaries cluster is described. The implication of the clustering highlighted by our model suggests that for species whose dispersal is dominated by long-distance planktonic periods, climate change induced changes in the relative competitiveness of species will lead to abrupt changes in species range boundaries and not gradual range extension.
... In this work, we extend the case study of the 2012 extreme warming event to multiple years during the winter and spring. As stated in earlier studies [e.g., Lentz, 2010;, the limited subsurface observations make it difficult to accurately quantify the heat balance in the MAB, particularly over interannual and longer time scales. Therefore, we employ numerical modeling to investigate the interannual variability of winter-spring temperature in the MAB, specifically to address the following scientific questions. ...
Relative contributions between the local atmospheric and oceanic processes on the interannual variability of winter-spring shelf temperature in the Middle Atlantic Bight (MAB) are investigated based on a regional ocean model. The model demonstrates sufficient capability to realistically simulate the interannual temperature changes during 2003-2014. On interannual time scales, the mean winter/spring temperature in the MAB is determined by the combination of the initial temperature at the beginning of the season and the mean cumulative air-sea flux, while the mean cumulative ocean advective flux plays a secondary role. In spite of the overall importance of air-sea flux in determining the winter and spring temperature, the relative contributions between air-sea flux and ocean advective flux on the evolution of the temperature anomaly in each individual year varies. The predictability of spring (April-June) temperature based on winter (January-March) temperature is weak because the temporal decorrelation time scale changes significantly from year to year. Both the highly variable shelf temperature and its decorrelation time scale are affected by the changes in the relative contributions between the air-sea flux and ocean advective flux.
... This work indicates that air-sea heat exchange plays an important role in regulating heat content on the shelf. However, previous studies have demonstrated that long-term changes in ocean temperature in the region are best explained by along-shelf ocean advection [e.g., Lentz, 2010;Shearman and Lentz, 2010]. In addition, shelf-slope exchange processes at the edge of the continental shelf [e.g., Chen et al., 2014b;Garvine et al., 1989;Gawarkiewicz et al., 2004], particularly under the strong influence of the Gulf Stream , are another potentially important player in determining the shelf temperature budget. ...
In the coastal ocean off the Northeast U.S., the sea surface temperature (SST) in the first half of 2012 was the highest on the record for the past roughly 150 years of recorded observations. The underlying dynamical processes responsible for this extreme event are examined using a numerical model, and the relative contributions of air-sea heat flux versus lateral ocean advective heat flux are quantified. The model accurately reproduces the observed vertical structure and the spatiotemporal characteristics of the thermohaline condition of the Gulf of Maine and the Middle Atlantic Bight waters during the anomalous warming period. Analysis of the model results show that the warming event was primarily driven by the anomalous air-sea heat flux, while the smaller contribution by the ocean advection worked against this flux by acting to cool the shelf. The anomalous air-sea heat flux exhibited a shelf-wide coherence, consistent with the shelf-wide warming pattern, while the ocean advective heat flux was dominated by localized, relatively smaller-scale processes. The anomalous cooling due to advection primarily resulted from the along-shelf heat flux divergence in the Gulf of Maine, while in the Middle Atlantic Bight the advective contribution from the along-shelf and cross-shelf heat flux divergences was comparable. The modeling results confirm the conclusion of the recent analysis of in situ data by Chen et al. (2014a) that the changes in the large-scale atmospheric circulation in the winter of 2011–2012 primarily caused the extreme warm anomaly in the spring of 2012. The effect of along-shelf or cross-shelf ocean advection on the warm anomalies from either the Scotian Shelf or adjacent continental slope was secondary.
... Near the site of the EPSONDE measurements, temperature and salinity gradients in the cross-isobath direction have been well documented . Lentz [2010] also showed that there existed a temperature gradient in the along-isobath direction, increasing from southeast to northwest. Figure 4 shows the time-depth variations of velocities and progressive vector diagrams at four selected heights above the seabed. ...
The layered structure of stratification and mixing on the New England Shelf (NES) in summer is examined by analyzing a comprehensive set of observations of hydrography, currents and turbulence. A clear distinction in mixing characteristics between the mid-column water (consisting of sub-surface stratification, mid-depth weak stratification and lower-layer stratification) and a well-mixed bottom boundary layer (BBL) is revealed. The combination of subtidal Ekman onshore bottom transport and cross-shore density gradient created a lower-layer stratification that inhibited the upward extension of the BBL turbulence. The BBL mixing was related to strong shear generated by bottom stress, and the magnitude and periodic variation of BBL mixing was determined by both the tidal and subtidal flows. Mixing in the mid-column water occurred under stably-stratified conditions and showed correspondence with the occurrence of near-inertial and semi-diurnal internal waves. Positive correlations between buoyancy frequency squared (N2) and shear variance (S2), S2 and dissipation rate (ε), N2 and ε are established in the mid-column, but not in the BBL. The mid-column ε was reasonably described by a slightly modified MacKinnon-Gregg (MG) model.
... This contrasts with some midlatitude shelves, which, often characterized by a surplus in surface heat fluxes, require ocean heat flux divergence as a source of cooling. For instance, Lentz [2010] estimated that the Middle Atlantic Bight's long-term averaged surface heat flux surplus (~10 W m À2 ) is primarily balanced by cold southward (along-shelf) currents. On the outer New England shelf, Lentz et al. [2010] showed that surface fluxes in summer are balanced by cross-shelf heat removal, while in winter, along-shelf advection imposes strong cooling on the region's waters. ...
We constructed annual cycles of National Centers for Environmental
Prediction air-sea fluxes and temporal oceanic heat content change from
Seward Line hydrographic surveys to quantify the different contributions
to the oceanic heat budget within the Alaska Coastal Current (ACC) on
the northern Gulf of Alaska shelf. The deficit between air-sea fluxes
and the temporal change in oceanic heat content throughout the cooling
season (October-April) varies from ~40 to 110 W m-2 and is
balanced by ocean heat flux convergence. Cross-shelf heat flux
convergence is insignificant on annual average, and the nearshore heat
budget is likely entirely balanced by the ACC, which resupplies ~15%-50%
of the heat removed by air-sea fluxes during the cooling season.
Furthermore, we estimated spatial heat flux gradients and conclude that
air-sea fluxes increase from east to west and from offshore to onshore.
The cross-shore gradients are governed by wind speed gradients, likely
due to ageostrophic nearshore wind events during the cooling season,
while the along-shelf heat flux gradients are governed by the occurrence
of low-pressure systems in the northern GOA that result in cold
northerly winds over the northwestern GOA. These results underline the
ACC's role as the dominant oceanic heat source to the northern GOA shelf
and further imply an increased cooling rate of the ACC west of the
Seward Line. Furthermore, our analysis showed that nearshore regions,
particularly waters in the ACC, are subjected to stronger winter cooling
than the middle and outer shelves.
... Bignami and Hopkins [2003] inferred from surface heat fluxes and temperature changes that advective heat fluxes at the southern end of the Middle Atlantic Bight, near Cape Hatteras, must be large. Recently, Lentz [2010] used historical data to show that surface heat fluxes and along-shelf advection could account for the observed along-isobath temperature increase from northeast to southwest in the MAB. However, this result was sensitive to the uncertainty in the surface heat flux climatologies. ...
Heat and salt balances over the New England shelf are examined using 10 month time series of currents, temperature, and salinity from a four element moored array and surface heat and freshwater fluxes from a meteorological buoy. A principal result is closure of the heat budget to 10 W m-2. The seasonal variation in depth-average temperature, from 14°C in September to 5°C in March, was primarily due to the seasonal variation in surface heat flux and a heat loss in winter caused by along-shelf advection of colder water from the northeast. Conductivity sensor drifts precluded closing the salt balance on time scales of months or longer. For time scales of days to weeks, depth-average temperature and salinity variability were primarily due to advection. Advective heat and salt flux divergences were strongest and most complex in winter, when there were large cross-shelf temperature and salinity gradients at the site due to the shelf-slope front that separates cooler, fresher shelf water from warmer, saltier slope water. Onshore flow of warm, salty slope water near the bottom and offshore flow of cooler, fresher shelf water due to persistent eastward (upwelling-favorable) winds caused a temperature increase of nearly 3°C and a salinity increase of 0.8 in winter. Along-shelf barotropic tidal currents caused a temperature decrease of 1.5°C and a salinity decrease of 0.7. Wave-driven Stokes drift caused a temperature increase of 0.5°C and a salinity increase of 0.4 from mid December to January when there were large waves and large near-surface cross-shelf temperature and salinity gradients.
The characteristics and dynamics of depth-average along-shelf currents at monthly and longer time scales are examined using 17 years of observations from the Martha’s Vineyard Coastal Observatory on the southern New England inner shelf. Monthly averages of the depth-averaged along-shelf current are almost always westward, with the largest interannual variability in winter. There is a consistent annual cycle with westward currents of 5 cm s ⁻¹ in summer decreasing to 1–2 cm s ⁻¹ in winter. Both the annual cycle and interannual variability in the depth-average along-shelf current are predominantly driven by the along-shelf wind stress. In the absence of wind forcing, there is a westward flow of ∼5 cm s ⁻¹ throughout the year. At monthly time scales, the depth-average along-shelf momentum balance is primarily between the wind stress, surface gravity wave–enhanced bottom stress, and an opposing pressure gradient that sets up along the southern New England shelf in response to the wind. Surface gravity wave enhancement of bottom stress is substantial over the inner shelf and is essential to accurately estimating the bottom stress variation across the inner shelf.
Significance Statement
Seventeen years of observations from the Martha’s Vineyard Coastal Observatory on the inner continental shelf of southern New England reveal that the depth-average along-shelf current is almost always westward and stronger in summer than in winter. Both the annual cycle and variations around the annual cycle are primarily driven by the along-shelf wind stress. The wind stress is opposed by a pressure gradient that sets up along the southern New England shelf and a surface gravity wave–enhanced bottom stress. The surface gravity wave enhancement of bottom stress is substantial in less than 30 m of water and is essential in determining the variation of the along-shelf current across the inner shelf.
Submarine canyons provide a conduit for shelf-slope exchange via topographically induced processes such as upwelling and downwelling. These processes in the Wilmington Canyon, located along the shelf-break of the Mid-Atlantic Bight (MAB), have not been previously studied, and the associated hydrographic variability inside the canyon and on the adjacent shelf are largely unknown. Observations from an underwater glider deployed in Wilmington Canyon (February 27 - March 8, 2016), along with wind and satellite altimetry data, showed evidence for a wind-driven canyon upwelling event followed by a subsequent downwelling event. Next, a numerical model of the MAB was developed to more fully represent these two events. Modeled results showed that under upwelling-favorable winds during February 25 - March 3, sea level increased seaward, shelf currents flowed northeastward, and canyon upwelling developed. Then under downwelling-favorable winds during March 4-7, sea level increased landward, shelf currents flowed southwestward, and canyon downwelling developed. Modeling experiments showed that canyon upwelling and downwelling were sub-tidal processes driven by winds and pressure gradients (associated with SSH gradients), and they would occur with or without tidal forcing. During the upwelling period, slope water originating from 150-215 m depths within the canyon (75 m below the canyon rim), was advected onto the shelf, forming a cold and dense canyon-upwelled slope-originated overflow water at the bottom of the outer shelf (75-150 m isobaths). The dense overflow current flowed was directed northeastward and expanded in the cross-shelf direction. It was 5-20 km wide and 10-30 m thick. The estimated volume of the plume overflow water exceeded 6×10⁹ m³ at peak. The density front at the shoreward side of the dense overflow water caused a subsurface baroclinic frontal jet, which flowed northeastward and along-shelf with maximum speed exceeding 0.5 m/s. In the ensuing downwelling event, a portion of the previously upwelled dense water was advected back to the canyon, and then flowed down-slope in the upper canyon in ~0.3 m/s bottom-intensified currents. Dynamical investigation of the overflow current showed that its evolution was governed by unbalanced horizontal pressure gradient force in the cross-shelf direction and that the current was geostrophic.
The Lagrangian characteristics of the surface flow field arising when an idealized, anticyclonic, mesoscale, isolated deep-ocean eddy collides with continental slope and shelf topography are explored. In addition to fluid parcel trajectories, we consider the trajectories of biological organisms that are able to navigate and swim, and for which shallow water is a destination. Of particular interest is the movement of organisms initially located in the offshore eddy, the manner in which the eddy influences the ability of the organisms to reach the shelf break, and the spatial and temporal distributions of organisms that do so. For non-swimmers or very slow swimmers, the organisms arrive at the shelf break in distinct pulses, with different pulses occurring at different locations along the shelf break. This phenomenon is closely related to the episodic formation of trailing vortices that are formed after the eddy collides with the continental slope, turns and travels parallel to the coast. Analysis based on finite-time Lyapunov exponents reveals initial locations of all successful trajectories reaching the shoreline, and provides maps of the transport pathways showing that much of the cross-shelf-break transport occurs in the lee of the eddy as it moves parallel to the shore. The same analysis shows that the onshore transport is interrupted after a trailing vortex detaches. As the swimming speeds are increased, the organisms are influenced less by the eddy and tend to show up en mass and in a single pulse.
The Cold Pool is a 20-60 m thick band of cold, near-bottom water that persists from spring to fall over the midshelf and outer shelf of the Middle Atlantic Bight (MAB) and Southern Flank of Georges Bank. The Cold Pool is remnant winter water bounded above by the seasonal thermocline and offshore by warmer slope water. Historical temperature profiles are used to characterize the average annual evolution and spatial structure of the Cold Pool. The Cold Pool gradually warms from spring to summer at a rate of order 1°C month⁻¹. The warming rate is faster in shallower water where the Cold Pool is thinner, consistent with a vertical turbulent heat flux from the thermocline to the Cold Pool. The Cold Pool warming rate also varies along the shelf; it is larger over Georges Bank and smaller in the southern MAB. The mean turbulent diffusivities at the top of the Cold Pool, estimated from the spring to summer mean heat balance, are an order of magnitude larger over Georges Bank than in the southern MAB, consistent with much stronger tidal mixing over Georges Bank than in the southern MAB. The stronger tidal mixing causes the Cold Pool to warm more rapidly over Georges Bank and the eastern New England shelf than in the New York Bight or southern MAB. Consequently, the coldest Cold Pool water is located in the New York Bight from late spring to summer.
At continental margins, energetic deep-ocean eddies can transport shelf water offshore in filaments that wrap around the eddy. One example is that of Gulf Stream warm-core rings interacting with the Mid-Atlantic Bight shelf. The rate at which shelf water is exported in these filaments is a major unknown in regional budgets of volume, heat, and salt. This unknown transport is constrained using a series of idealized primitive equation numerical experiments wherein a surface-intensified anticyclonic eddy interacts with idealized shelf-slope topography. There is no shelfbreak front in these experiments, and shelf water is tracked using a passive tracer. When anticyclones interact with shelf-slope topography, they suffer apparent intrusions of shelf-slope water, resulting in a subsurface maximum in offshore transport. The simulations help construct an approximate model for the filament of exported water that originates inshore of any given isobath. This model is then used to derive an expression for the total volume of shelf-slope water transported by the eddy across that isobath. The transport scales with water depth, radius, and azimuthal velocity scale of the eddy. The resulting expression can be used with satellite-derived eddy properties to estimate approximate real-world transports ignoring the presence of a shelfbreak front. The expression assumes that the eddy's edge is at the shelf break, a condition not always satisfied by real eddies.
Continental shelf waters located off the east coast of Canada and the United States are part of a long shelf current system that is partly comprised of colder, less-saline waters originating from high latitudes, including waters from the North Atlantic sub-polar gyre, along with ice-melt and freshwater input from local rivers. A 41-year analysis (1973–2013) of near-surface salinity (NSS) using three hydrographic datasets (Bedford Institute of Oceanography “Climate”, NOAA/ESDIM, and Canadian Marine Environmental Data Service (MEDS)) allowed an examination of NSS variability within 11 continental shelf sub-regions, extending from the southern Newfoundland Shelf of eastern Canada to the DelMarVa/Hatteras Shelf of the United States. Although the periods of record containing sufficient data vary between sub-regions, regional mean NSS values are lowest within the Gulf of St. Lawrence and highest on the DelMarVa/Hatteras shelf, with largest annual variability within the Gulf of St. Lawrence. After removal of outliers, long-term linear trends computed from annual mean NSS were detected along the Newfoundland Shelf (+0.011 y⁻¹), Western Scotian Shelf (−0.007 y⁻¹), Gulf of Maine (−0.014 y⁻¹), Georges Bank (−0.011 y⁻¹), and DelMarVa/Hatteras Shelf (+0.024 y⁻¹). A long-term quadratic fit to annual mean NSS from the Eastern Scotian Shelf displays a salinity increase through 1992 of +0.026 y⁻¹, decreasing thereafter until 2013 by −0.028 y⁻¹. A quadratic fit for the Western Grand Banks displays a NSS increase through 2007 of +0.022 y⁻¹, decreasing thereafter through 2013 by −0.006 y⁻¹. Annual mean NSS from the Eastern Grand Banks, Tail of the Grand Banks, Gulf of St. Lawrence, and Middle Atlantic Bight display no long-term trends. Inter-annual variability (IAV) of NSS residuals shows similar small mean squared error (mse) of 0.02–0.04 for the four northern-most sub-regions (Newfoundland Shelf, Eastern, Tail and Western Grand Banks) and are correlated at 0-year lag. IAV of NSS residuals (mse) are larger for the Gulf of St. Lawrence (~0.19), Eastern and Western Scotian Shelf (~0.09–0.06), Gulf of Maine and Georges Bank (~0.08–0.06), Middle Atlantic Bight (~0.19), and maximal for the DelMarVa/Hatteras Shelf (~0.36), and are also correlated at 0-year lag, but are uncorrelated with the four northern-most sub-regions. Consideration of a simple “flux variation” model that includes along-shelf, altimeter-derived velocity anomalies measured upstream on the Western Scotian Shelf and the positive along-shelf mean salinity gradient between the Eastern Scotian Shelf and the DelMarVa/Hatteras Shelf, may explain the synchronous nature of NSS residuals for the southern-most 6 sub-regions. Furthermore, the flux variation model results in calculated NSS residuals that are within a factor of two of observed NSS residuals for the southern-most DelMarVa/Hatteras Shelf. Co-varying broad-scale coastal sea level and shelf break front position anomalies also support the flux variation model, as do CMV Oleander temperature anomalies across a limited Middle Atlantic Bight shelf region. Overall, the relationships between along-shelf observations of NSS and other shelf parameters support an existing wind-driven dynamical shelf model. Specifically, a flux variation model is able to describe IAV of NSS along a section of the Canadian and U.S shelf for periods greater than one year. In the future, this model may be able to provide useful indices of regime change as noted within the Northeast Shelf Large Marine Ecosystem by other workers.
On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic
Bight and warm salty South Atlantic Bight shelf waters converge
alongshelf 90% of the time, causing strong alongshelf gradients in
temperature, salinity, and density known as the ‘Hatteras
Front’. Mechanisms of shoreward transport in this region have long
been a topic of interest, since many commercially important species
spawn on the outer shelf, but utilize the adjacent Albemarle and Pamlico
Sounds for nurseries, requiring some physical transport mechanism to
move the eggs and larvae from the outer shelf to these nursery areas.
One mechanism providing such shoreward transport is strong shoreward
velocity along the cross-shelf oriented ‘nose’ of the
Hatteras Front. The Frontal Interactions near Cape Hatteras (FINCH)
project used shipboard ADCP and a towed undulating CTD to examine
Hatteras Front property, density and velocity fields in August 2004,
January 2005, and July 2005. Strong property gradients were encountered
across the nose of the Hatteras Front in all cases, but the density
gradient evolved in time, and along with it the dynamic height gradient
driving the observed along-front cross-shelf velocities in the nose of
the Front. In August and January FINCH data, MAB shelf waters on the
north side of the Hatteras Front are less dense than SAB shelf waters,
driving shoreward velocities along the Hatteras Front. By July, MAB
shelf waters are slightly more dense than SAB shelf waters, with areas
of weak seaward and shoreward velocities within the Hatteras Front. As
Part 1 of a pair of contributions, this article focuses on FINCH data to
illustrate the range of density gradients encountered and resulting
cross-shelf velocities. Whether these observations are typical of
variability in the Hatteras Front is explored in a second article, Part
2.
On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic
Bight and warm salty South Atlantic Bight shelf waters converge
alongshelf 90% of the time, causing strong alongshelf gradients in
temperature and salinity known as the 'Hatteras Front'. The resulting
density gradient supports strong shoreward velocities in the cross-shelf
oriented 'nose', of the Front in wintertime. To investigate further, the
Frontal Interactions near Cape Hatteras (FINCH) project used shipboard
ADCP and a towed undulating CTD to examine Hatteras Front property,
density and velocity fields in August 2004, January 2005, and July 2005.
Strong property gradients were encountered across the nose of the
Hatteras Front in all cases, but the density gradient, dynamic height
gradient, and observed along-front cross-shelf velocities evolved in
time. FINCH along-Front velocities were strong and shoreward in fall and
winter, and weakly mixed shoreward and seaward in July. Several archived
data sets were examined, and demonstrate that the density evolution and
wind forcing seen in FINCH are characteristic of other years. Evidence
suggests the width of the Hatteras Front does not vary dramatically in
time, so that consistently large fall and winter density contrast across
the Front implies consistently large shoreward velocities along it in
winter. Weak density contrasts across the Hatteras Front in spring
suggest the magnitude and sign of springtime density gradients and
along-Front velocities could vary interannually. Recruitment success of
commercially important stocks on the shelf that depend on cross-shelf
transport may thus be affected year to year.
It is quite widely accepted that the along-shelf pressure gradient (ASPG) contributes in driving shelf currents in the Middle Atlantic Bight (MAB) off the northeastern U.S. coast; its origin, however, remains a subject for debate. Based on analyses of 16 yr (1993–2008) of satellite, tide gauge, river, and wind data and numerical experiments, the authors suggest that river and Coastal Labrador Sea Water (CLSW) transport contribute to a positive mean ASPG (tilt up northward) in the ratio of approximately 1:7 (i.e., CLSW dominates), whereas wind and the Gulf Stream tend to produce a negative mean ASPG in the ratio of approximately 1:6.
Data also indicate seasonal and interannual variations of ASPG that correlate with the Gulf Stream’s shift and eddy kinetic energy north of the Gulf Stream (N-EKE) due to warm-core rings. A southward shift in the Gulf Stream produces a sea level drop north of Cape Hatteras, which is most rapid in winter. The N-EKE peaks in late spring to early summer and is larger in some years than others. A process model is used to show that ring propagation along the MAB slope and ring impingement upon the shelf break north of Cape Hatteras generate along-isobath density gradients and cross-shelfbreak transports that produce sea level change on the shelf; the dominant ageostrophic term in the depth-integrated vorticity balance is the joint effect of baroclinicity and relief (JEBAR) term. In particular, the shelf’s sea surface slopes down to the north when rings approach Cape Hatteras.
Subtidal current and sea leavel response to wind and Gulf Stream forcing are investigated for the South Atlantic Bight shelf during winter conditions. Low-frequency flow variability in the outer shelf results primarily from wavelike meanders and eddies in the Gulf Stream front that occur in a 2-day to 2-week period band. Current meter derived vertically integrated momentum balances indicated that these large amplitude flow events are in approximate geostropic balance with baroclinic pressure gradients induced by northward propagating Gulf Stream disturbances.
A two-dimensional cross-shelf model of the New England continental shelf and slope is used to investigate the mean cross-shelf and vertical circulation at the shelf break and their seasonal variation. The model temperature and salinity fields are nudged toward climatology. Annual and seasonal mean wind stresses are applied on the surface in separate equilibrium simulations. The along-shelf pressure gradient force associated with the along-shelf sea level tilt is tuned to match the modeled and observed depth-averaged along-shelf velocity. Steady-state model solutions show strong seasonal variation in along-shelf and cross-shelf velocity, with the strongest along-shelf jet and interior onshore flow in winter, consistent with observations. Along-shelf sea level tilt associated with the tuned along-shelf pressure gradient increases shoreward because of decreasing water depth. The along-shelf sea level tilt varies seasonally with the wind and is the strongest in winter and weakest in summer. A persistent upwelling is generated at the shelf break with a maximum strength of 2 m day 21 at 50-m depth in winter. The modeled shelfbreak upwelling differs from the traditional view in that most of the upwelled water is from the upper continental slope instead of from the shelf in the form of a detached bottom boundary layer.
Sea surface temperature variations along the entire U.S. East Coast from 1875 to 2007 are characterized using a collection of historical observations from lighthouses and lightships combined with recent buoy and shore-based measurements. Long-term coastal temperature trends are warming in the Gulf of Maine [1.0° ± 0.3°C (100 yr)−1] and Middle Atlantic Bight [0.7° ± 0.3°C (100 yr)−1], whereas trends are weakly cooling or not significant in the South Atlantic Bight [−0.1° ± 0.3°C (100 yr)−1] and off Florida [−0.3° ± 0.2°C (100 yr)−1]. Over the last century, temperatures along the northeastern U.S. coast have warmed at a rate 1.8–2.5 times the regional atmospheric temperature trend but are comparable to warming rates for the Arctic and Labrador, the source of coastal ocean waters north of Cape Hatteras (36°N). South of Cape Hatteras, coastal ocean temperature trends match the regional atmospheric temperature trend. The observations and a simple model show that along-shelf transport, associated with the mean coastal current system running from Labrador to Cape Hatteras, is the mechanism controlling long-term temperature changes for this region and not the local air–sea exchange of heat. Author Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 40 (2010): 1004-1017, doi:10.1175/2009JPO4300.1. This work was supported by NSF Grant OCE-0220773.
Develops a set of approximate dynamics describing the subinertial motions in a vertically homogeneous and rapidly rotating layer of fluid with a free lower boundary. This is a very idealized model of the ocean mixed layer (ML). The simplified equations of motion, called the subinertial mixed layer (SML) approximation, are obtained by applying standard "filtering' arguments to the primitive equations. The analysis, and the resulting approximate dynamics, is similar to the quasigeostrophic set. -from Author
Seasonal-mean currents in fall, winter, and spring on the bathymetrically complex continental shelf 15-65 m deep off Montauk Point, outside Block Island Sound, are analyzed using moored profiling current-meter records from a 2.5-yr period. A sharp boundary, or coastal front, occurs where strong, shallow, generally southwestward flow that weakens nearly linearly with increasing depth meets deeper flow nearly opposite in direction (fall/winter) or markedly weaker (spring). Velocities veer clockwise (counterclockwise) with increasing depth inshore (offshore) of the front. Evidence is presented that thermal wind balance holds without major frictional modification: it accounts for the veering, and the seasonal-mean horizontal density gradients it implies are generally toward the southeast quadrant in agreement with limited hydrographic measurements. Substantial seasonal changes in flow and frontal attributes occur because of the interplay of annual cycles in wind forcing and buoyant discharge. In fall and winter, upwelling-favorable wind causes nearshore setdown of sea surface height and an inshore-directed barotropic pressure gradient: shallow down-coast (alongshore to the south and west) currents weaken, and deep currents are up-coast. In spring, buoyant discharge peaks and winds weaken: shallow down-coast flow strengthens and deep flow weakens. The front extends to the seafloor, with attachment depth shallowest in winter, deepest in spring, and intermediate in fall; its width is smaller in fall than in winter. Buoyant discharge theory based on bottom boundary layer density advection dynamics captures these seasonal shifts. Cross-shelf circulation includes offshore (onshore) shallow (deep) motion; deep onshore motion appears enhanced near a canyon and persistent through all three seasons, suggesting wind-driven upwelling is not solely responsible.
Velocity time series are used to study cross-shelf circulation on the northern California shelf and to examine classical ideas of locally wind-forced cross-shelf circulation. A simple linear two-dimensional model of cross-shelf transport is compared to estimates of cross-shelf transport in the near surface, interior, and near bottom. In winter, when wind forcing is brief and episodic, model transports are highly correlated to the total surface flow and show some skill in predicting subsurface cross-shelf flow. The same model does not work well below the surface in summer when persistent upwelling is observed. This suggests a two-dimensional wind-forced model of cross-shelf circulation may have more applicability to the brief wind events observed in winter than to the persistent wind events observed in summer. The reason for this is unclear. Numerous factors not included in the simple linear wind-forced model such as mesoscale features, upwelling fronts, the interaction of flow with topography, baroclinic pressure gradients, remote forcing, and small-scale wind stress all affect cross-shelf circulation. It is possible some of these are more pronounced on the northern California shelf in summer.
Surface wind stress, heat, and freshwater fluxes were estimated over the southern flank of Georges Bank during February-August 1995 using moored measurements made at ST1 located on the 76-m isobath, roughly halfway between the tidal mixing and shelf/slope fronts. Wind stress variability was dominated by a succession of atmospheric lows that passed Georges Bank during the deployment. A transition between frequent lows and strong wind stress events (``winter'') to less frequent lows and weaker wind stresses (``summer'') occurred in mid-May. In winter, wind stress fluctuations tended to be omnidirectional, with maximum stresses above 0.5 N/m2 during four storms, one a classic ``nor'easter'', while summer fluctuations were weaker but strongly polarized in the along-bank direction. The ST1 surface heat flux was dominated by shortwave heating, which increased from a winter mean of 130 W/m2 to 230 W/m2 in summer. Long-wave cooling decreased from 50 W/m2 (winter) to 30 W/m2 (summer), while mean sensible and latent fluxes increased from -20 and -40 W/m2 (winter) to +10 and 0 (summer) respectively. Overall, winter was characterized by weak net heating (30 W/m2) with shortwave gain offset by long-wave, latent, and sensible heat loss. In summer, increased shortwave gain and reduced long-wave loss and weak sensible and latent fluxes combined to produce strong net heating (210 W/m2). ST1 precipitation was highly episodic with little seasonality while evaporation occurred mostly during winter, resulting in a net evaporation of -15 cm and net freshwater flux of +48 cm over the deployment.
A modeling study of summer ocean circulation on the inner shelf south of Cape Cod, Massachusetts, has been conducted. The influences of winds, air-sea heat fluxes, tides, and shelfwide circulation are all incorporated. The model reproduces recognized features of the regional summer circulation: warm temperatures and weak eastward flow in Nantucket Sound, cool tidally mixed waters and an associated anticyclonic flow encircling the Nantucket Shoals, and strong stratification south of Martha's Vineyard. Comparisons with satellite and in situ observations show the model simulates the major features of the temperature patterns that develop during summer 2002. The evolution of the summer heat budget is characterized by three regimes: Nantucket Sound heats rapidly in June and then maintains warm temperatures with little net air-sea heat flux; tidal mixing on the Nantucket Shoals maintains perpetually cool ocean temperatures despite significant air-sea heating; and midshelf south of Martha's Vineyard the surface waters warm steadily through July and August because of sustained air-sea heating with only modest cooling resulting from the mean circulation. In the environs of the Martha's Vineyard Coastal Observatory tidal eddy heat flux emanating from Nantucket Sound produces a bowl of warm water trapped against the coast and significant local variability in the net role of advection in the heat budget. A suite of idealized simulations with forcing dynamics restricted, in turn, to only one of winds, tides, or shelfwide inflows shows that tidal dynamics dominate the regional circulation.
A plume of low-salinity water was observed along the North Carolina coast 100 km south of the mouth of Chesapeake Bay during the Coastal Ocean Processes Pilot field program conducted from August through October 1994. The presence of the plume was episodic, occurring every 3 to 8 days. The timing was related to wind patterns, which influence both the delivery of estuarine discharge to the shelf and the plume's passage down the coast. When not affected by local winds, the low-salinity water was confined to within 7-9 km of the coast and was about 8 m deep. Downwelling winds narrow and deepen the plume, whereas upwelling winds cause it to thin and spread offshore, eventually detaching from the coast. The low-salinity plume propagated along the coast at speeds comparable to linear internal wave phase speed, except when strong downwelling wind conditions caused the plume to be in contact with the bottom. The observed propagation speeds are faster than those predicted by previous numerical modeling efforts. The plumes slowed with distance from the source, as mixing with ambient shelf water reduced the density contrast. This buoyancy source was balanced by an alongshore current with a southward velocity of 0.3 to 0.7 ms-1, bounded by a region of high horizontal velocity shear at the offshore salinity front. The currents observed in the nose of the plume are consistent with properties of an internal gravity current under rotation.
To investigate the processes influencing the evolution of stratification over continental shelves a moored array was deployed on the New England shelf from August 1996 to June 1997. Temperature, salinity, and current observations spanning the water column were obtained at four midshelf sites, along with meteorological measurements at a central site to estimate the wind stress and the surface heat and freshwater fluxes. Four processes contributed to the seasonal evolution of the stratification. (1) The breakdown of the seasonal thermocline in fall was primarily due to wind forcing, not surface cooling, and occurred in four discrete steps associated with westward, along-coast wind stress events. Eastward wind stress events of similar magnitude did not reduce the stratification. (2) The water at midshelf remained stratified throughout most of the winter due to saltier shelf-slope front water displaced onshore by anomalously strong and persistent eastward alongcoast wind stresses. (3) The gradual redevelopment of the thermocline, beginning in April, was primarily a one-dimensional response to increasing surface heat flux. (4) Stratification in early April and throughout May was substantially enhanced by low-salinity water associated with river runoff from southern New England that was driven eastward and offshore by upwelling-favorable (eastward) wind stresses.
A 25-yr (1981–2005) time series of daily latent and sensible heat fluxes over the global ice-free oceans has been produced by synthesizing surface meteorology obtained from satellite remote sensing and atmospheric model reanalyses outputs. The project, named Objectively Analyzed Air–Sea Fluxes (OAFlux), was developed from an initial study of the Atlantic Ocean that demonstrated that such data synthesis improves daily flux estimates over the basin scale. This paper introduces the 25-yr heat flux analysis and documents variability of the global ocean heat flux fields on seasonal, interannual, decadal, and longer time scales suggested by the new dataset.
The study showed that, among all the climate signals investigated, the most striking is a long-term increase in latent heat flux that dominates the data record. The globally averaged latent heat flux increased by roughly 9 W m−2 between the low in 1981 and the peak in 2002, which amounted to about a 10% increase in the mean value over the 25-yr period. Positive linear trends appeared on a global scale, and were most significant over the tropical Indian and western Pacific warm pool and the boundary current regions. The increase in latent heat flux was in concert with the rise of sea surface temperature, suggesting a response of the atmosphere to oceanic forcing.
Nearly 2200 days of current and temperature data were collected at a midshelf location in the Middle Atlantic Bight between June 1974 and March 1977. These data were examined for the average conditions and seasonal cycles of water circulation and temperature and some statistical properties of their variation at higher frequency. The average flow is found to be toward the south-southwest at about 5 cm/s near the surface, diminishing to about 1 cm/s near the bottom. The occurrence of energetic wind-driven transient current events which can exceed a 2-month duration makes it impossible to determine a clear seasonal pattern in the sequence of monthly mean flows. There is, however, a clear seasonal pattern in the distribution of higher-frequency fluctuations. Storm wind-induced transient currents of 3- to 10-day duration appear prominently in winter records. Inertial currents appear selectively in the summer, that is, in parts of the water column well insulated from the bottom by a strong thermocline. For low-frequency motions (periods from 3 to 10 days) in both summer and winter records the preferred directions of motion throughout the water column appear to be consistent with equilibrium (mean) Ekman veering arguments. The water column response was found also to be most sensitive to the component of wind stress parallel to the bathymetry. The temperature follows a well-known se
A persistent salinity maximum is observed in the upper part of the pycnocline in summer-autumn over the outer shelf of the New York Bight, induced by a nearly isopycnal transfer of slope water to the outer shelf. The transfer is possible only in the stratified seasons when isopycnals are continuous across the shelf-slope front. Estimates of the slope-shelf salt flux required to produce the pycnocline s-max suggest that it may provide about half of the salt required annually in the shelf region to balance input of river water.
New estimates of rainfall over the Atlantic Ocean between 30oS to 70oN have been constructed based on a technique that uses the present weather observations taken by ships. Annual and quarterly rainfall maps are presented. Compared to the Pacific, the Atlantic is significantly drier and has less extreme values. -from Authors
Repeated hydrographic surveys off New Jersey about 100 km south of the Hudson River mouth were conducted during spring and summer 2006. Glider observations reveal a strong seasonal variation in the surface salinity. Buoyant water is restricted to an area close to the coast during spring, but spans the entire shelf width during summer. During late July and August, freshwater lenses with large density anomaly are found up to 100 km from the coast. Surface velocity maps, satellite imagery and drifters' trajectories revealed the existence of a jet directed offshore and to the south, from near the river mouth toward the study region. This provides a direct pathway for transporting freshwater and any biogeochemical material it contains, including phytoplankton, dissolved organic and nonalgal particulate matter, across the shelf. The highest frequency of observation of the freshwater lenses offshore occurs when the jet transport is large, and the river discharge is relatively high. The transport in the jet is correlated with upwelling winds on scales of a few days.
With the completion of the NCEP-NCAR and ECMWF reanalyses there are now global representations of air-sea surface heat fluxes with sufficient spatial and temporal resolution to be useful in characterizing the air-sea interaction associated with individual weather systems, as well as in developing global-scale oceanic heat and moisture budgets. However, these fluxes are strongly dependent on the numerical models used, and, as a result, there is a clear need to validate them against observations. Accurate air-sea heat flux estimates require a realistic representation of the atmospheric boundary layer, and the implementation of an appropriate surface flux parameterization. Previous work at high latitudes has highlighted the shortcomings of the surface turbulent heat flux parameterization used in the NCEP-NCAR reanalysis during high wind speed conditions, especially when combined with large air-sea temperature differences. Here the authors extend this result through an examination of the air-sea heat fluxes over the western boundary currents of the North Atlantic and North Pacific Oceans. These are also regions where large transfers of heat and moisture from the ocean to the atmosphere take place. A comparison with in situ data shows that the surface layer meteorological fields are reasonably well represented in the NCEP-NCAR reanalysis, but the turbulent heat flux fields contain significant systematic errors. It is argued that these errors are associated with shortcomings in the bulk flux algorithm employed in the reanalysis. Using the NCEP-NCAR reanalysis surface layer meteorological fields and a more appropriate bulk flux algorithm, `adjusted' fields for the sensible and latent heat fluxes are presented that more accurately represent the air-sea exchange of heat and moisture over the western boundary currents.
Description of the shelfbreak front in the Middle Atlantic Bight is hampered by the extreme variability of the front. In order to gain more insight into the mean frontal structure and associated baroclinic jet, historical data is used to produce two dimensional climatological fields of temperature and salinity for the region south of Nantucket shoals. Associated cross-shelf fields of density, geostrophic velocity, relative vorticity, and shallow water potential vorticity have also been computed. Historical data from a quality-controlled database (HydroBase) in the region 69-72 deg W, 39.5-41 deg N is included. Cross-shelf sections are obtained by averaging the data in nine depth bins with an average cross-shelf spacing of 10 km but an increased resolution of 4 km near the shelfbreak. The vertical averaging interval was 10 m over the shelf and upper slope waters, increasing to 50 m in the deep slope waters. The data were averaged in bimonthly periods to study seasonal trends. For inter-regional comparison, similar analyses were performed for the south flank of Georges Bank and the shelf off New Jersey. The climatological temperature and salinity are consistent with previous descriptions of the frontal hydrography. Most importantly, features such as the cold pool, the upper slope pycnostad, and the frontal boundary are well resolved when compared with synoptic sections. The temperature contrast across the front varies seasonally between 2-6 deg C near the surface and at depths of 45-65 m. The salinity contrast is 1.5-2 PSS, with little seasonal variation. The resulting cross-frontal near surface density gradients are strongest during the winter and weakest during the summer, when the seasonal thermocline is established.
Estimates of evaporation (E) over the North Atlantic Ocean by Bunker have been combined with estimates of precipitation (P) by Dorman and Bourke to produce new annual and seasonal maps of E–P and surface density flux. Although uncertainties about precipitation algorithms and exchange coefficients still presist, it is felt that the high spatial resolution of these data set permits an estimate of E–P that is a significant improvement over previous work. The maps of E–P show considerably more detail than earlier maps, including a previously uncharted minimum with a northeast to southwest trend across the subtropical gyre. The two regions of maximal E–P display a close connection with continental air flows off Africal and North America, suggesting that the relative narrowness of the North Atlantic contributes to its status as a net evaporation basin. The zonally integrated E–P values are combined with river runoff estimates to obtain the meridional flux of freshwater, which can be compared with fluxes calculated from oceanographic sections.
Local winds and lateral buoyancy fluxes from estuaries constitute two major forcing mechanisms on the inner continental shelf of the Mid Atlantic Bight on the eastern seaboard of the U.S.A. We report observations of the resulting coastal current that suggest a linear superposition of the wind and buoyancy forced motions. This current, which we term the Delaware Coastal Current, has a mean flow of about 10 cm/s in the direction of Kelvin wave phase propagation. It opposes the generally upwelling favorable local winds there. The same winds, however, force important across-shelf flows that agree qualitatively with Ekman dynamics with Ekman numbers that are O(1). Velocity fluctuations at current meter mooring are consistent with the above dynamics, and explain the local hydrography well. Trajectories from drifters and derived velocity fields, too, reveal consistent flow patterns. We further find that Lagrangian and Eulerian integral time scales are similar, implying a linear flow field. We estimate dispersion coefficients for this buoyancy driven coastal current to be about 2000 and 200 m2/s in the along- and across-shelf direction, respectively. Our results disagree both qualitatively and quantitatively with those of a recent numerical model of the study area.
Comparisons are made between a time series of meteorological surface layer observational data taken on board the R/V Knorr, and model analysis data from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction (NCEP). The observational data were gathered during a winter cruise of the R/V Knorr, from 6 February to 13 March 1997, as part of the Labrador Sea Deep Convection Experiment. The surface layer observations generally compare well with both model representations of the wintertime atmosphere. The biases that exist are mainly related to discrepancies in the sea surface temperature or the relative humidity of the analyses. The surface layer observations are used to generate bulk estimates of the surface momentum flux, and the surface sensible and latent heat fluxes. These are then compared with the model-generated turbulent surface fluxes. The ECMWF surface sensible and latent heat flux time series compare reasonably well, with overestimates of only 13% and 10%, respectively. In contrast, the NCEP model overestimates the bulk fluxes by 51% and 27%, respectively. The differences between the bulk estimates and those of the two models are due to different surface heat flux algorithms. It is shown that the roughness length formula used in the NCEP reanalysis project is inappropriate for moderate to high wind speeds. Its failings are acute for situations of large air–sea temperature difference and high wind speed, that is, for areas of high sensible heat fluxes such as the Labrador Sea, the Norwegian Sea, the Gulf Stream, and the Kuroshio. The new operational NCEP bulk algorithm is found to be more appropriate for such areas. It is concluded that surface turbulent flux fields from the ECMWF are within the bounds of observational uncertainty and therefore suitable for driving ocean models. This is in contrast to the surface flux fields from the NCEP reanalysis project, where the application of a more suitable algorithm to the model surface-layer meteorological data is recommended.
This work reports on the evolution of water masses within the southern portion of the Middle-Atlantic Bight (MAB) and their exchange with the slope waters based upon the Ocean Margins Program hydrographic dataset (February–October 1996). Water mass distributions were quantified in terms of their content of freshwater and of Gulf Stream Water, with the Cold Pool Water (CPW) as the core shelf water. The CPW entered the area during the early spring, migrated offshore during the early summer and disappeared by early fall. An analysis of the salt and heat balance was estimated from the observed data, extended over an annual cycle. Surface heat and evaporative exchanges were obtained using an atmospheric Eta-coordinate model. This analysis supported the concept that the southernmost portion of the MAB circulates more as a positive estuary, exchanging with the slope waters, than as a shelf conduit exporting MAB waters to the Southern Atlantic Bight. Thus, the primary disposition of the shelf waters, entering the SMAB from the north, is in an offshore surface flow. This offshore flow requires, in turn, a sub-surface onshore flow. Significantly, the type of estuarine circulation switches during the year. During the stratified period, the circulation was analogous to that of a ‘highly stratified’ estuary; during the unstratified period, it resembled that of a ‘well-mixed’ estuary. The important difference between these two modes is that the winter exchange is enhanced, relative to that during the summer period; for example, the offshore flow increased from 2.8 to 3.4 105 m s−1 and the onshore flow from 0.44 to 1.0 105 m s−1, respectively. It is suggested that the enhanced exchange is linked to the reduced density gradients, in both the vertical and horizontal, characteristic of the winter convective season. The slower exchange during summer favored freshwater retention in the SMAB volume, with salinities decreasing by 2 psu over the period; the shorter, more intense exchange during the winter favored freshwater loss (salting). In addition to providing salt, the onshore flow brought enough heat to dominate the heat budgets; for example, the advective heat gain increased from 46 to 135 W m−2, between the summer and winter periods. With regard to the OMP objectives, these results suggest a significantly enhanced potential for carbon loss off the shelf during the winter period, compared to that of summer.
Some 19,000 bathythermograms and 1600 oceanographic stations in the region 39 degree to 41 degree N, 69 degree to 72 degree W were examined for evidence of changes in the character and position of the shelf water/slope water boundary. On each plot were noted the positions and values of important hydrographic features; the cold water minimum on the shelf and its off-shore extension; evidence of shelf-water patches in the slope water; the intersection of the shelf water/slope water boundary with both the sea surface and the sea floor; intrusions of Gulf Stream water into the region. Results show that the boundary, identified by the 10 degree isotherm, intersects the bottom within 16 km of the 100-m curve about 80 percent of the time, with a seasonal progression from the south in the winter to north in the fall; at the sea surface the boundary position is much more variable, averaging 52 km seaward of the 100-m curve in winter and 72 km seaward in late summer and the combination of entrainment at Hatteras and production of detached parcels appears to account for the self water/slope water exchange required by salt balance considerations.
The seasonal and interannual variability in the temperature, salinity, and volume of Shelf Water (SHW) in the Middle Atlantic Bight (MAB) is described for the period 1977-1999. Large interannual variations in the volume, salinity, and, to a lesser extent, temperature of the SHW occurred that were coherent over multiple year time periods. The variations in volume and salinity originated through processes acting outside of the MAB and were advected into the region from the Gulf of Maine. On a decadal average the SHW observed in the 1990s was approximately 1°C warmer, 0.25 PSU fresher, and 1000 km3 more abundant than during the 1977 - 1987 period. The warming during the 1990s was largest in the southern part of the bight during the winter, when the SHW was more than 2°C warmer then during the earlier decade.
[1] Temperature and salinity variability over the crest and southern flank of Georges Bank are investigated using moored observations obtained from February to August 1995 and historical data. There was a large seasonal variation in water temperature, which decreased 1°C–2°C in February to a minimum of 5°C due to surface cooling and wind-forced cross-bank advection, and then increased steadily due to surface heating, reaching 10°C (southern flank) to 17°C (crest) in August. The crest warmed more than the southern flank because it is shallower. Temperature variability at shorter timescales (days to weeks) was primarily due to surface heating on the crest and horizontal advection on the southern flank. Salinity variability over the southern flank was primarily associated with two processes. Intrusions of warm, salty, shelf-slope-front water in May and August were associated with Gulf Stream warm-core rings, but did not cause longer-term changes on the southern flank or penetrate onto the crest. Alongbank advection brought low-salinity, cool Scotian Shelf Water to the southern flank sites in March and early May, about three weeks after crossing the Northeast Channel onto the northeastern flank of Georges Bank (≈130 km away). This low-salinity water on the southern flank in the spring did not penetrate immediately onto the crest. Instead, the crest salinity steadily decreased from April to August due to both precipitation (evaporation was small) and, based on historical data, a tidally forced, cross-frontal exchange flow (0.01–0.02 m s−1) that both freshens and cools the crest.
[1] The seasonal and interannual variability in the temperature, salinity, and volume of Shelf Water (SHW) in the Middle Atlantic Bight (MAB) is described for the period 1977–1999. Large interannual variations in the volume, salinity, and, to a lesser extent, temperature of the SHW occurred that were coherent over multiple year time periods. The variations in volume and salinity originated through processes acting outside of the MAB and were advected into the region from the Gulf of Maine. On a decadal average the SHW observed in the 1990s was approximately 1°C warmer, 0.25 PSU fresher, and 1000 km3 more abundant than during the 1977–1987 period. The warming during the 1990s was largest in the southern part of the bight during the winter, when the SHW was more than 2°C warmer then during the earlier decade.
A Reynolds stress-based model is used to derive algebraic expressions for the vertical diffusivities K α(α = m, h, s) for momentum, heat, and salt. The diffusivities are expressed as K a(R ρ, N, Ri T, ∈) in terms of the density ratio R p = α,∂S/∂z(α 1∂T/∂z) -1, the Brunt-Väisälä frequency N 2 = -gρ -1o∂p/∂z, the Richardson number Ri T = N 2/Σ∑ 2 (∑ is the shear), and the dissipation rate of kinetic energy ∈. The model is valid both in the mixed layer (ML) and below it. Here R p and N are computed everywhere using the large-scale field from an ocean general circulation model while Ri T is contributed by resolved and unresolved shear. In the ML, the wind-generated large-scale shear dominates and can be computed within an OGCM. Below the ML, the wind is no longer felt and small-scale shear dominates. In this region, the model provides a new relation Ri T = cf(R ρ) with c ≈ l in lieu of Munk's suggestion Ri T ≈ c. Thus, below the ML, the K α become functions of R p, N, and ∈. The dissipation ∈ representing the physical processes responsible for the mixing, which are different in different parts of the ocean, must also be expressed in terms of the large-scale fields. In the ML, the main source of stirring is the wind but below the ML there is more than one possible source of stirring. For regions away from topography, one can compute ∈ using a model for internal waves. On the other hand, near topography, one must employ different expressions for ∈. In agreement with the data, the resulting diffusivities are location dependent rather than universal values. Using North Atlantic Tracer Release Experiment (NATRE) data, the authors test the new diffusivities with and without an OGCM. The measured diffusivities are well reproduced. Also, a set of global T and S profiles is computed using this model and the KPP model. The profiles are compared with Levitus data. In the North Atlantic, at 24°N, the meridional overturning is close to the measured values of 17 ∓ 4 Sv and 16 ∓ 5 Sv (Sv e5 10 6 m 3 S -1). The polar heat transport for the North Atlantic Ocean, the Indo-Pacific Ocean, and the global ocean are generally lowered by double diffusion. The freshwater budget is computed and compared with available data.
Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5-10 m thick, and propagated alongshore at ∼50 cm s-1. The density decreased 2-3 kg m-3 over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in ∼8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s-2, respectively). The southward alongshore current increased abruptly to ∼50 cm s-1 at the nose and continued to increase to a supercritical maximum of ∼70 cm s-1 about 1 km behind the nose. An onshore flow of between 5 and 15 cm s-1, which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore flow. The observed flow structure is qualitatively similar to theoretical predictions and laboratory measurements of buoyant gravity currents propagating along a sloping bottom.
Along-shelf transports across three cross-shelf lines on the continental shelf near Cape Hatteras have been calculated from moored current meter data over a continuous 24 month period in 1992-1994. The along-shelf convergence has been used to infer off-shelf export. Transport and transport convergence have been related to wind and Gulf Stream forcing and to variability in sea level at the coast. The along-shelf transport variability is primarily wind-driven and highly correlated with sea level fluctuations at the coast. Both winds and along-shelf transport exhibit a near-annual period variability. Along-shelf transport is not well correlated with Gulf Stream offshore position. Along-shelf transport convergence is highly correlated with Gulf Stream position offshore, with a more shoreward Gulf Stream position leading increased along-shelf convergence by hours to a few days. Long-period variability of 14-16 months and 1-3 months is apparent in both Gulf Stream position and transport convergence. Variability in along-shelf convergence is poorly correlated with wind, wind convergence, or coastal sea level. A likely hypothesis accounting for the observed relationship between Gulf Stream position and along-shelf transport convergence is that the Gulf Stream is directly influencing cross-shelf export processes along the outer boundary of the study site. Despite predominantly convergent flow on the shelf at Cape Hatteras, brief periods of along-shelf divergence and shoreward cross-shelf transport exist (~10% of the time just north of Cape Hatteras and ~34% of the time just south of Cape Hatteras during episodes of up to 3-8 days duration). Implied onshore flows of a few cm s-1 are tentatively identified in the moored current meter data for these periods. Satellite imagery for an extended along-shelf divergent period clearly suggests that shelf edge parcels could be advected a significant fraction of the way across the shelf.
Seawater temperatures have been measured at United States tide gauges throughout most of the 20th century. All available records have been digitized, and the longest 14 have been analyzed by linear least-square regression. The largest positive trend is from Boston MA (+3.6±0.4°C per century), and the largest negative trend is at Charleston SC (−0.1±0.3°C per century). No consistent latitudinal or east-coast vs. west-coast patterns are discernable, but air temperature trends are typically greater than seawater changes.
Observations taken during the Lagrangian Transport and Transformation Experiment (LaTTE) in 2005 indicated that the Hudson's river outflow formed a bulge of recirculating fluid that limits the volume of fresh water that is advected away in a coastal current. Focusing on an event that began with downwelling winds we made estimates of the fresh-water flux in the coastal current and the fresh water inventory of the bulge. The coastal current was characterized by a surface advected plume in thermal wind balance. However, the freshwater transport in the coastal current was less than 1/2 of the total freshwater outflow. The bulge extended 30 km from the coast and 40 km in the along-shore direction and was evident in ocean color imagery. Recirculation in the bulge region was also apparent in daily averaged surface current radar data, but this flow pattern was obscured in the hourly data by tidal and wind-forcing even in the diurnal band. Nevertheless, many aspects of the Hudson's outflow are consistent with recent laboratory experiments and numerical simulations of buoyant discharges. The growing bulge transports the river's outflow to the head of the Hudson shelf valley where it crosses the 50 m isobath. Previous work in this region indicates that frontal features reside along this isobath. We observed fresh water being transported along this isobath and is suggestive of a rapid cross-shelf transport pathway for fresh water. Both the bulge formation and cross-shelf transport have significant biogeochemical implications.
Intrusions or lens of anomalously salty slope water (Smax intrusions) are often found over the continental shelf of the Middle Atlantic Bight (MAB). Salty intrusions were identified in 11% of 10,652 historical hydrographic profiles. Intrusions occurred primarily in summer, were observed across the entire shelf, but were most common over the outer shelf, and were concentrated at the depth of the seasonal pycnocline. The percentage of profiles with intrusions increased linearly along the MAB shelf from Georges Bank (3%) to Chesapeake Bay (15%), suggesting either a north to south increase in generation or an along-shelf accumulation associated with a decay timescale of 90 days or longer. Intrusions were typically less than 30 m thick, with a salinity anomaly of less than 0.5, though 10% of the anomalies were greater than 1. The thickness increased as the stratification decreased in a manner consistent with double-diffusively driven lateral intrusions. Intrusions did not preferentially occur during certain wind conditions. Salty intrusions increase the average salinity of the MAB shelf during summer by 0.3 or more, depending on how rapidly intrusions mix with the surrounding shelf water.
A strong relationship is observed between synoptic weather systems and atmospheric forcing of the ocean as estimated from buoy measurements made on the North Carolina inner shelf during August and October-November 1994 as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study. Synoptic variation (timescales of days to weeks) in the meteorological time series was primarily associated with the passage of atmospheric frontal systems. The most common synoptic weather pattern observed was the passage of a low-pressure center to the north of the study site, which caused the associated cold front to pass over the study region. Before passage of the cold front, warm, moist northeastward winds increased the heat flux into the ocean, whereas after the cold front passed, cold, dry southwestward winds decreased the heat flux into the ocean. In addition, in the presence of oceanic stratification, northeastward winds drove coastal upwelling, bringing colder water to the surface, further increasing the air-sea temperature contrast and hence the heat flux into the ocean inshore of the surface front between cool upwelled water and warmer water offshore. The decrease in surface heat flux during the passage of a cold front was of order 400 W m−2, due primarily to a decrease in latent heat flux. Although other synoptic patterns were observed, including one warm front passage and two tropical storm systems, the dominance of cold fronts as a source of variability resulted in a strong positive correlation between the along-shelf component of wind stress and the surface heat flux. To address the issue of spatial variation in the surface heat fluxes, data from several different sources located along a cross-shelf transect were analyzed. This analysis suggests that the temperature of the atmospheric boundary layer undergoes adjustment when warm air blows over cold water but not when cold air blows over warm water. This produces cross-shelf gradients in the bulk estimates of turbulent heat fluxes during offshore winds but not during onshore winds.
We present a quantitative analysis of properties of the eddy field associated with the shelfbreak front south of New England off the U.S. east coast. We employ records from current meters, drifting buoys, and hydrography obtained during June 1984. Mean currents during the study period showed the across-shelf cyclonic shear in the alongshelf velocity to be similar to that found in earlier studies when eddies were probably absent most of the time. Our mean across-shelf currents, however, differed from earlier findings and showed a marked divergence of the across-shelf flow for the upper 65 m of water that implied high rates of upwelling from beneath. This across-shelf divergence was also consistent with observed spreading of surface frontal isotherms in the study area. Records from both current meters and drifting buoys enabled us to compute both the lagged Eulerian and Lagrangian velocity autocorrelations. The former showed a decorrelation time of about 4 days and lacked any negative lobe, but the latter showed a strong negative lobe. Using a simple model for a band-limited process in frequency, we inferred a rotational period for fluid particles in trajectories within eddies of 4 days and a Lagrangian decorrelation time of 2.6 days, comparable to the Eulerian time. The corresponding estimate of horizontal diffusivity was zero. These results were consistent with the generation close upstream of discrete, coherent vortices with dominantly deterministic character. We could find no evidence that across-shelf exchange of heat or salt by the eddies was enhanced above estimates of climatic mean exchange, despite having a detailed alongshelf hydrographic transect across the strongest eddy features that we found.
This study covers sea-surface temperature anomalies at 27 Atlantic coast stations from Eastport, Maine, to Key West, Florida, for the years 1873-1961. The anomalies were computed using as a common base period the years 1950-1959. A secular warming trend peak- ing in the early 1950's is demonstrated. This trend was strongest in the north and diminished progressively toward the south, where a slight cooling trend was present at Key West. SeasonM differences in the trends are also pointed out. A comparison between the east and west coasts of the United States, and between the English Channel and the east coast of the United States, shows that a partial negative correlation exists between the anomalies of the east and west sides of the oceans. This negative correlation is greatest between northern stations and be- comes less toward the south, where a positive correlation may exist at times. A short review is given of previous work on climatic change in the North Atlantic, and of work on the differences between the two sides of an ocean and their causes. Introduction. For many years a climatic change has been evident in the North Atlantic region. Kincer (1933) noted a general trend to- ward higher mean annual air temperatures over North America. Changes in the marine biota of the European Arctic during the 1920's and 1930's, especially the development of a cod fish- ery in west Greenland, prompted many investi- gations of the causes; these were summarized by Jensen (1939), who concluded that both sea and air temperatures had risen. Smed (1947) strengethened this conclusion by an analysis of sea-surface temperature records around south Greenland; he noted that warming had occurred, beginning about 1920. Further evidence of cli- matic change in the Arctic, and of concurrent changes in the biota, was given in 1949 in Rapports et Procbs-Verbaux des Reunions, vol. 125, part 1, Conseil Permanent International pour l'Exploration de la Mer. Ahlmann (1948) brought together much in- formation indicating a general climatic warming and stated that the temperature of the North Atlantic had increased since the beginning of the century. Hachey and McLellan (1948) presented data showing a warming trend in the surface waters of the Canadian Atlantic, as did Smed (1949) for the waters of the northeast Atlantic. Smed (1952) later presented data showing a general warming in the Faroes. Lauzier (1952, 1953, 1954) compiled data on the sea-surface temperatures at 17 Canadian Atlantic coast sta- tions and commented on the general trend to higher temperatures in the early 1950's. Rode- wald (1956) also demonstrated this warming of Canadian waters and extended his analysis to cover the area as far south as Charleston, S.C. Changes in the commercial fisheries and the marine biota of New England were analyzed in relation to a climatic warming by Taylor et al. (1957), who also showed that the entire water column in the Gulf of Maine had increased 1-5øF since 1912-1926. This warming of deep waters was also demonstrated by Lauzier and Trites (1957) for the Laurenttan Channel. Bjerknes (1959) wrote on the warming in the North Atlantic. Using the means of the periods 1890-1897 and 1926-1933, he found a maximum surface temperature increase along the Gulf Stream axis from Cape Hatteras to the Grand Banks and a belt from Ireland to the Labrador Current that had not been warmed; north of this belt warming had started about 1920. Southward (1960) later noted a rise of the mean annual surface temperature in the English Chan- nel. Lauzier and Hull (1961) brought the Canadian Atlantic coast station data up to 1959 and showed that a downward trend in surface
Current measurements on the southern flank of Georges Bank at depths of 45 and 75 m between May 1975 and March 1979 are analyzed. Seasonal differences in strength and direction are discussed. Most of the seasonal change is explained in terms of the seasonal change in the cross bank density field. There was no significant seasonal change in the monthly mean along bank flow at 75 m. In winter, only about 21% of the along bank flow at 45 m can be explained in terms of tidal rectification, the density field, and wind stress. In late summer, however, almost all of the flow at 45 m can be explained by these three mechanisms. (D.W.T.)
From 1974 through 1978 studies were made of temperature, salinity, and water velocity, by hydrographic surveys and from moored instruments, in the waters within 15 km of the southern Long Island coastline. Data of late summer reveal frequent intrusions of anomalous water which appear to be of slope or outer shelf origin. Three intrusions of notably differing character are described. Two occurred at mid-depth, within the pycnocline. Concurrent hydrographic and current meter data suggest that these water masses were transported shoreward due to circulation forced by wind stress and by longshore density gradients. The third intrusion was initially observed near the surface and was of anomalously high temperature. Its T/S relation was similar to a warm and salty bolus detected 12 days earlier in the midshelf region. Hydrographic data suggest that this bolus may have been a detached parcel of slope water. Such intrusions may commonly occur during summer and fall and may be related to the appearance of tropical fish in the Long Island vicinity during these seasons.
RUSTRACT Plastic and brass sea-bed drifters have been used to investigate residual drift along the bottom on the continental shelf in the Middle Atlantic Bight area.
The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability. In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed ove...
Energy fluxes through the surface of the North Atlantic Ocean have been calculated using bulk aerodynamic equations with exchange coefficients which vary with wind speed and stability, and radiation equations. Values of the coefficients were determined from published results of many experiments made at sea, coastal installations and in laboratories. Eight million ship weather reports were used for flux calculations. Reliabilities of monthly meteorological and flux averages are found to be good in areas having SW or move observations per average. A chart of the net annual heat gain by the ocean shows regions with characteristic gains or losses such as the Gulf Stream, Norwegian Sea, Labrador Current, trade winds and upwelling areas. Annual cycles of fluxes, winds, temperatures, clouds and rainfall in eight regions are presented and discussed from the point of view of predominating air–sea interaction processes. Contributions of meteorological variables, oceanic currents, atmospheric circulations a...
The presence of Gulf Stream water on the continental shelf as much as 60 km north of Cape Hatteras was observed during a hydrographic cruise in the summer of 1990. Gulf Stream water was concentrated at mid-depth between 10 and 30 m and penetrated the shelfbreak front which normally separates the shelf water from slope water and Gulf Stream water. Velocities of Gulf Stream water in the upper 110 m of the water column along the 1000 m isobath indicated a flow of 18 to 25 cm/s directed towards the northwest. Gulf Stream water on the shelf is considered to be associated with low values of fluorescence, transmissivity, and nutrient concentrations relative to adjacent shelf water.
1] Auroral electrons are field-aligned current carriers and thus changes of their flux are related to the magnetospheric substorm current systems. Therefore a study of processes associated with substorms can provide insight into a source of auroral electrons. This paper attempts to identify basic processes associated with substorm onset by synthesizing/ integrating the three well-established observations at substorm onset: a sudden brightening of an auroral arc, a sudden growth of the westward electrojet, and dipolarization. It is demonstrated that when these observations are combined, they reveal basic processes associated with substorms and a source of auroral electrons. First of all, it is shown that, together with the southward-directed electric field E s , the associated Pedersen current I p and its equatorial closing component I r , the pair of the upward/downward field-aligned current sheets in the night sector forms a meridional current system (Boström's Case 2 current system); since E s Á I p > 0, driving electric field E d must be present to drive I r on the equatorial plane, so that E d Á I r < 0. Thus, an enhancement of Boström's Case 2 current system can increase the upward current sheet, explaining a sudden brightening of auroral arcs. Also, it is noted that E d drives the westward electrojet, which is mainly the Hall current. It is argued that the westward electrojet cannot be the diverted cross-tail current, and the dipolarization must be caused by the ''return current'' from the westward electrojet; the return current must be located in the equatorial plane of the magnetosphere. In fact, the diversion of the cross-tail currents explains little about the aurora. Further, the dipolarization sometimes results in an overdipolarization and even a negative dipolarization, which cannot be explained by the diversion of the cross-tail current. Therefore there must be an internal dynamo process that enhances Boström's current system on the equatorial plane. This explains many aspects of auroral arcs during substorms and provides a source of auroral electrons. The enhanced internal dynamo process during the expansive phase must be closely related to a short-lived unloading process. The disruption (not the diversion) of the cross-tail current can release magnetic energy stored during the growth phase for the unloading process. A chain of processes associated with the substorm warrants further study. INDEX TERMS: 2407 Ionosphere: Auroral ionosphere (2704); 2409 Ionosphere: Current systems (2708); 2788 Magnetospheric Physics: Storms and substorms; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; KEYWORDS: Substorm onset, auroral electrons, substorm current system, dipolarization Citation: Akasofu, S.-I., A source of auroral electrons and the magnetospheric substorm current systems, J. Geophys. Res., 108(A4), 8006, doi:10.1029/2002JA009547, 2003.
We have compiled what we believe is the longest coherent coastal sea surface temperature record in North America. Near-surface
water temperature measurements have been made almost daily at Great Harbon, Woods Hole, Massachusetts, since 1886 with remarkably
few gaps. The record shows that there was no significant trend in water temperature at this site for the first 60 yr of observation.
There was some cooling during the 1960s that was followed by a significant warming from 1970–2002 at a rate of 0.04°C yr−1. During the 1990s annual mean temperatures averaged approximately 1.2°C warmer than they had been on average between 1890
and 1970; winter (December, January, and February) temperatures were 1.7°C warmer and summer (June, July, and August) temperatures
were 1.0°C warmer. There has not been a statistically significant decrease in the annual number of winter days below 1°C or
an increase in the annual number of winter days above 5°C. The number of summer days each year with water temperature above
21°C has not increased significantly. The dates of first observations of 10°C and 20°C water in the spring have not changed
sufficiently to be statistically significant. There is a weak positive correlation between annual and winter water temperature
and the annual and winter North Atlantic Oscillation index, respectively, during the period of record.
Although intrusions of water from the Gulf Stream have often been observed over the Carolina shelf, there has been no published report of Gulf Stream water near the continental margin north of Cape Hatteras. By examining sea surface temperature distributions and hydrographic data collected over an 11-yr period, we have found that water discharged from the Gulf Stream often appears over the shelf and upper slope north of Cape Hatteras. For example, in the band between 36 and 38°N, roughly 80–300 km north of the tip of Cape Hatteras, Gulf Stream water was detected 13–27% of the time at sites on the upper slope and 3–9% of the time at locations on the shelf. In most instances, sea surface temperature distributions suggest that the Gulf Stream water which appeared near the continental margin was not part of the Gulf Stream's main current but was fluid which had been expelled from the Gulf Stream. The Gulf Stream water found on the shelf did not appear to significantly influence the local circulation or alongshelf density structure. Its subtidal flow was largely driven by the alongshelf wind stress; and in most instances its vertical density profile nearly matched that of adjacent shelf water situated alongshore. In contrast, the discharged Gulf Stream water observed over the upper slope was significantly less dense than abutting fluid of equivalent depth located along the slope to the northeast. The circulation associated with this density contrast often conveyed Middle Atlantic Bight shelf water seaward of the continental margin. The resulting rate of shelf water export is not well resolved by the available data. Some rough estimates put it at 0.1 Sv, comparable with the estimated mean alongshore transport of shelf water over the Middle Atlantic Bight shelf.
Temperature and current data derived from the SEEP—I array moored on the New England continental margin during 1983–1984 are used to study the structure and motion of the shelf-slope water front and the cross-shelf exchange of heat. During the winter frontal motion is associated primarily with wind-driven Ekman forcing across the shelf-break. The foot of the front undergoes 10–20 km cross-shelf excursions which are not correlated with current fluctuations over the slope. The cross-shelf eddy flux of heat has a large variance but zero mean.During the summer frontal displacements diminish as does the wind stress. As the seasonal thermocline develops shelf and slope regions become connected by isopycnal surfaces. This appears to facilitate diffusive exchange of shelf and slope water and the formation of submesoscale eddies at the frontal boundary. As the thermocline deepens late in the summer there appears to be an increase in the eddy flux of heat across the front from the slope to the deep shelf water.In the autumn intrusions of slope water are observed which may be related to a surface convergence and sinking along the front associated with geostrophic adjustment following a mixing event.During the year-long deployment one warm-core ring passed through the SEEP—I array. Little of the heat flux observed over the slope penetrated onto the shelf.
Twenty-eight hydrographic surveys of the Middle Atlantic Bight (MAB) were conducted during 1977–1987 as part of the National Marine Fisheries Service's Marine Resources, Monitoring, Assessment, and Prediction program. The average temperature and salinity for Shelf Water (<34psu) in the MAB were determined for each survey. While temperature followed a well-defined seasonal cycle, the Shelf Water salinity exhibited large interannual variations. A stepwise multiple linear regression model is used to show that variations in river discharge and in precipitation can explain 70% of the year-to-year changes in the Shelf Water salinity.
The Ocean Margins Program was a major multi-disciplinary observational effort in the southern Middle Atlantic Bight (MAB), focusing on the transformation, retention, and export of biogenic materials from the shelf. The observational effort peaked in the spring and summer of 1996 with four ship-based surveys, augmented by an array of 26 moorings supporting 126 temperature and 118 salinity sensors. The data from the cruises and moored array are used to describe how the springtime hydrographic evolution takes place in an area in which five water masses locally vie for dominance and that is subject to strong wind stress, heat flux, and offshore forcing. The results show that the region is subject to large-scale intrusions from both the north and south, which materially affect the timing and development of stratified conditions. The intrusions from the north are wind driven and provide cold, moderately saline, unstratifed water, delaying the development of stratified conditions. Intrusions also occur from the south, where warmer and generally more saline waters from the South Atlantic Bight are driven into the area by alongshore winds and/or intrusions of Gulf Stream waters pushed shoreward by Gulf Stream frontal eddies. In 1996, an intrusion of saline water from the south, combined with a reversal of the alongshore winds from the north, slowed the southward flow of cold MAB water, and subsequently caused low-salinity Virginia Coastal Waters to spread out from the coastal plume over the denser water from the north. With the reduced alongshelf flow and an initial stratification provided by the low-salinity coastal water, solar insolation and sensible heat fluxes were then able to warm the surface layer, permanently establishing the seasonal thermocline/halocline for the area not directly impacted by intrusions from the Gulf Stream and South Atlantic Bight.
Mooring data with high vertical resolution have been combined with hydrographic data to investigate the character of high salinity thermocline intrusions in the shelf water-slope water frontal zone in the southern Mid-Atlantic Bight. The coincidence of current, temperature and salinity data has allowed a detailed investigation of how the intrusion process is initiated. The data show that the time scale for both the onset and duration of the intrusions is very short. Most intrusions at a single location seem to last no more than a day, while the onshore flow associated with many, but not all, intrusions accelerates very quickly attaining amplitudes of 10–20 cm s−1. As has been noted in earlier studies, there is a correlation between the intrusions and upwelling favorable winds, but the magnitude of the wind stress is not sufficient to account for the onshore transport of the intrusions. The combination of current measurements with density profiles has allowed the calculation of gradient Richardson numbers associated with the intrusions. These calculations show that during the strong onshore flow periods there is significant mixing at the boundaries of the intrusions. At other times when the high salinity water is being passively advected along the shelf, there appears to be little shear turbulence at the intrusion boundaries. Continuous data coverage during the summer suggests that the intrusion process is actually fairly infrequent, apparently requiring an offshore preconditioning in which high salinity waters of the correct density are located just offshore in addition to upwelling favorable winds.