Wind forcing controls on Antarctic Bottom Water export from the Weddell Sea via bottom boundary layer processes

Abstract

The Antarctic Bottom Water (AABW) exported from the Weddell Sea has experienced warming and contraction in the past 30 yrs. Superposed on this decadal trend is substantial annual and interannual variability in the volume and properties of Weddell-sourced AABW. Several mechanisms have been suggested to explain these variations, many of which highlight a role of wind stress, but the comparative importance and possible simultaneity of the different mechanisms remains unclear. Using data from two mooring sites within the Weddell Sea, we find a rapid intensification of the abyssal boundary current carrying AABW through Orkney Passage (OP), the most direct export pathway of AABW from the Weddell Sea, in response to periods of strong zonal wind stress and anomalous wind stress curl along the South Scotia Ridge upstream of OP. This acceleration is concomitant with a 40% reduction in northward AABW transport in late 2015. The changes in transport follow anomalous wind forcing by approximately 3 months, with the short timescale indicative of a barotropic response in the flow through OP. The bottom boundary layer over the OP’s sloping topography is found to have a key role in regulating export on monthly to interannual timescales. Increased boundary current velocity leading up to the passage forms a thickened bottom boundary layer, resulting in reduced AABW thickness and density, and thus restricting northward transport of AABW through the passage. Whilst other processes are likely to dominate on longer (decadal) periods, the dynamics identified here can explain significant variability on timescales up to interannual.