Coherent Seasonal Acceleration of the Weddell Sea Boundary Current System Driven by Upstream Winds

Abstract

The Weddell Sea is of global importance in the formation of dense bottom waters associated with sea ice formation and ocean-ice sheet interaction occurring on the shelf areas. In this context, the Weddell Sea boundary current system (BCS) presents a major conduit for transporting relatively warm water to the Weddell Sea ice shelves and for exporting some modified form of Wedell Sea deep and bottom waters into the open ocean. This study investigates the downstream evolution of the structure and the seasonality of the BCS along the Weddell Sea continental slope, combining ocean data collected for the past two decades at three study locations. The interannual-mean geostrophic flow, which follows planetary potential vorticity contours, shifts from being surface intensified to bottom intensified along stream. The shift occurs due to the densification of water masses and the decreasing surface stress that occurs westward, toward the Antarctic Peninsula. A coherent along-slope seasonal acceleration of the barotropic flow exists, with maximum speed in austral autumn and minimum speed in austral summer. The barotropic flow significantly contributes to the seasonal variability in bottom velocity along the tip of the Antarctic Peninsula. Our analysis suggests that the winds on the eastern/northeastern side of the gyre determines the seasonal acceleration of the barotropic flow. In turn, they might control the export of Weddell Sea Bottom Water on seasonal time scales. The processes controlling the baroclinic seasonality of the flow need further investigation. Plain Language Summary In the Weddell Sea, large amounts of seawater are cooled to become dense and sink, carrying signals of human-induced changes such as atmospheric carbon into the abyss of the ocean. Understanding the variability of the ocean currents at the Antarctic continental margin is critical because it controls both the export of the dense water formed in these areas and the access of warm water that may melt the Antarctic ice sheet. This study investigates the structure and the seasonality of the flow at the continental margin in the Atlantic sector of the Southern Ocean, using in situ observations upstream and downstream of the dense water formation regions. Following the bathymetry, ocean currents flow from East to West along the continental shelf edge. As water densifies along this path, the flow speed changes from being maximum at the ocean surface to be maximum at the bottom. The depth-averaged current varies with a synchronized seasonality along the continental shelf break, reaching a maximum in austral autumn. Our analysis suggests that the winds on the eastern/northeastern margin drives the seasonality of the depth-averaged flow along the shelf break, significantly contributing to changes in bottom velocity near the export region.

Heat transport across the Antarctic Slope Front controlled by cross-slope salinity gradients

“…S1A) to a bottom-intensified current in the dense-shelf case (fig. S1C), consistent with observations ( 53 ).…”

“…Figure S1 shows the time- and zonal-mean zonal circulation, potential temperature, and salinity in the fresh-shelf, reference, and dense-shelf cases, overlaid by neutral density contours. As reported by previous studies ( 52 , 53 ), the slope current is surface-intensified in the fresh-shelf case (fig. S1A), nearly barotropic in the reference case (fig.…”

Phenology and Environmental Control of Phytoplankton Blooms in the Kong Håkon VII Hav in the Southern Ocean

“…The mechanisms causing vertical mixing and surface nutrient supply are, e.g., ice-shelf meltwater driven circulation, dynamic instabilities and storms, and ocean currents interacting with bottom topography or land masses (e.g., Sokolov and Rintoul, 2007;Nicholson et al, 2016;Dinniman et al, 2020). Important current patterns in the region of interest include the Antarctic Slope Current flowing westward along the continental shelf break in the southern part of the study area (Le Paih et al, 2020) and the eastern inflow of the Weddell Gyre with southward and westward currents in the deep ocean further north in the study area (Vernet et al, 2019). In addition, observations and models reveal hot-spots of enhanced vertical mixing along the Antarctic continental shelf break due to interactions of tides with the sloping topography (Pereira et al, 2002;Fer et al, 2016).…”

The Deep‐Water Plume in the Northwestern Weddell Sea, Antarctica: Mean State, Seasonal Cycle and Interannual Variability Influenced by Climate Modes

“…The fluctuations found at seasonal timescales in WSBW transports may either be caused by changes in the volume of dense precursors exported further upstream, by changes in the strength of the Weddell Gyre imposed by the curl of the winds, or by a combination of both (Fahrbach et al., 2001; Gordon et al., 2010; le Paih et al., 2020). In this regard, the anticorrelation that we observed in November between the change in WSBW area and velocity suggests a seasonal weakening of the wind‐driven component of the WSBW flow (i.e., a weakening of the Weddell Gyre), as this decrease in WSBW velocity would increase its cross‐section area (Fahrbach et al., 2001).…”