Mixing and Water Mass Transformation Over Discovery Bank, in the Weddell-Scotia Confluence of the Southern Ocean

Abstract

The South Scotia Ridge, in the Atlantic sector of the Southern Ocean, is a key region for water mass modification. It is the location of the Weddell-Scotia Confluence, an area of reduced stratification which separates the Weddell Gyre to the south and the Antarctic Circumpolar Current to the north, and which receives input of shelf waters from the tip of the Antarctic Peninsula. To elucidate the transformations over the ridge, we focus on one of its largest seamounts, Discovery Bank, which has previously been observed as hosting a stratified Taylor column that retains water for months to years, during which time water masses are entrained from north and south of the Weddell Front and steadily mixed. Data from ship-deployed sensors and autonomous platforms are analyzed to quantify and understand the diapycnal mixing, heat fluxes and water mass transformations over the bank. Ocean glider and free-profiling drifting float data show that the mid-depth temperature maximum of the Circumpolar Deep Water (CDW) is eroded between the northern and southern sides of the bank, while diapycnal diffusivity is enhanced by up to an order-of-magnitude over its steeply sloping portions. This is accompanied by heat fluxes from the CDW layer being increased by up to a factor of six, which may contribute to a reduction in mid-depth stratification. Tidal model analysis shows that the southern side of the bank hosts strong barotropic to baroclinic energy conversion (>150 N m⁻²), emphasizing the role of internal tides in modulating water mass transformations in the Confluence.