Dynamics of the Weddell Sea Boundary Current System (BCS) on seasonal time-scales

Abstract

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.