Fig 6 - uploaded by Alexander E. Yankovsky
Content may be subject to copyright.
Ageostrophic vertical shear v 0 a vs. geostrophic (TWB) shear v 0 g from all analyzed transects. Positive/negative values of the geostrophic shear correspond to the upwelling/downwelling-favorable alongshelf current. The gray line represents the linear regression.
Source publication
The validity of thermal wind balance (TWB) as an approximation for the observed vertical shear in coastal currents (depth 10–30m) is analyzed based on the ADCP and CTD transects conducted off the New Jersey coast in early August of 1996. At that time, the study area was forced simultaneously by a brief upwelling-favorable wind event and the arrival...
Contexts in source publication
Context 1
... value near the transect's offshore edge, where temporal variations in the pycnocline vertical position were smaller. The ageostrophic shear derived from the averaged data (Fig. 5 bottom panel) is of the opposite sign relative to the TWB shear within the pycnocline (as on the individual transects), although its magnitude is now much reduced. Fig. 6 compares v 0 a vs. v 0 g from all the individual transects. Negative/positive geostrophic shear cor- responds to downwelling/upwelling-favorable cur- rent. The absolute values of the horizontal density gradient and corresponding TWB shear are higher for the upwelling-favorable current; the latter exceeds 0.08 s À1 compared to 0.04 s À1 ...
Context 2
... the upwelling flow and thus was more affected by bottom friction; (ii) in general, downwelling frontal structures are more diffuse compared to their upwelling counterparts because the lighter water is advected through the bottom boundary layer underneath the denser water offshore inducing some vertical mixing. The linear regression slope is À0.49 (Fig. 6) indicating that on average TWB shear overestimates the observed by approximately a factor of 2. The discrepancy between the geostrophic and ageostrophic shear grows with the v 0 g absolute values. Indeed, when a higher-order polynomial is specified for the best least-squares fit (not shown), the maximum slopes occur at the edges of the ...
Similar publications
With the purpose of studying the hydrography and flow structure off the mouth of the Chesapeake Bay, a series of transects were sampled continuously with a 600 kHz acoustic Doppler current profiler (ADCP) and a thermosalinograph during late September 1995. Hydrographic (CTD) stations were combined with underway measurements and occupied every 4 km...
The oceanographic processes in the region of fluvial influence (ROFI) of the ItajaÃ-Açu river were assessed through fourteen monthly surveys from November 2002 until December 2003. The main objective of this study was to investigate the effects of the fluvial riverine in the physical, biogeochemical and biological processes in the ROFI. Twenty ei...
Citations
... In these downwelling scenarios, the plumes remain bottom-attached, characterized by nearly vertical isohalines. The vertical shear within the plume here is driven by two factors: the thermal wind resulting from lateral density gradients (Mazzini et al., 2019;Pimenta et al., 2011;Yankovsky, 2006) and the shear associated with the Ekman flows (Fisher et al., 2018;Fong & Geyer, 2001). Figure 5 The Rossby number is notably high in the narrow cape scenario, indicating that inertia terms in the momentum balance surpass Coriolis terms by an order of magnitude. ...
Plain Language Summary
The interaction of river plumes with capes and headlands can affect the circulation and transport of substances and organisms in the coastal ocean. However, not much is known regarding the impact of capes on the retention of freshwater and any substances or organisms present in river plumes. To address this research gap, we conducted simulations, exploring various scenarios with differing cape shapes, constant river flow, and variable winds. Our findings underscore the roles of capes, wind, and the Earth's rotation in freshwater retention dynamics. At narrow capes without wind, freshwater accumulates predominantly on the lee side, forming a bulge. In contrast, broader capes lead to reduced freshwater retention. This research advances our understanding of plume‐cape interactions in the coastal ocean, with implications for marine ecosystems dynamics and water quality.
... Across-shelf plume structures are influenced by wind strength and direction via wind-driven Ekman circulation in stratified water columns (Hallock & Marmorino, 2002;Lentz, 2001;Whitney & Garvine, 2005). Upwelling-favorable winds force plume water to spread offshore and mix with ambient seawater due to shear instability generated by the vertical shear of currents, and wind-induced stresses are trapped within the surface layer (Fong & Geyer, 2001;Yankovsky, 2006). In response to upwelling-favorable winds, entrainment occurs continually at the edge of a river plume due to competition between lateral buoyancy force and wind-driven mixing (Lentz, 2004). ...
... The baroclinic thermal wind shear is a prominent balance in a plume during weak wind conditions, but this balance breaks down during strong wind conditions (Lentz et al., 1999;Mazzini et al., 2019). In the pycnocline, the thermal wind balance shear by buoyant plume water is counteracted by the ageostrophic shear (Yankovsky, 2006). ...
... The estimate of the buoyancy flux used here is based on the flux Richardson number proposed by Fisher et al. (2018), the validity of which is debated. In addition, thermal wind shear affects the pycnocline of a river plume (Yankovsky, 2006), causing TKE production there. How much turbulence is transferred from TKE production to mix the river plume depends on the value of the flux Richardson number. ...
An idealized three‐dimensional numerical model is used to investigate turbulent kinetic energy (TKE) production in a far‐field river plume under upwelling‐favorable winds. TKE production decreases over longer length scales as the river plume thickens. Maximum TKE production appears in the surface layer and is mainly generated by the alongshore component of the velocity shear. The large velocity shear and weak stratification in the surface layer result in a gradient Richardson number (Ri) of <0.25, which corresponds to the locally high TKE production. We find that asymmetrical TKE production occurs at the two edges of the river plume, due to the opposite nonlinear interaction of the Ekman and geostrophic effects at the shoreward and seaward edges of the river plume. This asymmetrical TKE production combines with secondary upwelling circulation in the river plume under upwelling‐favorable winds, which may generate more intense biological activity at the shoreward edge of the river plume than at the seaward edge. Numerical model experiments are performed to examine the effects of wind, river discharge, and stratified conditions on TKE production in the river plume. Finally, we propose a conceptual model in which the depth‐averaged TKE production in the river plume is proportional to the alongshore wind stress (τy2.5 ${{\tau }_{y}}^{2.5}$) and inversely proportional to the cube of the surface boundary layer thickness (D³), which is consistent with the results of numerical experiments.
... The geostrophic velocity in Figure 10B is compared against the observed v-component (average of velocity profiles from the two stations). Clearly, there is significant geostrophic component in the observed velocity field at stations 11-12, although turbulent stresses tend to "smooth" the velocity profile and make it more linear with depth, as was also reported by Yankovsky (2006) and Mazzini et al. (2019). Now we discuss the most seaward station 16 which lacks near-surface information as no drone or MicroCTD sampling took place. ...
In this study, we present observations of the Winyah Bay (WB) plume (SC, United States) formed by high freshwater discharge and a moderate upwelling-favorable wind acting continuously for ∼1.5 days prior to the shipboard survey. If a similar wind forcing persists over a longer period, the plume turns upstream (against its natural propagation) and curves offshore forming a “filament” with minimal transverse spreading, as seen in numerous satellite images. The observed plume comprises a train of tidal sub-plumes undergoing rotational adjustment and being transported offshore by Ekman dynamics. The WB outflow is supercritical in terms of the interior Froude number. Moderate wind extends this supercritical regime farther offshore. The plume is characterized by interior fronts associated with consecutive tidal pulses. Age of the buoyant water can be distinguished by the buoyant layer mixing (evident in the layer’s thickness and salinity anomaly) along with the transformation of its TS properties. However, relatively little transverse (lateral) spreading of buoyant water occurs: the equivalent freshwater layer thickness remains surprisingly consistent, approximately 0.8 m, over more than 20 km in the direction of the bulge extension. It is hypothesized that the supercritical regime constrains the transverse spreading of a plume. Microstructure measurements reveal higher dissipation rates below the base of the older (offshore) part of the plume. This is attributed to internal wave radiation from a newly discharged tidal pulse into an older plume, with the buoyant layer acting as a waveguide. Theoretical estimations of the internal wave properties show that the interior front is highly supercritical, while the observed dissipation maximum agrees with the theoretical wave structure.
... By selecting 15 profiles, Garvine (2004) showed that depth averaged values of OBS were significantly correlated with their TWS counterparts (correlation coefficient r = 0.69). Yankovsky (2006) conducted perhaps the most in-depth study of the topic up to date. Using data from the same field campaign as in Garvine (2004), Yankovsky (2006) combined the shipboard ADCP data with the density measurements from the Scanfish, to estimate the spatial distribution of the TWS. ...
... Yankovsky (2006) conducted perhaps the most in-depth study of the topic up to date. Using data from the same field campaign as in Garvine (2004), Yankovsky (2006) combined the shipboard ADCP data with the density measurements from the Scanfish, to estimate the spatial distribution of the TWS. The data set was limited to only seven transects obtained during 4 days under weak wind conditions. ...
... The TWS, v g ∕ z (equation (1)), was calculated by applying centered differences to the low-passed density data from the WWs at S2 and S3, located 1.7 km apart, and using a reference density 0 of 1,025 kg/m 3 , gravitational acceleration g of 9.81 m/s 2 , and the Coriolis parameter f = 8.65×10 −5 s −1 (36 • 21.7 ′ N). Following Yankovsky (2006), we will define the ageostrophic shear (AGEOS) v a ∕ z, as simply the difference between OBS and TWS: (2) Density is contoured in intervals of 0.5 t , thick gray line denotes 24 t which is an approximate limit for the CBP, and surface mixed layer depth is plotted in black. Panels d-h are overlayed with contours of average density between S2 and S3 and mixed layer depth of the averaged density field. ...
Plain Language Summary
Historically, oceanographers have been using a theoretical framework that allows us to estimate current velocities from water density profiles, commonly obtained during oceanographic cruises. This technique has been used to calculate transport of water and heat carried by the Gulf Stream toward the poles and to study other major currents in the deep ocean. The same theoretical framework has also been applied to study currents in shallow coastal regions (10‐20‐m), however, without a thorough validation. In this work, we test and validate this theoretical framework in the Chesapeake Bay Plume, which is brackish water originated from the Chesapeake Bay off the coasts of Virginia and North Carolina. We use novel high‐resolution measurements obtained by a series of moored instruments that include current meters and two Wirewalkers, which are wave‐powered profiling platforms that were equipped with salinity and temperature sensors and that allowed us to obtain a rich data set with thousands of density profiles. We found that theory and observations agree when winds are weak or moderate. However, during strong winds, we found that the theory breaks down due to wind‐generated turbulence (irregular motion resulting from eddies), and a correction for using this theory in shallow areas is necessary.
... Vertical shears on the shoreward side of the USJ, where water depth decreases to 50 m and less, are less than predicted by the cross-shelf density gradients. Examination of the thermal wind balance on the New Jersey shelf by Yankovsky (2006) also found observed shears to be weaker than geostrophic shears and they suggest turbulent mixing to explain the difference. ...
A 3-month-long field program conducted in winter 2012 inshore of the seaward deflection of the Gulf Stream at the Charleston Bump observed several 7–21-day periods of strong (>0.5 m s ⁻¹ ) equatorward along-shelf flow over the upper continental slope. In sea surface temperature images, these phenomena resemble and appear linked to warm filaments, features known to be associated with meanders of the Gulf Stream as it traverses the southeast coast of North America. However, the character of these upper-slope features differs from previous descriptions of filaments, hence we describe them as “upper-slope jets.” We document the characteristics of the jets, which are approximately 30 km in width, centered on the 200-m isobath, with a maximum temperature variation at depth, and reasonably long-lived. Southwestward flow within the jet extends to 200 m and is in approximate thermal wind balance below a surface mixed layer. Maximum transport is estimated to be about 2.0 Sv (1 Sv ≡ 10 ⁶ m ³ s ⁻¹ ), driving a net equatorward along-shelf velocity over the deployment period. For this time period, at least, the jets form the equatorward flow of the shoreward flank of the Charleston Gyre. We suggest the features resemble the Pinocchio’s Nose Intrusion recently described by Zhang and Gawarkiewicz. Large-amplitude meander crests with sufficiently strong curvature vorticity are a plausible source of initiation of the upper-slope jets.
... -The domain extends from the coast to the open ocean. It thus encompasses on the one hand areas directly under the continental influence (large runoffs), with very shallow waters clearly in highly frictional regimes and on the other hand deeper ocean closer to the geostrophic equilibrium (Yankovsky, 2006;Hill et al., 2008) showing strong seasonal haline and thermal stratifications (Puillat et al., 2004). The fast moving limits between the two may coincide with thermal fronts (e.g. ...
... Thermal balances have been shown to hold down to depths as shallow as 10 m (Yankovsky, 2006;Lentz et al., 1999). In this section terms we compare on each sides of Equation 3.6 with a least squares fit of the vertical shear by the horizontal density gradient. ...
Ph. D thesis, Scripps Institution of Oceanography, University of California San Diego
... In the absence of external forcing, two dynamically distinct regions arise: the bulge, near the river mouth, and the coastal current, farther downstream where the flow is confined to the coast (Chao and Boicourt 1986). The bulge is an unsteady, continuously growing feature, where both advection and rotational effects are major terms in the momentum balance, while the coastal current, which is the focus of this work, is a predominantly steady feature characterized by a semigeostrophic balance to first order (away from the bulge and the nose of the coastal current) (e.g., Blanton 1981;Garvine 2004;Yankovsky 2006). ...
During fall/winter off the Oregon coast, oceanographic surveys are relatively scarce due to rough weather conditions. This challenge has been overcome by the use of autonomous underwater gliders deployed along the Newport Hydrographic Line (NH-Line) nearly continuously since 2006. The discharge from the coastal rivers between northern California and the NH-Line reach several thousands of m3 s−1, and peaks are comparable to the discharge from the Columbia River. This freshwater input creates cross-shelf density gradients, that together with the wind-forcing and the large-scale Davidson Current, results in strong northward velocities over the shelf. A persistent coastal current during fall/winter, which we call the Oregon Coastal Current (OCC), has been revealed by the glider data set. Based on a 2-layer model, the dominant forcing mechanism of the OCC is buoyancy, followed by the Davidson Current, and then the wind stress, accounting for 61% (±22.6%), 26% (±18.6%) and 13% (±11.7%) of the along-shore transports, respectively. The OCC average velocities vary from 0.1 to over 0.5 m s−1, and transports are on average 0.08 (±0.07) Sv, with maximum observed value of 0.49 Sv, comparable to the summertime upwelling jet off the Oregon coast. The OCC is a surface-trapped coastal current and its geometry is highly affected by the wind-stress, consistent with Ekman dynamics. The wind stress has an overall small direct contribution to the along-shore transport, however it plays a primary role in modifying the OCC structure. The OCC is a persistent, key component of the fall/winter shelf dynamics and influences the ocean biogeochemistry off the Oregon coast.
... Observations of the current response to wind forcing in open coastal environments (Yankovsky 2006;Winant et al. 1987) have shown that lateral density gradients tend to be balanced by the vertical shear, as predicted by the thermal wind equation, ...
Moored current and pressure observations were obtained at Bahia Concepcion, a semienclosed bay located on the eastern side of the Baja California peninsula in Mexico, to describe the wind-driven subinertial circulation. In winter and early spring, the bay is well mixed and forced by persistent winds toward the southeast, aligned with the central axis. The authors' observations show that the sea surface rises downwind in response to wind stress and that there exists a crosswind drift at the surface that is consistent with Ekman dynamics. This feature is typical of a bay that is deeper than one Ekman depth and hence affected by the rotation of the earth. There is a persistent along-bay circulation toward the end of the bay along its western side with return flow on the opposite side. Drifters released near the surface across a transect move westward and downwind toward the closed end, where they recirculate cyclonically. Wind-driven linear theoretical models successfully predict the observed cross-bay circulation but fail to predict the along-bay flow pattern. The role of spatial inhomogeneities of wind stress (suggested by synoptic observations of the wind) and nonlinearities related to advection of momentum is investigated with theoretical and numerical modeling. Both mechanisms can contribute to the observed pattern of along-bay circulation. Even though the observations discussed were taken during the relatively well-mixed season, density fluctuations are shown to play, at times, an active role dynamically.
... Our results suggest the breakdown of the acrossshelf thermal wind assumption on time scales of a few days during winter offshore wind events. It is interesting to note that Yankovsky [2006], in a summertime study in the region inshore of our mooring array, also concluded that the thermal wind balance was not valid within the pycnocline. However, whereas we observe stronger shear than the thermal wind estimate, Yankovsky [2006] found exactly the opposite. ...
... It is interesting to note that Yankovsky [2006], in a summertime study in the region inshore of our mooring array, also concluded that the thermal wind balance was not valid within the pycnocline. However, whereas we observe stronger shear than the thermal wind estimate, Yankovsky [2006] found exactly the opposite. He attributed reduced shear in the pycnocline to the homogenizing effect of vertical mixing driven by the breaking of remotely generated internal waves. ...
The structure and variability of the wintertime midshelf front in the
New York Bight is examined using moored observations of currents and
hydrography during 2007. The front is located near the 50 m isobath,
inshore of the shelf break front and offshore of estuarine outflow plume
fronts. It spans the water column and is the boundary between cooler,
fresher, and less dense inner shelf water and warmer, saltier, and
denser outer shelf water. The mean hydrographic front slopes upward
offshore, and there is an associated equatorward along-shelf 5-10 cm/s
surface-intensified current jet. The mean across-shelf circulation is
offshore near the surface and onshore at depth. The across-shelf
velocity is convergent and strengthens across-shelf property gradients
with a time scale for gradient doubling of approximately 8 days. Tidal
analysis of currents and temperatures suggests that tidal shear
dispersion is not an important mechanism for midshelf front formation.
The frontal structure and location are roughly consistent with the
theory of bottom boundary layer advected fronts, suggesting that
multiple estuarine outflows are collectively responsible. Pulses of
strong offshore wind cause the breakdown of the across-shelf thermal
wind balance, which has been commonly assumed to hold in shelf
environments. Observed shear significantly exceeds thermal wind shear at
middepths, where observation of low-gradient Richardson number indicates
the importance of vertical mixing. The response of currents to wind
fluctuations is asymmetric, whereby the combination of upwelling
favorable and offshore winds or downwelling favorable and onshore winds
produces a stronger response than winds from the remaining quadrants.
