Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) and Antarctic shelf properties are investigated within a realistic bathymetry coupled ocean-ice sector model of the Atlantic Ocean. In this model, AABW formation is dominated by dense shelf water, with only a small contribution from open-ocean convection. The inclusion of shelf-sourced AABW (absent from most current generation climate models that rely on open-ocean convection) leads to cooling and freshening of dense shelf waters. Such responses are facilitated via changes in the on-shelf/off-shelf exchange flow under increased surface buoyancy fluxes and hence provide a mechanism explaining recent observational trends of Antarctic dense shelf waters.

The overflow of newly formed AABW propagates the shelf cooling response to the abyssal ocean, with the dominant cooling occurring under increased freshwater fluxes from Antarctic runoff. With increased surface heat fluxes however, the abyssal cooling reverses after a few decades, consistent with increased penetration of warming deep waters that entrain into the overflowing shelf waters. Hence, a decadal scale variability of the abyssal ocean is revealed in this case. Without inclusion of shelf-sourced AABW, climate models will not be able to reproduce the shelf exchange responses driven by surface buoyancy flux changes, and hence will not capture the complete spectrum of climate change responses attainable.

This study emphasizes the importance of inclusion of shelf-sourced AABW, as well as the need to attain increased understanding of the drivers of the cross-shelf exchange processes and the role it plays in the global climate.