Water Mass Exchange Between the Weddell Gyre and the Antarctic Circumpolar Current

The Weddell Sea is a primary bottom water formation region, where newly formed bottom water is exported to all the ocean basins. This water mass is partly supplied from the upper layers of the Antarctic Circumpolar Current (ACC), as well as from sources within the Antarctic Slope Current (ASC). In addition to the mean meridional overturning circulation (MOC) in the Weddell Gyre, observations support the idea that the water mass exchange between the Weddell Gyre and the ACC is also due to two other mechanisms, meso-scale eddies and large-scale fluctuations in forcing. The Ocean model and Lagrangian method are used to study the water mass exchange by these mechanisms. For the exchange caused by large-scale fluctuations, only the chaotic transport caused by the seasonal cycle (CTSC) is studied because of computational limitations. By the Lagrangian method, the three different mechanisms for water mass exchange are separately found. Based on the model, considering the Weddell Gyre as a whole system, the water mass exchange by the total velocity field is 10.9 Sv, of which 7.2 Sv is due to the mean velocity field, 1.3 Sv due to CTSC and 2.4 Sv due to eddies. In total, one third of the mass exchange is caused by the time-dependent part of the velocity field. According to the model, the transport of the ASC along 30°E is estimated as 4.2 Sv from the Antarctic coast northward to the 3000 m isobath, but only 1.0 Sv is from outside of the Weddell Gyre east of 94°E. The other 3.2 Sv transport is due to gyre circulation along the eastern boundary crossing 30°E, which then flows back along the Antarctic coast. Additionally, this work also shows the strength of the Lagrangian method in calculating water mass exchange.