Bottom Ekman Transport Research Articles

Responses of Coastal Upwelling to Tidally Induced Bottom Friction Dynamics and Plume-Modulated Geostrophy: A Process-Oriented Modeling Study

Abstract We used a high-resolution cross-shelf two-dimensional numerical model to investigate the response of coastal wind-driven upwelling circulation to barotropic tidal forcing and lateral buoyant discharge over a broad continental shelf. We found that the tidally amplified asymmetric friction effect arising from the interaction between tidal and subtidal currents modulated the upwelling structure across the shelf. The interaction weakened the water outcropping (upwelling) in the inner shelf due to tidally amplified mixing, but enhanced cross-shore velocity offshore due to tidally induced asymmetric friction effect and nonlinear advection. The enhanced mixing changed the density in the bottom boundary layer and subsequently in the upwelling front, which eventually weakened the geostrophic alongshore flow. The mass and stratification inputs of the lateral buoyant discharge weakened or even reversed geostrophic dynamics for alongshore and upslope transports. The reversed cross-shore density and elevation gradient induced by the buoyant influx weakened the alongshore current and the associated bottom friction effect. The upslope cross-shore transport was reduced due to weakened alongshore flow and the associated bottom Ekman transport. The mass of buoyant influx compensated for the wind-driven offshore transport in the upper layer. The upwelling could be reversed to downwelling when the transport of lateral influx exceeded the wind-driven offshore transport. The responses of upwelling circulation to tidal and lateral buoyancy forcing highlighted in this process-oriented study are fundamental for interpreting more complex wind-driven shelf circulation.

Triggering mechanisms of the Yucatan upwelling

The main physical processes involved with the origin, development, intensity, and seasonality of Yucatan upwelling events are investigated using 7 years of direct (in situ) observations of the near-bottom temperature and velocity profiles of the Yucatan Current. Time series from three moorings deployed over the shelf break in the western part of the Yucatan Channel between February/2008 and July/2014 were analyzed, together with one year (May/2010 to May/2011) of temperature and velocity data from two moorings in the northeastern part of the Yucatan shelf. In particular, current measurements 10 m above the sea bed are used to calculate bottom Ekman transport on the shelf break. Low temperature events occur throughout the year in the upwelling origin area, but spring-summer events (March–August) are stronger and present dynamical features quite different from those in fall-winter (September–February). During spring-summer, bottom Ekman transport towards the shelf triggers upwelling events, which are followed by changes in sea level anomalies along and across the Yucatan Channel. The offshore movement of the Yucatan Current favors the development of cyclonic vorticity and divergence that help maintain the upwelling for long periods of time. Occasionally, in the northern part of the channel where the isobaths diverge (an area called the “notch”), cooling temperatures associated with cyclonic circulation also lead to persistent upwelling conditions. By contrast, during fall-winter, the bottom Ekman transport decreases the temperature over the Yucatan shelf, but the cooling is now strongly modulated by variability at the 6–10 days period band, linked to the passage of Coastal Trapped Waves that limit the duration of the upwelling events and their impact on shelf waters. Our study provides a plausible explanation of the seasonal strengthening of upwelling in the Yucatan system driven by current-topography interactions occurring throughout the year.

Instabilities in the Bottom Boundary Layer Reduce Boundary Layer Arrest and Stir Boundary Layer Water Into the Stratified Interior

AbstractAn along‐isobath current in stratified waters leads to a bottom boundary layer. In models with no alongshore variation, cross‐isobath density transport in this bottom boundary layer reduce the velocity in the bottom boundary layer via thermal wind, and thus the bottom friction experienced by the current above the boundary layer—this is bottom‐boundary‐layer arrest. If, however, alongshore variation of the flow is allowed, the bottom boundary layer is baroclinically unstable. We show with high resolution numerical models that these instabilities reduce this arrest and allow bottom friction to decelerate the flow above the bottom boundary layer when the flow is in the Kelvin wave direction (so that the bottom Ekman transport is downwelling). Both the arrest of the bottom boundary layer and the release from this arrest are asymmetric; the friction experienced by flows in the direction of Kelvin‐wave propagation (downwave) is much greater than flows in the opposite direction. The strength of the near bottom currents, and thus the magnitude of bottom friction, is found to be governed by the destruction of potential vorticity near the bottom balanced by the offshore along‐isopycnal transport of this anomalous potential vorticity. A simple model of this process is created and used to quantify the magnitude of this effect and the resulting reduction of arrest of the bottom boundary layer.

Mesoscale and wind-driven intra-annual variability in the East Auckland Current

Intra-annual variability in the East Auckland Current (EAuC) was studied using a year-long timeseries of in situ and remotely-sensed velocity, temperature and salinity observations. Satellite-derived velocities correlated well (hbox {r} > 0.75) with in situ observations and well-represent the long-term (> 30 days) variability of the upper ocean circulation. Four mesoscale eddies were observed during the year (for 260 days) which generated distinct flows between the continental slope and rise. The EAuC dominated the circulation in the continental shelf break, slope and rise for 110 days and generated the most energetic events associated with wind forcing. Current variability on the continental slope was coherent with along-slope wind stress (wind stress curl) at periods between 4 and 12 days (16 and 32 days). We suggest that along-slope winds generated offshore Ekman transport, uplift on the shelf-break, and a downwind geostrophic jet on the slope. In contrast, positive wind stress curl caused convergence of water, downwelling, and increased the current speed in the region. Bottom Ekman transport, generated by the EAuC, was suggested to have caused the largest temperature anomaly (-1.5 ^{circ }hbox {C}) at the continental shelf-break.

Modulation of Shelf Circulations Under Multiple River Discharges in the East China Sea

AbstractEast Asian Marginal Seas are a continuum receiving multiple large rivers including the Changjiang, the Yellow, and the Pearl Rivers. Fates of these river discharges are strongly influenced by the energetic shelf circulation system. Meanwhile, the massive freshwater discharge from these rivers also modulate the density structure and dynamic height of receiving seas, which could affect the shelf circulation as well. In this study, we developed a numerical model covering the East Asian Marginal Seas based on the Regional Ocean Modeling System to assess the influence of river plumes on the circulation over the entire shelf. The East China Sea was selected as a focus due to the massive freshwater influence and the strong and complicated shelf circulation therein. The model results suggested that both local (e.g., the Changjiang River) and remote (e.g., the Pearl River) rivers were important in regulating the shelf circulation. Overall, the riverine freshwater elevated the adjacent sea level and changed the density field, which weakened the Taiwan Warm Current (TWC), strengthened the southward coastal current, and even disturbed the Kuroshio. In the vertical direction, the TWC turned from surface‐intensified to bottom‐intensified under the river plume influences. In the cross‐shelf direction, the cross‐shelf exchange was enhanced and the Kuroshio subsurface water could more easily intrude to the coastal region, due to the enhanced bottom Ekman transport. The fundamental mechanism is the barotropic and baroclinic adjustments under the riverine influences, which could be considered as a shelf‐version of the estuarine circulation.

Progress of Studies on Circulation Dynamics in the East China Sea: The Kuroshio Exchanges With the Shelf Currents

This paper reviews recent advances in the circulation dynamics of the Kuroshio and its interaction with shelf currents in the East China Sea (ECS). The annually averaged Kuroshio volume transport varies between 19 and 24 Sv, based on different observations, but there is no consensus on which season its volume transport peaks. The Kuroshio is intensified over the central slope of the ECS from that off the northeast of Taiwan. The total Kuroshio intrusion into the ECS shelf is estimated to be 1.3–1.4 Sv, deduced from the observed volume transport of exchange flow in the Taiwan and Tsushima Straits, based on the assumption of volume conversation over the shelf. However, the uncertainty regarding this estimation remains due to the absence of sufficient observations and understanding of the Kuroshio dynamics. The Kuroshio intrusions over the shelf off the northeast of Taiwan and southwest of Kyushu are stimulated by planetary or topographic β -effect associated with the alongshore variations in the ECS slope topography and altered by variations in the Kuroshio intensity, shear stress, and baroclinicity. Multilayered exchanges between the Kuroshio and shelf currents were found between 100- and 200-m isobaths along the central ECS slope. The spatial variations in these exchanges are governed by cross-isobath transport by geostrophy, whereas bottom Ekman transport may play a predominant role in altering the integrated exchange flow along the slope. Although the intrusion is greatly modulated along the path of the Kuroshio in the ECS by variable slope topography, there are few observations on the spatial variations of these exchange flows. The characteristics and variations in the circulation and hydrographic properties of waters between 100- and 200-m isobaths significantly determine the general ECS circulation, about which consensus has still not been attained.

Dynamics of cross-shelf water exchanges off Pearl River Estuary in summer

Dynamics regulating the cross-isobath momentum and water exchanges over the shelf in the vicinity of the Pearl River Estuary (PRE) in July 2015 and 2017 are examined using comprehensive observations and multiple-level nested high-resolution numerical simulations. Although driven by the upwelling- and downwelling-favorable winds in July 2015 and 2017, respectively, the shelf currents and the cross-isobath exchanges illustrate great similarity during these two periods. This study finds that the southwesterly-induced flow downstream in the Taiwan Strait maintained the northeastward upwelling currents in the vicinity of the PRE that caused the extensive upslope invasion of the deep shelf waters. The cross-shelf exchanges of the shelf and coastal waters mainly occur over the irregular topography off the entrance of the HuangMaoHai estuary, to the west of the Dangan Island, and over a shallow trough to the east of Hong Kong. It is also rationalized that the intensified exchange flow is established by an along-shelf pressure gradient force that notably exceeds the contributions from the bottom and surface Ekman transport as well as the nonlinearity of the shelf current in the water column. The baroclinicity of the shelf and coastal waters plays minimal roles in altering this along-shore pressure gradient force, especially when upwelling-favorable winds prevail. The exchange flows in the water column thereby show great similarity with those in the bottom layer, where the bottom along-shore pressure gradient force is jointly imposed by the nonlinear advection of relative vorticity and frictional stress curl in both the surface and bottom layers. The spatial variation of these terms is regulated by the intensity of the shelf current and along-shelf variation of bottom topography.

Oceanic Density/Pressure Gradients and Slope Currents

AbstractEastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat, and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth integrate the linearized x and y momentum and continuity equations and assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow) and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified) and then derive an expression for the slope current velocity by integrating upward including thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses. This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution toward that balance. There is a direct relationship between the slope current strength, friction, and along-slope forcing (e.g., wind), and also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high-resolution hydrodynamic numerical model.

Dissolved inorganic nitrogen and particulate organic nitrogen budget in the Yucatán shelf: driving mechanisms through a physical–biogeochemical coupled model

Abstract. Continental shelves are the most productive areas in the seas with the strongest implications for global nitrogen cycling. The Yucatán shelf (YS) is the largest shelf in the Gulf of Mexico (GoM); however, its nitrogen budget has not been quantified. This is largely due to the lack of significant spatio-temporal in situ measurements and the complexity of the shelf dynamics, including coastal upwelling, coastal-trapped waves (CTWs), and influence of the Yucatán Current (YC) via bottom Ekman transport and dynamic uplift. In this paper, we investigate and quantify the nitrogen budget of dissolved inorganic nitrogen (DIN) and particulate organic nitrogen (PON) in the YS using a 9-year output from a coupled physical–biogeochemical model of the GoM. The sum of DIN and PON is here referred to as total nitrogen (TN). Results indicate that the main entrance of DIN is through its southern (continental) and eastern margins. The TN is then advected to the deep oligotrophic Bay of Campeche and central GoM. It is also shown that the inner shelf (bounded by the 50 m isobath) is “efficient” in terms of TN, since all DIN imported into this shelf is consumed by the phytoplankton. Submarine groundwater discharges (SGDs) contribute 20 % of the TN, while denitrification removes up to 53 % of TN that enters into the inner shelf. The high-frequency variability of the TN fluxes in the southern margin is modulated by fluxes from the YC due to enhanced bottom Ekman transport when the YC leans against the shelf break (250 m isobath) on the eastern margin. This current–topography interaction can help to maintain the upwelling of Cape Catoche, uplifting nutrient-rich water into the euphotic layer. The export of TN at both western and northwestern margins is modulated by CTWs with a mean period of about 10 d in agreement with recent observational and modelling studies.

Variability of bottom carbonate chemistry over the deep coral reefs in the Florida Straits and the impacts of mesoscale processes

Abundant and diverse cold-water coral and fish communities can be found in the deep waters of the Florida Straits, which are believed to be living under suboptimal conditions impacted by increasing oceanic CO2 levels. Yet, little is known regarding the spatial–temporal variability of bottom carbonate chemistry parameters and their dynamic drivers in this area. To address this issue, we present results from numerical simulations of a coupled physical-biogeochemical model for the south Florida shelf and Florida Straits. Our exploratory analysis focuses on two well-known deep-coral habitats: Pourtalès Terrace (200–450 m) and Miami Terrace (270–600 m). Results suggest that bottom waters along the northern/western slope of the Straits are comprised primarily of the North Atlantic Central Water (NWCW) and Antarctic Intermediate Water (AAIW), driven by upwelling associated with the bottom Ekman transport of the Florida Current. Over the Pourtalès Terrace, both the meandering of the Florida Current and mesoscale eddies modulate the upwelling (downwelling) of cold (warm) waters. In contrast, Florida Current makes a sharp turn at the southern end of the Miami Terrace leading to persistent island wakes, frequent occurrences of a transient eddy, and strong upwelling of deep waters toward the platform of the terrace. Passage of the transient eddy often accompanies strong downwelling of warm waters and a return (southward) flow on top of the platform. Overall, bottom water properties including temperature (T), dissolved inorganic carbon (DIC) and total alkalinity (TA) show strong variability on weekly to monthly time-scales over entire Pourtalès Terrace and on the platform of Miami Terrac, mostly driven by physics. In deeper areas (>400 m), bottom water properties are fairly stable with both DIC and TA showing narrow ranges. Interestingly, waters over the southeastern portion of the Pourtalès Terrace show consistently warmer temperature, lower DIC, and higher TA than those on top of this terrace. The aragonite saturation state (Ω) ranges 1.2-2 on top of the Pourtalès Terrace and 1.2-1.7 both on top of Miami Terrace and on the upper slope of Pourtalès Terrace. In the deeper slope areas (>400 m), it is nearly constant at 1.2-1.3. This modeling effort suggests that remote forcing and biogeochemical processes along the transport paths, from the Gulf of Mexico to the Straits, are significant but second-order contributors to the variability of bottom carbonate chemistry. The impacts of benthic biogeochemical processes along the transit paths are not resolved.

Deep Bottom Mixed Layer Drives Intrinsic Variability of the Antarctic Slope Front

AbstractThe Antarctic Slope Front (ASF) is located along much of the Antarctic continental shelf break and helps to maintain a barrier to the movement of Circumpolar Deep Water (CDW) onto the continental shelf. The stability of the ASF has a major control on cross-shelf heat transport and ocean-driven basal melting of Antarctic ice shelves. Here, the ASF dynamics are investigated for continental shelves with weak dense shelf water (DSW) formation, which are thought to have a stable ASF, common for regions in East Antarctica. Using an ocean process model, this study demonstrates how offshore bottom Ekman transport of shelf waters leads to the development of a deep bottom mixed layer at the lower continental slope, and subsequently determines an intrinsic variability of the ASF. The ASF variability is characterized by instability events that affect the entire water column and occur every 5–10 years and last for approximately half a year. During these instability events, the cross-shelf density gradient weakens and CDW moves closer to the continent. Stronger winds increase the formation rate of the bottom mixed layer, which causes a subsequent increase of instability events. If the observed freshening trend of continental shelf waters leads to weaker DSW formation, more regions might be vulnerable for the ASF variability to develop in the future.

Barotropic Kelvin Wave‐Induced Bottom Boundary Layer Warming Along the West Antarctic Peninsula

AbstractIntrusions of warm circumpolar deep water onto the Antarctic continental shelf are thought to drive accelerated loss of Antarctic glacial ice mass by triggering melt at the ice shelf grounding line. However, the mechanisms responsible for driving on‐shelf circumpolar deep water intrusions are not well understood. Here we examine how sea surface height (SSH) anomalies propagating around the Antarctic coastline as coastal‐trapped waves can drive warm water intrusions through changes in bottom Ekman transport. A wind perturbation motivated by the recent intensification and poleward shift of the southern annular mode during its positive phase is applied over Eastern Antarctica between 20°E and 120°E in two global ocean sea‐ice models (1/4° and 1/10°) and a single‐layer shallow water model. The changes in winds generate a drop in coastal SSH that propagates around Antarctica as a barotropic Kelvin wave. The SSH drop is accompanied by a barotropic flow, which alters the bottom stress, generating an onshore transport of warm water wherever thermal gradients are favorable. We estimate the resulting anomalous bottom Ekman flow and use temperature gradients calculated from the Southern Ocean State Estimate, along with the 1/4° and 1/10° models, to evaluate the resultant heat advection. We find that this mechanism can drive warming of up to 0.7 °C along the West Antarctic Peninsula within a year, depending on the mean state of the cross‐shelf temperature gradient and the barotropic flow strength. Over longer time scales, warming eventually ceases due to saturation of the SSH field and arrest of the Ekman transport by buoyancy forces.

Tidal Mixing Sustains a Bottom‐Trapped River Plume and Buoyant Coastal Current on an Energetic Continental Shelf

AbstractConventional wisdoms on river plume dynamics suggest that a down‐shelf buoyant coastal current will ultimately be trapped at a specific depth, that is, the trapping depth, as constrained by riverine outflow and offshore bottom Ekman transport. Theoretically, a prerequisite down‐shelf current is necessary to form a stable bottom‐trapped river plume. In this study an alternative is described by carrying out a modeling study on the Zhe‐Min Coastal Current (ZMCC). Buoyant water from the Changjiang River is a major factor driving the ZMCC, as is common in bottom‐trapped river plumes; however, the trapping depth is more determined by tidal mixing. When the plume water comes to the sloping topography, strong tidal mixing induces a mixing front, shoreward of which the bottom Ekman layer occupies the entire water column. Such a tidal‐induced front maintains a down‐shelf frontal current, which is intensified both at the surface due to the thermal wind balance and on the top of bottom boundary layer due to the tidal rectification. Direct wind‐induced transport only covers a small fraction of the ZMCC; however, it redistributes the plume water and, thus, affects the coastal current. The tide‐induced frontal trapping depth varies much less between seasons than that predicted by previous plume theories. Instead, it fluctuates strongly in the spring‐neap cycle. Even in summer when upwelling‐favorable winds prevail, the mixing front still sustains a down‐shelf coastal current. Intense tidal mixing exists in many coastal waters, which might be an alternative mechanism in forming bottom‐trapped river plumes and their associated buoyant coastal current.

Cross-Shelf Circulation Induced by the Kuroshio Shear Stress in the East China Sea

AbstractThis study solves two-dimensional (cross-shelf and depth directions) steady-state nonlinear primitive equations to infer the cross-shelf circulation induced by the Kuroshio shear stress in the East China Sea (ECS). The Kuroshio velocity data are estimated from hydrographic observations at the PN section in the ECS. Nonlinear momentum equations are solved using an iterative approach in a terrain-following coordinate system, which helps to adequately take into account the boundary conditions over complex topography. The vertical shear stress of the Kuroshio is shown to induce two offshore transport pathways over the continental shelf, which are related to the structure of the interior geostrophic current and bottom Ekman transport, respectively. As a result of the vertical shear stress, an upwelling is induced above the bottom Ekman layer on the continental slope. The horizontal shear stress of the Kuroshio has the effect of inducing onshore transport at the flow core. The advection terms in the primitive equations are found to amplify the cross-shelf velocity solved from the linear equations. This study reveals that the Kuroshio has a substantial effect on the cross-shelf circulation and that it might drive multiple transport pathways.

Buoyancy shutdown process for the development of the baroclinic jet structure of the Soya Warm Current during summer

The Soya Warm Current (SWC), which is the coastal current along the northeastern part of Hokkaido, Japan, has a notable baroclinic jet structure during summer. This study addresses the formation mechanism of the baroclinic jet by analyzing a realistic numerical model and conducting its sensitivity experiment. The key process is the interaction between the seasonal thermocline and the bottom Ekman layer on the slope off the northeastern coast of Hokkaido; the bottom Ekman transport causes subduction of the warm seasonal thermocline water below the cold lower-layer water, so the bottom mixed layer develops with a remarkable cross-isobath density gradient. Consequently, the buoyancy transport vanishes as a result of the thermal wind balance in the mixed layer. The SWC area is divided into two regions during summer: upstream, the adjustment toward the buoyancy shutdown is in progress; downstream, the buoyancy shutdown occurs. The buoyancy shutdown theory assesses the bottom-mixed-layer thickness to be 50 m, consistent with observations and our numerical results. The seasonal thermocline from June to September is strong enough to establish the dominance of the buoyancy shutdown process over the frictional spindown.

Cross‐shelf water exchange in the East China Sea as estimated by satellite altimetry and in situ hydrographic measurement

AbstractCombining satellite altimetry and in situ hydrographic measurement, we estimated the cross‐shelf transport (CST) and its spatial and temporal variations across 200 m isobath in the East China Sea (ECS) from 1993 to 2014. The vertically integrated CST can be dynamically divided into three parts: surface Ekman transport, geostrophic transport, and bottom Ekman transport. The results show that the 22 year‐mean, sectionally integrated CST to be 1.7 ± 2.0 Sv (positive in the on‐shelf direction), comprised of bottom and surface Ekman transports of 2.7 ± 1.0 Sv and 0.6 ± 0.6 Sv, respectively, that are partially offset by a geostrophic transport of −1.5 ± 1.7 Sv. The sectionally integrated CST shows significantly high power at roughly annual period from 1999 to 2001, with lower power at intra‐annual period. The vertically integrated CST to the northeast of Taiwan is the main source of sectionally integrated CST. The vertically integrated CST also shows significant variations in the 6–15 month period band to the northeast of Taiwan as well. The temporal variations of the sectionally integrated and vertically integrated CST are both controlled primarily by geostrophic transport and modulated by bottom Ekman transport. In the upper 50 m, the geostrophic current to the northeast of Taiwan exhibits large mean and significant variability. The empirical orthogonal function analysis of vertical structure of geostrophic current shows two significant modes with strong annual signal. The first mode is associated with the migration of Kuroshio axis near Taiwan, while the second mode is associated with the variation of the meander of the Kuroshio to the northeast of Taiwan.

Distribution and cross-front transport of suspended particulate matter over the inner shelf of the East China Sea

An obvious shear front between the Taiwan Warm Current and the Zhe–Min (Zhejiang–Fujian) Coastal Current exists over the inner shelf of the East China Sea (ECS) in winter. Although the cross-front spread of surface water in the ECS has been reported based on satellite images, the cross-front transport of suspended particulate matter (SPM) has not been well studied. To reveal the detailed characteristics of SPM in vertical direction, in terms of different fractions, and to study its cross-front transport and mechanism, high-vertical resolution profiles of SPM concentrations observed by Laser In-Situ Scattering and Transmissometry (LISST) and derived from water samples at various levels, together with salinity and temperature, were measured in February 2007, from 34 stations in the ECS. Results showed that volume concentration (VC) of SPM finer than 280.07µm has a good correlation with mass concentration (MC), because coarser factions were thought to be biological components. The VC was then converted to the MC of different fractions at high resolution, by reducing the influence of biological sources. Both temperature and salinity data showed an obvious front along 50m-isobath in the bottom layer, tilting seaward toward the surface layer. Within 50m-isobath, the MC was higher than 2mg/L in the surface layer and 10mg/L in the bottom layer. The SPM could be transported across the front to the sea in the bottom layer, even to the east of 100m-isobath, where the MC of SPM was less than 1mg/L in the surface layer and 2mg/L in the bottom layer. The SPM across the front was mainly silt fraction with some fraction of clay. An idealized numerical model was established. Numerical experiment results showed that bottom Ekman transport produced by the northerly wind in winter is the key factor controlling cross-front transport, and strong northerly wind is responsible for its expansion. This study helps our understanding of material exchange in the coastal and shelf seas.

Processes controlling mid‐water column oxygen minima over the Texas‐Louisiana shelf

AbstractWe investigate distributions of dissolved oxygen over the Texas‐Louisiana shelf using spatially highly resolved observations in combination with a regional circulation model with simple oxygen dynamics. The observations were collected using a towed, undulating CTD during the Mechanisms Controlling Hypoxia (MCH) program. Mid‐water oxygen minimum layers (dissolved oxygen lower than 3.2 mL L−1) were detected in many transects. These oxygen minimum layers are connected with the bottom boundary layer and follow the pycnocline seaward as a tongue of low oxygen into the mid‐water column. T‐S diagrams highlighting the low oxygen minima in both observations and simulations imply direct connections between low‐oxygen bottom water and the oxygen minimum layer. The dynamics of these oxygen minimum layers in the mid‐water column are examined using a three‐dimensional hydrodynamic model, based on the Regional Ocean Modeling System (ROMS). Convergence within the bottom boundary layer relative to density surfaces is calculated, results show that there is a convergence in the bottom boundary layer at the location where the pycnocline intersects the bottom. Buoyancy advection forced by bottom Ekman transport creates this convergent flow, and the corresponding low‐oxygen intrusion. Similar intrusions of near‐bottom water into the pycnocline are observed in other regions. The presence of hypoxia within the bottom boundary layer in the northern Gulf of Mexico creates a unique situation in which these intrusions are also associated with low dissolved oxygen.

The role of turbulent mixing in the modified Shelf Water overflows that produce Cape Darnley Bottom Water

AbstractThe mixing process associated with modified Shelf Water (mSW) overflows that eventually mix to form Cape Darnley Bottom Water (CDBW) was investigated by hydrographic and microstructure observations off the Cape Darnley Polynya (CDP), East Antarctica, in January 2009. Closely spaced microstructure observations revealed that mSW properties varied considerably within a distance of ∼4 km across the shelf edge. Near the bottom, the rate of turbulent kinetic energy dissipation was enhanced to values greater than 10−7 W kg−1, and the vertical scale of the bottom boundary layer (BBL) was on the order of 10 m. The observed BBL around the shelf edge was characterized by strong vertical mixing with turbulent eddy diffusivities of (10−3−10−2) m2 s−1. A geostrophically balanced density current, which resulted from the presence of mSW over the continental shelf, is considered the primary energy source for the turbulent mixing in the BBL. This turbulent mixing transforms the overflowing mSW through mixing with ambient water masses, specifically with the overlying modified Circumpolar Deep Water. The BBL is also thought to partly contribute to the gradual descent of mSW down the continental slope through bottom Ekman transport. We conclude that turbulent mixing, primarily caused by a density current, plays an important role in CDBW formation, by modifying the mSW overflowing from the CDP.

Rapid Generation of Upwelling at a Shelf Break Caused by Buoyancy Shutdown

AbstractModel analyses of an alongshelf flow over a continental shelf and slope reveal upwelling near the shelf break. A stratified, initially uniform, alongshelf flow undergoes a rapid adjustment with notable differences onshore and offshore of the shelf break. Over the shelf, a bottom boundary layer and an offshore bottom Ekman transport develop within an inertial period. Over the slope, the bottom offshore transport is reduced from the shelf’s bottom transport by two processes. First, advection of buoyancy downslope induces vertical mixing, destratifying, and thickening the bottom boundary layer. The downward-tilting isopycnals reduce the geostrophic speed near the bottom. The reduced bottom stress weakens the offshore Ekman transport, a process known as buoyancy shutdown of the Ekman transport. Second, the thickening bottom boundary layer and weakening near-bottom speeds are balanced by an upslope ageostrophic transport. The convergence in the bottom transport induces adiabatic upwelling offshore of the shelf break. For a time period after the initial adjustment, scalings are identified for the upwelling speed and the length scale over which it occurs. Numerical experiments are used to test the scalings for a range of initial speeds and stratifications. Upwelling occurs within an inertial period, reaching values of up to 10 m day−1 within 2 to 7 km offshore of the shelf break. Upwelling drives an interior secondary circulation that accelerates the alongshelf flow over the slope, forming a shelfbreak jet. The model results are compared with upwelling estimates from other models and observations near the Middle Atlantic Bight shelf break.