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Spatial coherence of interannual variability in water properties on the U.S. northeast shelf
Abstract
Interannual variability in the surface and bottom temperature and salinity on the U.S. northeast shelf is described by using hydrographic data from the Northeast Fisheries Science Center (NEFSC) MARMAP program. This 10 year data set provides the spatial resolution to describe spatial patterns in the variability of the shelf water properties. An empirical orthogonal function analysis is used to determine the primary modes of variability. All parameters investigated have significant first modes, which contain 40–50% of the variance in the winter and 25–35% in the summer period. All parameters, except the summer surface temperature, exhibit coherent variability across the entire shelf region from near Cape Hatteras to the central Gulf of Maine. The summer surface temperature appears to have two regions of coherent variability: in the central Middle Atlantic Bight and in the western Gulf of Maine. This regionalization is believed due to the effect of local sources of coastal runoff.
... The warm and saline northward-flowing Gulf Stream and the colder and fresher southward-flowing Labrador Current converge and interact in this region, forming the source waters for the continental shelf (Loder et al., 1998;Fratantoni and Pickart, 2007;Greene et al., 2013;Richaud et al., 2016). The surface layers are dominated by shelf waters entering the Gulf of Maine (GoM) from the north, while deeper layers are sourced by a mixture of slope waters from the north and south entering the GoM through the Northeast Channel (Mountain and Manning, 1994;Mountain and Taylor, 1998;Mountain, 2012). The shelf circulation and annual cycle of heating, along with influxes of freshwater from riverine sources, result in local variability of water properties. ...
The Northeast U.S. (NEUS) continental shelf has experienced rapid warming in recent decades. Over the NEUS continental shelf, the circulation and annual cycle of heating and cooling lead to local variability of water properties. The mixed layer depth (MLD) is a key factor that determines the amount of upper ocean warming. A detailed description of the MLD, particularly its seasonal cycle and spatial patterns, has not been developed for the NEUS continental shelf. We compute the MLD using an observational dataset from the Northeast Fisheries Science Center hydrographic monitoring program. The MLD exhibits clear seasonal cycles across five eco-regions on the NEUS continental shelf, with maxima in January–March and minima in July or August. The seasonal cycle is largest in the western Gulf of Maine (71.9 ± 24.4 m), and smallest in the southern Mid-Atlantic Bight (34.0 ± 7.3 m). Spatial variations are seasonally dependent, with greatest homogeneity in summer. Interannual variability dominates long-term linear trends in most regions and seasons. To evaluate the sensitivity of our results, we compare the MLDs calculated using a 0.03 kg/m³ density threshold with those using a 0.2 °C temperature threshold. Temperature-based MLDs are generally consistent with density-based MLDs, although a small number of temperature-based MLDs are biased deep compared to density-based MLDs particularly in spring and fall. Finally, we compare observational MLDs to the MLDs from a high-resolution ocean reanalysis GLORYS12V1. While the mean values of GLORYS12V1 MLDs compare well with the observed MLDs, their interannual variability are not highly correlated, particularly in summer. These results can be a starting point for future studies on the drivers of temporal and spatial MLD variability on the NEUS continental shelf.
... Moreover, the Gulf lies within the greater Northwest Atlantic (NWA) shelf circulation and its southwestward flow from the Labrador Sea and Grand Banks, to the Gulf of St. Lawrence and Scotian Shelf, and then equatorward toward Cape Hatteras. Thus investigation of local and remote forcing impacts on internal Gulf circulation needs upstream information [Csanady, 1978;Csanady and Hamilton, 1988;Xue et al., 2000;Smith et al., 2001;Loder et al., 2003;Shen, 2012, 2014] and will also have implications for sites downstream [Chapman et al., 1986;Mountain, 2003;Mountain and Taylor, 1998]. ...
In the Gulf of Maine (GoM), a network of buoy hydrography measurements collected since 2001 provide a subsurface salinity time series showing a strong seasonal cycle and interannual variations that are both consistent with remote forcing of Gulf hydrography by upstream advection. These long-term mooring data are combined with satellite altimeter estimates of upper ocean current anomaly on the adjoining Scotian Shelf (SS) in a new attempt to use disparate regional observations as proxies to detect and evaluate remote forcing of water mass change inside the Gulf from 2002-2015. Focusing on buoys moored along the Maine coastal current (MCC), lagged cross correlations with upstream altimeter-derived SS current anomalies are found to be as high as 0.84 and explain 50-70% of variance in the MCC subsurface salinity data at both seasonal and interannual time scales. Significant MCC freshening in 2004-2005 and 2010-2011 follow SS velocity strengthening, while salting events in 2002-2004 and 2012-2015 are associated with relaxation of SS currents. Estimated time lags translate to advective SS inflow velocity estimates of 6±2 cm/s that are consistent with past modeling and observational work. Investigation of wind stress control on SS velocity anomalies indicates that wind directions away from the along-shore can factor into flow modulation. Overall, the study findings are consistent with past freshwater flux observations and modeling examining southwest SS inflow to the GoM, provide a new empirical means to diagnose GoM hydrographic change, and point to one potential application of an altimeter measurement record that extends from 1992 into the future. This article is protected by copyright. All rights reserved.
... Previous studies have identified inter-annual salinity anomalies using hydrographic station data within the Gulf and time-series observations at the boundaries of the Gulf, e.g. in the SSW and SLW inflow regions, but have not had the advantage of extended salinity time series observations within the Gulf [Mountain and Taylor, 1998;Ramp, et al, 1985;Smith, et al, 2001]. The high temporal resolution GoMOOS time series allow the first opportunity to track the development of an inter-annual salinity anomaly using time series from multiple locations throughout the Gulf of Maine. ...
The salinity and vertical density structure (stratification) of the Gulf of Maine strongly influence the physical and biological character of the region including: circulation and transport, vertical mixing, and primary productivity. Variability in salinity and stratification also provides insights into the character and timing of the oceanic waters entering the region, a key to predicting regional climate change. This thesis addresses outstanding questions related to variability in salinity and the relative role of salinity and temperature in creating stratification. Hourly observations from Ocean Observing System buoys throughout the Gulf provide the primary data source for this investigation. Analysis of estimated annual cycles, cross-correlations, and short-term (10-30 day) events indicate that Scotian Shelf Water (SSW) moves cyclonically around the Gulf, typically traveling from the southwest Scotian Shelf to Jordan Basin over one to two months and to the western Maine shelf over three to four months. SSW contributes to both freshening and salting, depending on season and location. River waters contribute to localized freshening of the Maine coastal shelf, especially during spring and autumn, and shelf waters contribute to near-surface (1m) freshening in Jordan Basin during the summer. The SSW, river inflows, and Slope Water contribute to seasonally and spatially variable stratification. Surface layer stratification is strongest in late July or early August, except on the western Maine shelf where peak stratification occurs in April, May, or June during years with large river inflows. Salinity is the primary determinant of stratification throughout the region. Salinity dominates surface layer stratification during winter, spring, and fall, and contributes 35-45% of the stratification during summer. Below 50m depth in Jordan Basin and the Northeast Channel mean temperature gradients are inverted and salinity completely supports stratification. A negative salinity anomaly of 0.6-0.8 in the surface, intermediate, and deep waters during 2004 and early 2005 occurred because of cold fresh Shelf Water inflows in February-April 2004 and September-November 2004, along with either less Slope Water or fresher Slope Water than average. St. Lawrence River discharge during 2003, unusual northerly winds, and unusually cold intermediate water on the Scotian Shelf may have contributed to these conditions.
... Regression slopes m (not shown) and their 95% confidence intervals are estimated using a geometric mean regression [Ricker , 1984]. This pattern of enhanced variability in winter ocean temperatures at the southern end of the MAB is consistent with previous analysis of hydrographic data [Mountain and Taylor , 1998]. ...
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.
... These papers provide an indication of low-salinity episodes transported from north to south. Periods of low-salinity appear for several months at a time and can be tracked at several locations along the coast (Mountain and Taylor, 1998). The objective of eMOLT II was to extend this idea to include several sites within the Western Gulf of Maine. ...
... Although defined as one ecosystem, there are discrete regions that differ in both physical processes (e.g. the extent of tidal mixing and stratification) and biological processes (e.g., timing of peak phytoplankton productivity and fishery species). Since the 1970s, temperature variability in the ecosystem has been well documented based both on in situ survey data (Mountain and Taylor, 1998) and satellite-derived surface data (Yoder et al., 2002). Longer-term examinations of temperature in the region typically include point measurements from single (Nixon et al., 2004) or several (o10) locations (Petrie and Drinkwater, 1993). ...
We investigated sea surface temperature (SST) variability over large spatial and temporal scales for the continental shelf region located off the northeast coast of the United States between Cape Hatteras, North Carolina, and the Gulf of Maine using the extended reconstruction sea surface temperature (ERSST) dataset. The ERSST dataset consists of 2°×2° (latitude and longitude) monthly mean values computed from in situ data derived from the International Comprehensive Ocean Atmosphere Data Set (ICOADS). Nineteen 2°×2° bins were chosen that cover the shelf region of interest between the years of 1854 and 2005. Mean annual and range of SST were examined using dynamic factor analysis to estimate trends in both parameters, while chronological clustering was used to determine temporal SST patterns and breakpoints in the time series that are believed to signal regime shifts in SST. Both SST and SST trend analysis show that interannual variability of SST fluctuations shows strong coherence between bins, with declining SST at the beginning of the last century, followed by increasing SST through 1950, and then rapidly decreasing between 1950 and mid-1960s, with somewhat warmer SST thereafter to present. Annual SST range decreases in a seaward direction for all bins, with strong coherence for interannual variability of range fluctuations between bins. The trend in SST range shows a decreasing range at the beginning of the last century followed by an increase in range from 1920 to the late-1980s, remaining high through present with some spatial variability. A more detailed spatial analysis was conducted by grouping the data into 7 regions using principal component analysis. We analyzed regional trends in mean annual SST, seasonal SST range (summer SST−winter SST), and normalized SST minima and maxima. Both the summer and winter seasons were also analyzed using the length of each season and amplitude of the warming and cooling season, respectively, along with the spring warming and fall cooling rates. Trends in all of the parameters were examined after low-pass filtering using a 10-point convolution filter (n=10 years) and regime shifts were identified using the sequential t-test analysis of regime shifts (STARS) method. The analysis shows some difference between regions in the timing of minimum SST with minima being reached 1 month earlier in the south (February) relative to more northern regions (March). Regional annual SST range decreased in a seaward direction. Amplitude of summer warming and the length of summer have shown fluctuations with recent years showing stronger warming and longer summers but generally not exceeding past levels. Overall, the difference in SST range, with recent larger values may be the most significant finding of this work. SST range changes have the potential to disrupt species important to local fisheries due to combinations of differing temperature tolerances, changes in reproduction potential, and changes in the distributional range of species.
... Combination of climatological physics with climatological biology has illuminated some of the controls on the January-June monthly mean distribution of C. finmarchicus for 1995-1999. However, both the physical environment and the distribution of C. finmarchicus are prone to significant departures from climatology (Mountain and Taylor, 1998; Sherman et al., 1998; Greene and Pershing, 2000; Conversi et al., 2001; Durbin and Casas, this volume). There are several opportunities to explore interannual variability in this context. ...
An adjoint data assimilation approach was used to quantify the physical and biological controls on Calanus finmarchicus N3–C6 stages on Georges Bank and its nearby environs. The mean seasonal cycle of vertically averaged distributions, from 5 years of the GLOBEC Georges Bank Broad-Scale Surveys between January and June, was assimilated into a physical–biological model based on the climatological circulation. Large seasonal and spatial variability is present in the inferred supply sources, mortality rates, computed molting fluxes, and physical transports. Estimated mortalities fall within the range of observed rates, and exhibit stage structure that is consistent with earlier findings. Inferred off-bank initial conditions indicate that the deep basins in the Gulf of Maine are source regions of early stage nauplii and late-stage copepodids in January. However, the population increase on Georges Bank from January to April is controlled mostly by local biological processes. Magnitudes of the physical transport terms are nearly as large as the mortality and molting fluxes, but their bank-wide averages are small in comparison to the biological terms. The hypothesis of local biological control is tested in a sensitivity experiment in which upstream sources are set to zero. In that solution, the lack of upstream sources is compensated by a decrease in mortality that is much smaller than the uncertainty in observational estimates.
... The timing and spatial extent of these resuspension events suggest both storm induced Styles and Glenn, 2002 and buoyancy instabilities Gargett et al., 2004 contribute to the sediment resuspension. These spatial physical and bio-optical time series on the MAB will provide critical spatial data in a sustained manner which data will be critical to understanding the processes underlying the changing water masses in the MAB Mountain and Taylor, 1998; Mountain, 2003. The success of maintaining the Glider time series has convinced operational entities within the state of New Jersey that these platforms are robust and can serve their operational needs. ...
Buoyancy driven Slocum Gliders were a vision of Douglas Webb, which Henry Stommel championed in a futuristic vision published in 1989. Slocum Gliders have transitioned from a concept to a technology serving basic research and environmental stewardship. The long duration and low operating costs of Gliders allow them to anchor spatial time series. Large distances, over 600 km, can be covered using a single set of alkaline batteries. Since the initial tests, a wide range of physical and optical sensors have been integrated into the Glider allowing measurements of temperature, salinity, depth averaged currents, surface currents, fluorescence, apparent and inherent optical properties. A command/control center, entitled Dockserver, has been developed that allows users to fly fleets of gliders simultaneously in multiple places around the world via the Internet. Over the last 2.5 years, Rutgers Gliders have logged 27 056 kilometers, and flown 1357 days at sea. Gliders call into the automated Glider Command Center at the Rutgers campus via satellite phone to provide a status update, download data, and receive new mission commands. The ability to operate Gliders for extended periods of time are making them the central in situ technology for the evolving ocean observatories. Off shore New Jersey Gliders have occupied a cross shelf transect and have documented the annual variability in shelf wide stratification on the Mid-Atlantic Bight and the role of storms in sediment resuspension. The sustained data permits scientists to gather regional data critical to addressing if, and how, the oceans are changing. © 2007 Wiley Periodicals, Inc.
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.
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] 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.
Atlantic bluefish exhibit cohort splitting, whereby two modes of juvenile recruits originate from spatially distinct spring- and summer-spawning regions in US Atlantic shelf waters. We evaluate the pattern of cohort splitting in a transition area (US Maryland coastal region and Chesapeake Bay) between the two major spawning regions. Spring and summer cohorts were differentially represented in Maryland estuarine (Chesapeake Bay) and coastal waters. The spring cohort was dominant in Chesapeake Bay, but was not well represented in the ocean environment, and the converse true for the summer cohort. We hypothesized that ocean temperatures control the bimodal spawning behavior and extent of cohort splitting. As evidence, we observed an intervening early summer cohort produced in years when shelf temperatures during early summer were suitably warm for spawning. In most years however, two dominant cohorts were evident. We propose that vernal warming dynamics in the mid-Atlantic Bight influence spawning behavior and the resultant bimodal pattern of seasonal juvenile cohort production commonly observed along the US east coast.
Examination of temperature and salinity data from the Scotian Shelf, Gulf of Maine, Gulf of St. Lawrence, and the adjacent continental slope has shown that the dominant low-frequency event over the last 45 years was a cooling and subsurface freshening of the water masses from 1952 to 1967, followed by a rapid reversal of these trends. The largest temperature and salinity changes (1952-1967) were 4.6°C and 0.7, respectively, and occurred at about 100 m over the slope. Exchanges with shelf waters and vertical mixing gave rise to the surface manifestation of this variability. The westward transport of the Labrador Current was found to have similar variability, increasing from about 1×106 m3s-1 in the early 1950s to about 4×106 m3s-1 in the mid-1960s. A simple model that accounts for this variation of transport and has a constant entrainment of North Atlantic water indicates that changes of the westward flow of the Labrador Current could contribute significantly to the T-S fluctuations.
Two stable equilibria have been obtained from a global model of the coupled ocean-atmosphere system developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The model used for this study consists of general circulation models of the atmosphere and the world oceans and a simple model of land surface. Starting from two different initial conditions, `asynchronous' time integrations of the coupled model, under identical boundary conditions, lead to two stable equilibria. In one equilibrium, the North Atlantic Oman has a vigorous thermohaline circulation and relatively saline and warm surface water. In the other equilibrium, there is no thermohaline circulation, and an intense halocline exists in the surface layer at high latitudes. In both integration the, air-sea exchange of water is adjusted to remove a systematic bias of the model that surpresses the thermohaline circulation in the North Atlantic. Nevertheless these results raise the intriguing possibility that the coupled system may have at least two equilibria. They also suggest that the themohaline overturning in the North Atlantic is mainly responsible for making the surface salinity of the northern North Atlantic higher than that of the northern North Pacific. Finally, a discussion is made on the paleoclimatic implications of these results for the large and abrupt transition between the Alleröd and Younger Dryas events which occurred about 11 000 years ago.
A technique is presented for selection of principal components for which the geophysical signal is greater than the level of noise. By contrasting the application of principal components based upon the covariance matrix and correlation matrix for a given data set of cyclone frequencies, it is shown that the former is more suitable to fitting data and locating the individual variables that represent large variance in the record, while the latter is more suitable for resolving spatial oscillations such as the movement of primary storm tracks.-from Authors
The circulation pattern proposed by Bigelow (1927) included a major current of water flowing southward off the southern side of Georges Bank. This pattern is not consistent with recent long-term current measurements that show that the water there moves westward along the bank isobaths. The measurements made by Bigelow are reviewed to show that a Gulf Stream ring was located south of the bank during his observations and that the ring was likely entraining water off the Georges Bank, consistent with Bigelow's observations. The works of Henry B. Bigelow [Bigelow, 1926, 1927; Big,low and Welsh, 1925-] form a foundation upon which much of our knowledge of the physical oceanography and the biology of the Gulf of Maine-Georges Bank region is based. With very little data he described a complex oceanographic regime in considerable detail. Observations over the intervening half century have shown that the major aspects of Bigelow's descriptions were correct and still applicable today. In one area--the circulation along the southern side of Georges Bank--recent measurements, however, are contrary to Bigel
Uses two years of moored velocity and temperature data to describe the flow field in a passage to the Gulf of Maine. Outlines the instrumentation used and data processing applied. Tides dominate the current velocity field throughout the year. Presents the tidal current parameters observed and examines low frequency currents together with seasonal variation of wind stress. Transport estimates are presented. Current fluctuations appear to be driven by external pressure gradients set up by wind stress over the Gulf of Maine. Discusses the inflow and outflow structure. (C.J.U.)
Long-term measurements off southwest Nova Scotia reveal the following features of the mean circulation:
(a) a westward longshore coastal current (4–10 cm s⁻¹,
(b) an anticyclonic gyre around Brown Bank (5–15 cm s⁻¹, and
(c) an upwelling circulation off Cape Sable (1–2 cm s⁻¹, at bottom).
The gyre circulation appears permanent but the coastal current and upwelling exhibit annual variations of the same order as the means. There is a distinct annual signal in the longshore transport at Cape Sable (maximum westward in winter), whereas the mean transport (0.14 × 10⁶ m⁴ s⁻¹) is consistent with both geostrophic estimates and budget requirements in the Gulf of Marine. Strong seasonal cycles are also found in the salinity and density fields at Cape Sable which appear to be controlled both by buoyancy input from the coastal current and local mixing effects.
A linear diagnostic model indicates that the primary dynamical balance for the circulation is between a longshore pressure gradient and longshore mean density and stratification gradients which have summer maxima. Lesser contributions arise from longshore wind, offshore density gradient and centrifugal upwelling. Tidal rectification, deduced from coherent modulations of the semidiurnal tidal streams and low-frequency currents, supports the Browns Bank gyre circulation and drives both westward and offshore components of the near-bottom flow off Cape Sable. Thus the “centrifugal upwelling” hypothesis fails and the main driving force for Cape Sable upwelling appears to be the longshore density variations maintained by tidal mixing.
The surface heat flux in the Gulf of Maine is estimated on a daily basis for the period 1979–1987 by combining air temperature, water temperature and wind speed data measured at a NOAA buoy in the central Gulf with insolation data from coastal stations and climatological parameters. The heat flux is compared with water column temperatures measured during the same period. The mean annual cycle of heat flux is similar to that reported by Bunker (1976) and to the rate of change in the heat content of the upper 100 m of the water column across the Gulf. The interannual variability in heat flux is dominated by the latent and sensible heat flux terms during winter and by insolation variability during summer. In the western Gulf the interannual variability in water temperature is significantly correlated with the heat flux variations. In the eastern Gulf, no relationship is found and the temperature variability is believed dominated by advective changes.
There has been a growing scientific concern about the influence of climatic change on marine ecosystems. Oceanic factors, such as water temperature and salinity, have been shown to be of prime importance to resident and transient biota, because of direct and indirect environmental influences upon physiological processes such as metabolic rates and reproductive cycles. Monitoring of the waters of the Middle Atlantic Bight and Gulf of Maine has been conducted by the MARMAP Ships of Opportunity Program since the early 1970's. Presented in the atlas are portrayals of the temporal and spatial patterns of surface and bottom temperature and surface salinity for these areas during the period 1978-1990. These patterns are shown in the form of time-space diagrams for single-year and multi-year (base period) time frames. Each base period figure shows thirteen-year (1978-1990) mean conditions, sample variance in the form of standard deviations of the measured values, and data locations. Each single-year figure displays annual conditions, sampling locations, and departures of annual conditions from the thirteen-year means, expressed as algebraic anomalies and standardized anomalies.
A transect across southern Georges Bank in May 1983 showed higher levels of available prey for haddock (Melanogrammus aeglefinus) and cod (Gadus morhua) larvae at two stratified sites than at a well-mixed site. At the stratified sites, prey biomass was high (30–300 μg dry wt∙L ⁻¹ ) near the surface above the thermocline; values were lower and more uniform with depth (10–30 μg dry wt∙L ⁻¹ ) at the well-mixed site. Larval population centers generally coincided with prey biomass vertically. Recent growth in dry weight of haddock larvae as estimated by RNA–DNA ratio analysis was higher at the stratified sites (8–13%∙d ⁻¹ ) than at the well-mixed site (7%∙d ⁻¹ ). Larvae appeared to be in excellent condition at the stratified sites, but up to 50% of haddock larvae from the well-mixed site had RNA–DNA ratios in the range observed for starved larvae in the laboratory. Cod collected at the same site were in better condition and growing faster than haddock. The data support the hypotheses that (1) stratified conditions in the spring favor good growth and survival of haddock larvae and (2) cod larvae are better adapted to grow and survive in well-mixed waters at lower levels of available food than haddock larvae.
Sea surface temperature (SST) variability in the shelf‐slope region of the northwest Atlantic is described and then explained in terms of latent and sensible heat exchange with the atmosphere. The basic data are primarily engine‐intake temperature measurements made by merchant ships over the period 1946–80. The data have been grouped by month and area and an empirical orthogonal function analysis has been performed to determine the dominant modes of variation. The first two modes account for 44% of the total variance. The first mode corresponds to in‐phase changes of SST from the Grand Banks to Mid‐Atlantic Bight; the second mode corresponds to opposite changes of SST on the Grand Banks and Mid‐Atlantic Bight. The time‐dependent amplitudes of these large‐scale modes have pronounced low‐frequency components; the associated changes in SST are typically 3°C. It is also shown that winter anomalies last longer than summer anomalies; their typical decay scales are 6 and 3 months, respectively.The onshore component of geostrophic wind is significantly correlated with the amplitude of the first mode in winter. We note the strong land‐sea contrast of temperature and humidity in this region during winter and explain the wind‐SST correlation in terms of latent and sensible heat exchanges. The second mode (i.e. the difference in SST between the Grand Banks and Mid‐Atlantic Bight) also appears to be related to changes in atmospheric circulation during the winter. A stochastic model for mixed layer temperature is finally used to model the SST autocorrelation functions. Following Ruiz de Elvira and Lemke (1982), it includes a seasonally‐varying feedback coefficient. The model successfully reproduces the extended persistence of winter anomalies with physically realistic parameter values but it cannot account for the summer reinforcement of winter anomalies on the Scotian Shelf. We speculate that this is due to the occasional entrainment of water, cooled the previous winter, into the shallow summer mixed layer.
Historical data from 1912–1987 indicate that low salinity (less than 32.0 psu) near-surface water occurs occasionally on southern Georges Bank during May, while none occurs during April. Composite monthly plots of historical near-surface salinity and estimated advection rates show that the southwestern Scotian Shelf is the immediate upstream source of this low salinity water. Optimally interpolated satellite-derived SST and hydrographic data reveal that very cold (less than 2.0°C), low salinity (less than 32.0 psu) Scotian Shelf Water (SSW), initially located south of the 200-m isobath off southern Georges Bank in early March 1992, moved north onto southern Georges Bank during April 1992. SSW was not as extensive during the same period in 1993, as evidence by significantly higher temperatures and salinities. These differences show large interannual variability in the transport and/or properties of SSW flowing onto Georges Bank. Lower (higher) salinities measured during spring 1992 (1993) on southern Georges Bank are consistent with higher (lower) St Lawrence River discharge noted during spring 1991 (1992) and the ∼ nine month lag between annual discharge maxima from rivers located upstream and minimum salinity at Cape Sable in February. However, comparisons between historical occurrences of low salinity (less than 32.0 psu) SSW on southern Georges Bank noted for May 1966, 1971, and 1978, and cumulative St Lawrence River discharge from the spring prior to each occurrence, show no relationship. This suggests that the occurrence of low salinity water on southern Georges Bank is not directly related to variations in upstream river discharge.
The MARMAP hydrographic data set (1977–1987) is used to determine the mean annual cycle of temperature, salinity, and density structure of surface waters throughout the Gulf of Maine. The temperatures follow the expected seasonal warming and cooling pattern. In the eastern Gulf the salinity cycle is dominated by influx of low salinity Scotian Shelf water which enters near Cape Sable in the winter, and in the western Gulf by the local spring runoff. Phasing of temperature and salinity cycles in different parts of the Gulf results in the western Gulf of Maine being more strongly stratified in the summer and more vertically uniform in the winter than is the eastern Gulf.The interannual variability, derived by subtracting the annual cycles from the original data, reveals relatively little temperature variability (1–2°C) during the period 1977–1987, compared to observed changes of 4–6°C in previous decades. Large interannual changes in salinity (0.5 psu), however, are evident in the data. The salinity variability is shown to be due primarily to changes in local fresh water sources—precipitation and runoff. Comparison of salinity changes in the Gulf of Maine with data from Georges Bank and the Middle Atlantic Bight shows that the salinity variability is coherent over the northeast continental shelf region from the western Gulf (Wilkinson Basin) to Cape Hatteras.
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.
Surface and bottom temperature distributions from the Northeast Fisheries Center spring and fall bottom trawl survey program
- T Holzwarth
- D Mountain
- T Holzwarth
- D Mountain
a fisheries ecosystem study in the NW Atlantic: Fluctuations in ichthyoplankton-zooplankton components and their potential impact on the system, Advanced Concepts in Ocean Measurements for Marine BiologyF
- K Sherman
- Marmap
- K Sherman
- Marmap
Seasonal bottom-water temperature trends in the Gulf of Maine and on Georges Bank, 1967–1975NOAA Tech. Rep. NMFS-SSRF-725, 17Natl
- C W Davis
- C W Davis
Bottom-water temperature trends in the Middle Atlantic Bight during spring and autumn, 1964–1976NOAA Tech. Rep. NMFS-SSRF-738, 13Natl
- C W Davis
- C W Davis
Evidence of nearshore summer upwelling off Atlantic City, New JerseyNOAA Tech. Memo. NMFS-F/NEC-31, 10Natl
- M C Ingham
- J Eberwine
- M C Ingham
- J Eberwine
MARMAP, a fisheries ecosystem study in the NW At-lantic: Fluctuations in ichthyoplankton-zooplankton components and their potential impact on the system, in Advanced Concepts in Ocean Measurements for Marine Biology The mean and seasonal circulation off southwest Nova Scotia
- K Sherman
Sherman, K., MARMAP, a fisheries ecosystem study in the NW At-lantic: Fluctuations in ichthyoplankton-zooplankton components and their potential impact on the system, in Advanced Concepts in Ocean Measurements for Marine Biology, edited by F. P. Diemer, F. J. Vernberg, and D. Z. Mirkes, Belle W. Baruch Inst. for Mar. Biol. and Coastal Res., Univ. of S.C. Press, Columbia, 1980. Smith, P. C., The mean and seasonal circulation off southwest Nova Scotia, J. Phys. Oceanogr., 13, 1034-1054, 1983.
Evidence of nearshore summer up-welling off Atlantic City
- M C Ingham
- J Eberwine
Ingham, M. C., and J. Eberwine, Evidence of nearshore summer up-welling off Atlantic City, New Jersey, NOA,4 Tech. Memo. NMFS-F/NEC-31, 10 pp., Natl. Oceanic and Atmos. Admin., Washington, D.C., 1984.
Sea surface tem-perature variability in the shelf-slope region of the Northwest At-lantic, Atmos. Ocean
- K R Thompson
- R H Loucks
- R W Trites
- D G Mountain
- Maureen H Taylor
- Noaa Nmfs
Thompson, K. R., R. H. Loucks, and R. W. Trites, Sea surface tem-perature variability in the shelf-slope region of the Northwest At-lantic, Atmos. Ocean, 26, 282-299, 1988. D. G. Mountain and Maureen H. Taylor, NOAA/NMFS, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, MA 02543-1026. (e-mail: dmountain@whsun. l.wh.whoi.edu) (Received November 29, 1994; revised August 20, 1997; accepted September 15, 1997.)
Groundfish survey program of BCF Woods Hole Surface and bottom temperature distributions from the Northeast Fisheries Center spring and fall bottom trawl survey program
- M D Grosslein
- T Holzwarth
- D Mountain
Grosslein, M.D., Groundfish survey program of BCF Woods Hole, Commer. Fish. Rev., 31(8-9), 22-30, 1969. Holzwarth, T., and D. Mountain, Surface and bottom temperature distributions from the Northeast Fisheries Center spring and fall bottom trawl survey program, 1963-1987, NMFS/NEFSC Ref. Doc. 90-03, 62 pp., Natl. Oceanic and Atmos. Admin., Washington, D.C., 1990.
Hydrographic structure and variability, in Georges Bank Global Ocean Ecosystems Dynamics or Global Ocean-Ecosystem Cou-pling (GLOBEC), Northwest Atlantic implementation plan
- C Flagg
Flagg, C., Hydrographic structure and variability, in Georges Bank, edited by R. H. Backus, pp. 108-124, MIT Press, Cambridge, Mass., 1987. Global Ocean Ecosystems Dynamics or Global Ocean-Ecosystem Cou-pling (GLOBEC), Northwest Atlantic implementation plan, Rep. 6, June 1992.
A significance test for prin-cipal components applied to a cyclone climatology, Mon. Weather Rev Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine
- J E Overland
- R W Preisendorfer
- B Petrie
- K Drinkwater
Overland, J. E., and R. W. Preisendorfer, A significance test for prin-cipal components applied to a cyclone climatology, Mon. Weather Rev., 110, 1-4, 1982. Petrie, B., and K. Drinkwater, Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945-1990, J. Geophys. Res., 98, 20,079-20,089, 1993. 1808, 1985.
Hydrographic structure and variability, Georges BankR. H. Backus
- C Flagg
Seasonal bottom-water temperature trends in the Gulf of Maine and on Georges Bank 1967-1975NOAA Tech
- C W Davis
Mountain Surface and bottom temperature distributions from the Northeast Fisheries Center spring and fall bottom trawl survey program 1963-1987 NMFS/NEFSC Ref. Doc. 90-03 62 Natl. Oceanic and Atmos. Admin
- T D Holzwarth
Bottom-water temperature trends in the Middle Atlantic Bight during spring and autumn 1964-1976NOAA Tech
- C W Davis
Eberwine Evidence of nearshore summer upwelling off Atlantic City New JerseyNOAA Tech
- M C J Ingham