Dynamik des Wedell Meer Küstenströmungen System auf saisonalen Zeitskalen

The boundary current system in the Weddell Sea is composed of surface and gravity currents, which flow along the Antarctic margin’s, south of the Atlantic basin. These currents are central for the meridional transfer of heat and bio-geochemical properties between the Weddell Sea and the world ocean. They contribute to the transport of relatively warm water towards the Weddell Sea continental shelves, a major site where cold and dense shelf water forms, and the export of dense shelf water towards lower latitudes, feeding the lower branch of the meridional overturning circulation. Yet, the link between warm water inflow and export of dense shelf water still needs to be made on seasonal-time scales. The goal of my PhD is to provide the first coherent description of the seasonal evolution of the boundary current system along the continental slope. Combining oceanographic data, I find a synchronised seasonality of the barotropic flow along the Weddell Sea’s continental slope. The seasonal acceleration of the barotropic flow significantly contributes to the transport of dense shelf waters and correlates with the surface stress in the eastern side of the Weddell Sea. This finding suggests that the winds in the eastern Weddell Sea remotely contribute to the transport of dense shelf waters in the western Weddell Sea. How- ever, the mechanisms controlling the relationship between the surface stress and the barotropic flow remains unclear. Even though oceanographic data are generally insufficient to quantitatively compare the baroclinic variability along the continental slope, I observe a weakening of the baroclinic seasonality between the eastern and the western part of the continental slope. A conceptual model developed for this study supports an along-slope dampening of the baroclinic signals associated with (1) the presence of a density gradient between the dense shelf water on the south- and western continental shelves and the water masses on the continental slope and, (2) the widening of the continental slope between the eastern and southern Weddell Sea. The former implies the formation of eddies, which diffuses the seasonality along the continental shelf edge. The latter implies an along-slope decrease in flow strength, limiting the downstream advection of density anomalies from the eastern Weddell Sea. In the end, my analysis suggests that the region in front of the Ronne/Larsen Ice shelves in the southern/western Weddell Sea is less sensitive to the downstream advection of seasonal anomalies from the eastern Weddell Sea than the region in front of the Filchner Ice shelf in the southeastern Weddell Sea. However, data overlapping in time need to be collected in front of the eastern and the south-western continental shelves to quantify the downstream propagation of the baroclinic signals and corroborate this result

Submesoscale Instability in the Straits of Florida

The Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high-resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected, relatively unaffected by the current, when in the southern part of the straits. In the absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom-layer vorticity flux as previously demonstrated by Gula et al. in the GS. When the FC retreated farther south, negative-vorticity west Florida shelf waters overflowed into the SOF and led to the formation of submesoscale eddies by baroclinic instability. The instability regimes, that is, the submesoscale frontal eddies formation, appear to be controlled by the lateral "sloshing" of the FC in the SoF

Submesoscale Instability in the Straits of Florida

International audienceThe Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high-resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected, relatively unaffected by the current, when in the southern part of the straits. In the absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom-layer vorticity flux as previously demonstrated by Gula et al. in the GS. When the FC retreated farther south, negative-vorticity west Florida shelf waters overflowed into the SOF and led to the formation of submesoscale eddies by baroclinic instability. The instability regimes, that is, the submesoscale frontal eddies formation, appear to be controlled by the lateral "sloshing" of the FC in the SoF

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

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