Wind Forcing Controls on Antarctic Bottom Water Export from the Weddell Sea Via Bottom Boundary Layer Processes

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. Antarctic Bottom Water (AABW) is the coldest, densest water mass in the World Ocean, and plays a pivotal role in controlling the ocean's ability to store and capture heat and carbon from the atmosphere. The volume of AABW has both warmed and shrunk over the past decades, with significant variability on shorter timescales from a few months to several years. Different mechanisms have been suggestively linked to these changes; however, their relative importance remains unclear. Here, we use observational data from two moorings along the flow path of AABW from the Weddell Sea to the South Atlantic, to investigate changes in AABW properties and volume transport. Comparing temperatures at each mooring, we find a rapid acceleration of this current is driven by stronger winds across the region. This acceleration causes a warming and reduction in transport of AABW as lighter warmer waters are pushed deeper along the seabed creating a thick turbulent and unstable layer. On longer timescales, the relationship between winds and variations in AABW transport cannot be unambiguously attributed to this mechanism, as other processes could have comparable or greater impact on the transport of AABW, however the process identified here has strong relevance on shorter timescales. Increased zonal wind stress and wind stress curl along steep bathymetry leads to a reduction in interannual northward AABW transport Speed of abyssal boundary current transporting AABW responds rapidly to changes in wind forcing via barotropic dynamics Barotropic acceleration of boundary current induces strengthening of downslope bottom Ekman flow, which reduces AABW thickness and transport