Surface Ekman Transport Research Articles

Wind‐Enhanced Separation of Large‐Scale River Plumes From Coastal Corners

AbstractIdealized models are analyzed to quantify how large‐scale river plumes interact with coastal corners with and without wind‐driven currents. The configuration has a corner formed by two perpendicular shelves (with constant slope) that are joined with a coastal radius of curvature (rc). The buoyant plume originates from an upstream point source. The rc and wind forcing are varied among runs. Steep‐ and gentle‐slope runs are compared for some situations. Without winds, plumes separate from corners with rc smaller than two inertial radii (ri); this threshold is twice the rc < ri theoretical separation criterion. After separation, no‐wind plumes form an anticyclonic bulge, and reattach farther downstream. Offshore excursion increases as rc decreases. A downwelling‐favorable wind component along the upstream coast (τsx) favors separation by increasing total plume speed. An upwelling‐favorable wind component along the downstream coast (τsy) also increases offshore excursion. Winds blowing obliquely offshore with both these wind components advect the plume farther offshore. Wind‐driven currents that steer plumes in this situation include a downshelf jet originating on the upstream shelf and continuing around the coastal corner and beyond, offshore and upshelf surface transport downstream of the corner, and surface Ekman transport on the outer shelf. Multiple linear regressions quantify plume position sensitivity to rc, τsx, and τsy; results are discussed in a dynamical context. Globally, many river plumes interact with coastal corners under various wind conditions.

Using Shelf‐Edge Transport Composition and Sensitivity Experiments to Understand Processes Driving Sea Level on the Northwest European Shelf

AbstractVariability in ocean currents, temperature and salinity drive dynamic sea level (DSL) variability on the Northwest European Shelf (NWES). It is dominated by mass variations, with steric signals relatively small. A mechanistic explanation of how ocean dynamics relates to the mass component of NWES sea level variability is required. We use regional ocean model experiments to isolate sources of variability and then investigate the effect on monthly to‐interannual DSL variability together with the simulated momentum budgets along the shelfbreak. Regional (local) wind and non‐regional (remote) forcing are important on the NWES. For the local wind forcing, the net mass flux onto the shelf, which drives a shelf‐mean mode of DSL variability, results from a combination of surface Ekman, bottom Ekman and geostrophic flows and explains 73% of the variance in transport across the shelf‐edge. The geostrophic flow is closely related to wind stress with a flow about half that of surface Ekman transport but in the opposite direction. For the remotely forced mass‐flux across the shelf‐edge, the geostrophic component explains 62% of the variance and bottom friction plays an important indirect role. The remotely forced variability, while relatively spatially uniform, is more important for explaining DSL variance over the western NWES. This mode of variability is sensitive to signals propagating northward via a thin strip of the southern boundary near the Portuguese coast, consistent with coastal trapped wave signals. It also appears to drive steric height in the Bay of Biscay, which is related to DSL on the shelf.

Wind-Forced Seasonal Exchange between Marginal Seas and the Open Ocean

Abstract The circulation within marginal seas subject to periodic winds, and their exchange with the open ocean, are explored using idealized numerical models and theory. This is motivated by the strong seasonal cycle in winds over the Nordic Seas and the exchange with the subpolar North Atlantic Ocean through the Denmark Strait and Faroe Bank Channel. Two distinct regimes are identified: an interior with closed f/h contours and a shallow shelf region that connects to the open ocean. The interior develops a strong oscillating along-topography circulation with weaker ageostrophic radial flows. The relative importance of the bottom Ekman layer and interior ageostrophic flows depends only on ωh/Cd, where ω is the forcing frequency, h is the bottom depth, and Cd is a linear bottom drag coefficient. The dynamics on the shelf are controlled by the frictional decay of coastal waves over an along-shelf scale Ly = f0LsHs/Cd, where f0 is the Coriolis parameter, and Ls and Hs are the shelf width and depth. For Ly much less than the perimeter of the basin, the surface Ekman transport is provided primarily by overturning within the marginal sea and there is little exchange with the open ocean. For Ly on the order of the basin perimeter or larger, most of the Ekman transport is provided from outside the marginal sea with an opposite exchange through the deep part of the strait. This demonstrates a direct connection between the dynamics of coastal waves on the shelf and the exchange of deep waters through the strait, some of which is derived from below sill depth. Significance Statement The purpose of this study is to understand how winds over marginal seas, which are semienclosed bodies of water around the perimeter of ocean basins, can force an exchange of water, heat, salt, and other tracers through narrow straits between the marginal sea and the open ocean. Understanding this exchange is important because marginal seas are often regions of net heat, freshwater, and carbon exchange with the atmosphere. The present results identify a direct connection between processes along the coast of the marginal sea and the flow of waters through deep straits into the open ocean.

Optimum Multiparameter Analysis of Water Mass Structure off Western Guangdong during Spring Monsoon Transition

Water masses and their variability play vital roles in regulating ocean circulation, material exchanges and biogeochemical processes. However, there is still a lack of quantitative analysis of water mass distributions in coastal waters of the South China Sea. Here, two oceanographic cruise observations in April and May 2016 are used to quantify water mass distributions, pathways and mixture, and their intraseasonal variability off western Guangdong during the spring monsoon transition. Temperature and salinity observations qualitatively reveal that there are three types of water masses: the Pearl River diluted water (PRDW, salinity (S) = 22 psu, potential temperature (θ) = 25 °C), the South China Sea surface water (SCSSW, S = 34 psu, θ = 28 °C) and the South China Sea subsurface water mass (SCSSUW, S = 34.5 psu, θ = 17 °C). Their relative contributions and intraseasonal variability are quantified using the Optimum Multiparameter (OMP) method. The PRDW is largely confined to the upper 10 m layer in shallow nearshore waters (depths < 30 m), with a maximum contribution >90% near the Pearl River Estuary. The SCSSW mainly dominates the rest of the surface layer above 20 m, with a contribution >50% in offshore regions. The layer below 20 m is primarily composed of ~60% SCSSW and ~40% SCSSUW. A comparison between the two different observations suggests that the PRDW tends to expand southwestward and the SCSSUW spreads offshore, whereas the SCSSW moves landward and is situated underneath the surface fresh PRDW. These characteristics are very likely associated with the wind transition from weak southeasterly in April to strong northeasterly in May, which enhances the southwestward coastal current and the onshore surface Ekman transport from offshore waters.

Topographic Influences on the Wind-Driven Exchange between Marginal Seas and the Open Ocean

AbstractThe wind-driven exchange through complex ridges and islands between marginal seas and the open ocean is studied using both numerical and analytical models. The models are forced by a steady, spatially uniform northward wind stress intended to represent the large-scale, low-frequency wind patterns typical of the seasonal monsoons in the western Pacific Ocean. There is an eastward surface Ekman transport out of the marginal sea and westward geostrophic inflows into the marginal sea. The interaction between the Ekman transport and an island chain produces strong baroclinic flows along the island boundaries with a vertical depth that scales with the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. The throughflows in the gaps are characterized by maximum transport in the center gap and decreasing transports toward the southern and northern tips of the island chain. An extended island rule theory demonstrates that throughflows are determined by the collective balance between viscosity on the meridional boundaries and the eastern side boundary of the islands. The outflowing transport is balanced primarily by a shallow current that enters the marginal sea along its equatorward boundary. The islands can block some direct exchange and result in a wind-driven overturning cell within the marginal sea, but this is compensated for by eastward zonal jets around the southern and northern tips of the island chain. Topography in the form of a deep slope, a ridge, or shallow shelves around the islands alters the current pathways but ultimately is unable to limit the total wind-driven exchange between the marginal sea and the open ocean.

Wind-driven and buoyancy-driven circulation in the subtropical North Atlantic Ocean.

Continuous observations of ocean circulation at 26°N in the subtropical Atlantic Ocean have been made since April 2004 to quantify the strength and variability in the Atlantic Meridional overturning circulation (AMOC), in which warm, upper waters flow northward and colder deep waters below 1100 m depth return southward. The principal components of the AMOC are northward western boundary current transport in the Gulf Stream and Antilles Current, northward surface Ekman transport and southward thermocline recirculation, all of which are generally considered to be part of the wind-driven circulation. Southward flowing deep waters below 1100 m depth are usually considered to represent the buoyancy-driven circulation. We argue that the Gulf Stream is partially wind-driven but also partially buoyancy-driven as it returns upper waters upwelled in the global ocean back to water mass formation regions in the northern Atlantic. Seasonal to interannual variations in the circulation at 26°N are principally wind-driven. Variability in the buoyancy-driven circulation occurred in a sharp reduction in 2009 in the southward flow of Lower North Atlantic Deep Water when its transport decreased by 30% from pre-2009 values. Over the 14-year observational period from 2004 to 2018, the AMOC declined by 2.4 Sv from 18.3 to 15.9 Sv.

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.

Recent Warming and Freshening of the Norwegian Sea Observed by Argo Data

Abstract Climate variability in the Norwegian Sea, comprising the Norwegian and Lofoten Basins, was investigated based upon monthly estimates of ocean heat and freshwater contents using data from Argo floats during 2002–18. Both local air–sea exchange and advective processes were examined and quantified for monthly to interannual time scales. In the recent years, 2011–18, the Norwegian Sea experienced a decoupling of the temperature and salinity, with a simultaneous warming and freshening trend. This was mainly explained by two different processes; reduced ocean heat loss to the atmosphere and advection of fresher Atlantic water into the Norwegian Sea. The local air–sea heat fluxes are important in modifying the ocean heat content, although this relationship varied with time scale and basins. On time scales exceeding 4 months in the Lofoten Basin and 6 months in the Norwegian Basin, the air–sea heat flux explained half or even more of the local ocean heat content change. There were both a short-term and long-term response of the wind forcing on the ocean heat content. The monthly to seasonal response of increased southerly wind cooled and freshened the Norwegian Basin, due to eastward surface Ekman transport, and increased the influence of Arctic Water. However, after about a 1-yr delay the ocean warmed and became saltier due to an increased advection of Atlantic Water into the region. Increased westerly winds decreased the ocean heat content in both cases due to increased transport of Arctic Water into the Norwegian Sea.

Effects of Wind Stress and Surface Cooling on Cross-Shore Exchange

AbstractThe formation of coastal dense shelf water in winter provides the available potential energy (APE) to fuel baroclinic instability. The combined effects of baroclinic instability and wind forcing in driving cross-shelf exchange are investigated using idealized numerical simulations with varied bottom slope, wind stress, and heat loss rate. The results show that under upwelling-favorable winds, the intensity of the instability decreases as the wind stress increases. This is caused primarily by enhanced turbulence frictional dissipation. Under downwelling-favorable winds, an increase in wind stress and/or a decrease in heat loss rate tends to constrain the baroclinic instability, leading to a circulation resembling that driven purely by wind forcing. In the latter case, once a critical value of cross-shore density gradient is reached, isopycnal slumping is initiated, leading to increased vertical stratification and narrowing of the inner shelf. The change in depth of the inner-shelf outer boundary, defined as the location corresponding to the maximum cross-shore gradient of the surface Ekman transport, is proportional to an empirically derived multiparametric quantity , where a2 is a dimensional constant, B0 is a constant heat loss rate, γ = 0.43, f is the Coriolis parameter, α is the shelf slope, B is the heat loss rate, and τ is the wind stress. This relationship is found to hold for cases when instabilities are present.

Argo Observations of the Deep Mixing Band in the Southern Ocean: A Salinity Modeling Challenge

AbstractThe Southern Ocean plays an important role in mediating oceanic uptake of CO2 and heat due to a strong meridional overturning circulation. Gridded Argo float data for 2004–2017 were used to evaluate subsurface processes at the mixed layer depth (MLD) that occur in a narrow deep mixing band. Shifts in the value of the Turner Angle at the MLD indicate that early in the season the MLD deepens slowly as it encounters and is stabilized by a subsurface salt maximum. By September mixing has penetrated this salinity feature and the rate of deepening is faster once the MLD is deeper than the depth where the maximum salinity occurs (~150–200 m). This distinctive salinity layer is the result of surface Ekman transport of fresh water from the south and subsurface advection of high‐salinity water from the north. Two configurations of the Community Earth System Model (CESM) ocean‐ice forced hindcast experiments—one with 1° and the other with 0.1° horizontal resolution (Parallel Ocean Program low and high resolutions [POP‐LR and POP‐HR], respectively)—are compared with the Argo data for 2005–2009. POP‐LR has a shallow MLD bias common to many Fifth Coupled Models Intercomparison Project (CMIP5) models, while POP‐HR has a mix of deep and shallow MLD biases. While both models were able to replicate the large‐scale processes leading to formation of a high‐salinity layer, the salinity feature in POP‐HR is too strong and deep. Neither model was able to replicate the vertical mixing processes leading to penetration of the subsurface salt maximum.

On the role of the North Equatorial Counter Current during a strong El Niño

Abstract. An analysis of archived data from the NEMO 1∕12th degree global ocean model shows the importance of the North Equatorial Counter Current (NECC) in the development of the strong 1982–1983 and 1997–1998 El Niños. The model results indicate that in a normal year the core of warm water in the NECC is diluted by the surface Ekman transport, by geostrophic inflow and by tropical instability waves. During the development of the 1982–1983 and 1997–1998 El Niños, these processes had reduced effect at the longitudes of warmest equatorial temperatures and to the west. During the autumns of 1982 and 1997, the speed of the NECC was also increased by a stronger-than-normal annual Rossby wave. The increased transport of warm water by the NECC due to these changes resulted in warm water reaching the far eastern Pacific and appears to have been a major factor in moving the centre of deep atmospheric convection eastwards across the Pacific.

Characteristics and dynamics of wind-driven upwelling in the Alaskan Beaufort Sea based on six years of mooring data

Six years of mooring data from the Alaskan Beaufort Sea slope, together with meteorological observations and reanalysis fields, are used to quantify the occurrence of wind-driven upwelling and the associated atmospheric forcing. The canonical upwelling event, composited from 115 individual events, reveals that when the easterly wind is strongest the entire shelfbreak jet is reversed to the west. At the end of the event a bottom-intensified, eastward-flowing “rebound jet” spins up that is stronger than the normal shelfbreak jet. The cross-isobath flow has a three-layer structure with onshore flow in the surface layer, offshore flow in the middle of the water column, and onshore flow near the bottom. This is because the reversed shelfbreak jet is oriented slightly onshore which overwhelms the cross-isobath surface Ekman transport. The vertically-integrated along-isobath momentum balance supports this interpretation and indicates that the rebound jet is driven by the zonal gradient in sea surface height. During over two thirds of the events, Atlantic Water (AW) is upwelled to the shelfbreak, while for the remaining events only Pacific Water (PW) is upwelled. The primary driving factor behind this is the seasonal variation in the PW-AW interface depth offshore of the shelfbreak, which is controlled by the local wind stress curl. During summer, when PW-type events dominate, Ekman pumping associated with negative wind stress curl deepens the interface depth, limiting access to the Atlantic layer. Over the remainder of the year, when AW events dominate, Ekman suction associated with positive wind stress curl raises the interface. These variations are due to the influence of the two regional atmospheric centers of action — the Aleutian Low and the Beaufort High.

Wind-forced depth-dependent currents over the eastern Beaufort Sea continental slope: Implications for Pacific water transport

Pacific water contributes significantly to the Arctic Ocean freshwater budget. Recent increases in Arctic freshwater flux, also affected by the Pacific-derived Arctic water, impact the Atlantic overturning circulation with implications for global climate. The interannual variability of the Pacific water outflow remains poorly understood, partly due to different branches of the Pacific water flow in the Arctic Ocean. The shelfbreak current over the Beaufort Sea continental slope transports ~50% of the Pacific-derived water eastward along the Beaufort Sea continental slope towards the Canadian Archipelago. The oceanographic mooring deployed over the eastern Beaufort Sea continental slope in October 2003 recorded current velocities through depths of 28–108 m until September 2005. Data analysis revealed that these highly energetic currents have two different modes of depth-dependent behaviour. The downwelling-favourable wind associated with cyclones passing north of the Beaufort Sea continental slope toward the Canadian Archipelago generates depth-intensified shelfbreak currents with along-slope northeastward flow. A surface Ekman on-shore transport and associated increase of the sea surface heights over the shelf produce a cross-slope pressure gradient that drives an along-slope northeastward barotropic flow, in the same direction as the wind. In contrast, the upwelling-favourable wind associated with deep Aleutian Low cyclones over the Alaskan Peninsula and/or Aleutian Island Arc leads to surface-intensified currents with along-slope westward flow. This northeasterly wind generates a surface Ekman transport that moves surface waters offshore. The associated cross-slope pressure gradient drives an along-slope southwestward barotropic flow. The wind-driven barotropic flow generated by upwelling and downwelling is superimposed on the background bottom-intensified shelfbreak current. For downwelling, this flow amplifies the depth-intensified background baroclinic circulation with enhanced Pacific water transport towards the Canadian Archipelago. For upwelling, the shelfbreak current is reversed, which results in surface-intensified flow in the opposite direction. These results are supported by numerical simulations.

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.

Wind-driven interannual variability of sea ice algal production in the western Arctic Chukchi Borderland

Abstract. Seasonal and interannual variability in the biogenic particle sinking flux was recorded using multi-year bottom-tethered sediment trap mooring systems in the Northwind Abyssal Plain (Station NAP: 75° N, 162° W, 1975 m water depth) of the western Arctic Chukchi Borderland. Trapped particle flux at a median depth of 184 m had an obvious peak and dominance of sea ice-related diatom assemblages in August 2011. The observed particle flux was considerably suppressed throughout summer 2012. In the present study, the response of ice algal production and biomass to wind-driven changes in the physical environment was addressed using a pan-Arctic sea ice–ocean modeling approach. A sea ice ecosystem with ice algae was newly incorporated into the lower-trophic marine ecosystem model, which was previously coupled with a high-resolution (i.e., 5 km horizontal grid size) sea ice–ocean general circulation model. Seasonal model experiments covering 2-year mooring periods indicated that primary productivity of ice algae around the Chukchi Borderland depended on basin-scale wind patterns via various processes. Easterly winds in the southern part of a distinct Beaufort High supplied nutrient-rich water for euphotic zones of the NAP region via both surface Ekman transport of Chukchi shelf water and vertical turbulent mixing with underlying nutricline water in 2011. In contrast, northwesterly winds flowing in the northern part of an extended Siberian High transported oligotrophic water within the Beaufort Gyre circulation toward the NAP region in 2012. The modeled ice algal biomass during summer reflected the differences in nutrient distribution. The modeled sinking flux of particulate organic nitrogen (PON) was comparable with the time series obtained from sediment trap data in summer 2011. In contrast, lateral advection of ice algal patches of shelf origin during a great cyclone event may have caused a modeled PON flux bias in 2012. Sensitivity experiments revealed several uncertainties of model configurations of ice algal productivity, particle sinking speed, and grazing activities. Extending the year-long measurements is expected to help illustrate the more general features of ice-related biological processes in the Arctic Ocean.

Wind‐driven upwelling around grounded tabular icebergs

AbstractTemperature and salinity data collected around grounded tabular icebergs in Baffin Bay in 2011, 2012, and 2013 indicate wind‐induced upwelling at certain locations around the icebergs. These data suggest that along one side of the iceberg, wind forcing leads to Ekman transport away from the iceberg, which causes upwelling of the cool saline water from below. The upwelling water mixes with the water above the thermocline, causing the mixed layer to become cooler and more saline. Along the opposite side of the iceberg, the surface Ekman transport moves towards the iceberg, which causes a sharpening of the thermocline as warm fresh water is trapped near the surface. This results in higher mixed layer temperatures and lower mixed layer salinities on this side of the iceberg. Based on these in situ measurements, we hypothesize that the asymmetries in water properties around the iceberg, caused by the opposing effects of upwelling and sharpening of the thermocline, lead to differential deterioration around the iceberg. Analysis of satellite imagery around iceberg PII‐B‐1 reveals differential decay around the iceberg, in agreement with this mechanism.

Mesoscale eddies and Trichodesmium spp. distributions in the southwestern North Atlantic.

Correlations of Trichodesmium colony abundance with the eddy field emerged in two segments of Video Plankton Recorder observations made in the southwestern North Atlantic during fall 2010 and spring 2011. In fall 2010, local maxima in abundance were observed in cyclones. We hypothesized surface Ekman transport convergence as a mechanism for trapping buoyant colonies in cyclones. Idealized models supported the potential of this process to influence the distribution of buoyant colonies over time scales of several months. In spring 2011, the highest vertically integrated colony abundances were observed in anticyclones. These peaks in abundance correlated with anomalously fresh water, suggesting riverine input as a driver of the relationship. These contrasting results in cyclones and anticyclones highlight distinct mechanisms by which mesoscale eddies can influence the abundance and distribution of Trichodesmium populations of the southwestern North Atlantic.

Estimating the Atlantic overturning at 26°N using satellite altimetry and cable measurements

AbstractClimate simulations predict a slowing of the Atlantic meridional overturning circulation (MOC), a key oceanic component of the climate system, while continuous observations of the MOC from boundary arrays demonstrate substantial variability on weekly to interannual time scales. These arrays are necessarily limited to individual latitudes. A potential proxy for the MOC covering longer time scales and larger spatial scales is desirable. Here we use sea surface height data from satellites to estimate the interannual variability of transbasin ocean transports at 26°N. Combining this estimate with surface Ekman transport and cable measurements of the Florida Current, we construct a time series of the MOC from 1993 to 2014. This satellite‐based estimate recovers over 90% of the interannual variability of the MOC measured by the RAPID 26°N array. This analysis complements in situ observational efforts to measure the MOC at multiple latitudes and opens the door to a broader spatial understanding of the Atlantic circulation variability.

Observations of the shelf circulation dynamics along Ningaloo Reef, Western Australia during the austral spring and summer

The dynamics of the three-dimensional circulation on the shelf off Ningaloo Reef in Western Australia were investigated using observations from two field experiments (Nov 2009–Jan 2010 and Sep 2010–Nov 2010). The observations revealed an onshore geostrophic flow of ~0.015ms−1 along the relatively straight Ningaloo Peninsula, coinciding with a substantial poleward increase of the along-shelf flow along this short (∼60km) section of coast. The cross-shelf transport analysis distinguished between two cross-shelf advection scenarios prevalent along this coast: 1) a transient coastal upwelling scenario when the offshore surface Ekman transport was replenished by an onshore transport, supplied mainly from the interior of the water column due to the combined effects of the stratification and steep continental slope; and 2) a downwelling scenario when offshore bottom boundary layer Ekman transport occurred, associated with a deeper pressure-gradient driven flow. A detailed along-shelf momentum balance indicated that there was a southward increase in the along-shelf pressure gradient from the northern limit of the Ningaloo Peninsula (the North West Cape), which explains the increase in the along-shelf flow down the coast. The results indicate that the Ningaloo shelf can be considered as one of the important formation regions of the poleward Leeuwin Current which, while very weak on Australia North West Shelf, is the dominant current feature south of Ningaloo and along the remaining southwestern Australia coast.

Meridional overturning transports at 7.5N and 24.5N in the Atlantic Ocean during 1992–93 and 2010–11

Transatlantic hydrographic sections along latitudes 7.5N and 24.5N have been repeated with about 20years difference, at the beginning of the 1990s and 2010s. For each period, an inverse model is applied to the closed box bound by both sections. The model imposes mass conservation for individual layers, defined by isoneutral surfaces, and the whole water column, using surface Ekman transport and several transport constraints for specific ranges of longitudes and depths. As a result, the velocities at the reference layer for each station pair and the dianeutral velocities between layers are estimated, and the horizontal velocity fields and the water, heat and freshwater transports are calculated; in particular, we find that mass transport per stratum at 24.5N in 2011 is in good agreement with the transport estimates from the RAPID-Watch array. During both realizations the dianeutral velocities downwell from the Upper North Atlantic Deep Water (UNADW) to the Lower North Atlantic Deep Water (LNADW) strata, resulting in the merging of the two southward flowing strata at 24.5N into one deep southward-moving stratum at 7.5N. At 24.5N, there is an increase in southward UNADW transport between 1992 and 2011, compensated by a decrease of southward LNADW transport; a descent in the upper limit of the Antarctic Bottom Water (AABW) from 1992 to 2011 is also inferred. The Atlantic Meridional Overturning Circulation (AMOC) is larger in 1992–93 than in 2010–11, decreasing from 24.7±1.7 to 20.1±1.4Sv at 24.5N and from 29.2±1.7 to 16.9±1.5Sv at 7.5N. Much of this decrease arises because of the northward flow of Antarctic Intermediate Water (AAIW), which was much more intense in 1992–93 than in 2010–11. As a consequence, heat transport at 24.5N is not significantly different in 1992 (1.4±0.1 PW) and 2011 (1.2±0.1 PW). The estimation of heat transport at 7.5N strongly depends on the magnitude of the North Brazil Current over the American continental platform. The freshwater flux into the box bounded by the two transoceanic sections decreases significantly from 1992–93 (−0.45±0.05Sv) to 2010–11 (−0.36±0.3Sv), meaning a decrease in net evaporation over this same area.