The Export Variability of Weddell Gyre

The export of Antarctic Bottom Water (AABW) from the Weddell Sea is influenced by the location of its density surface outcropping on the South Scotia Ridge. Changes in the position of this outcropping can modulate the strength of the deepest cell of the global overturning circulation. Here we investigate the response of this export to a variable surface wind forcing, spanning a range of timescales. We construct an idealized, conceptual model for the position of the AABW upper interface using residual-mean theory (Su, Stewart and Thompson, 2014, J. Phys. Oceanogr., 44, 1671–1688) and test the model predictions against idealized eddy-resolving numerical simulations. Both systems highlight the dominant role of mesoscale eddies at the boundary of the Weddell gyre, and their rapid (~1 yr) response to wind-driven changes in the surface Ekman transport. There is an additional adjustment to the stratification that occurs over a ~10 yr timescale due to the slow response of the eddy kinetic energy, a form of eddy saturation. Introducing quasi-realistic bathymetric variations, e.g. a bowl-shaped gyre with a continental slope, produces a qualitatively different gyre circulation, with a strong boundary current at the northern edge of the gyre. This also modulates the stratification of the gyre by suppressing the eddy diffusivity at the boundary, breaking Sverdrup balance in the interior and reducing the influence of Rossby waves on stratifcation changes. Additionally, the bathymetry creates closed contours of planetary/topographic vorticity around the gyre, resulting in a circulation that more closely resembles the Antarctic Circumpolar Current than the classic Stommel–Munk gyre model. Our analytical model accurately predicts the observed phases of temperature fluctuations at the Weddell Gyre boundary associated with the seasonal cycle of surface wind forcing. We discuss the implications of our results for the long-term adjustment of the gyre and the export of AABW to anthropogenically-induced strengthening of the cyclonic winds over the Weddell Gyre.