Abstract

Processes on the Antarctic continental shelf control the exchange of water masses across the shelf break, the delivery of heat fluxes to adjacent ice-shelves, and the formation of Antarctic Bottom Water (AABW). Antarctic Bottom Water controls the abyssal limb of the global overturning circulation, and plays an important role in the ocean’s heat and carbon uptake. Understanding how changing surface buoyancy fluxes influence AABW and Antarctic shelf processes is thus important in defining future climate change.

Using a realistic bathymetry coupled ocean-ice sector model of the Atlantic Ocean, the sensitivity of AABW to changing surface buoyancy fluxes emulating a changing climate is assessed. The sector model, unlike most climate model, maintains Dense Shelf Water (DSW) sourced AABW as the dominant mode of AABW formation, and thus provides a tool to asses feedback’s and variability climate models may be lacking. Inclusion of shelf-scoured AABW leads to cooling and freshening of DSW under enhanced surface buoyancy fluxes. The cooling is facilitated via changes in the on-shelf/off-shelf exchange and highlights the importance of surface buoyancy fluxes in the exchange process. Such cooling, in concert with open-ocean warming, leads to a decadal scale variability of the abyssal ocean.

The role of buoyancy in controlling the cross shelf exchange and AABW formation is further revealed by analysing unique long-term time series of hydrographic and mooring observation on the Adelie Land continental shelf, an important AABW formation region. At seasonal time scales, changes in surface buoyancy fluxes maintain significant control on the strength of the cross-shelf exchange. With heightened buoyancy losses in winter, the exchange of DSW and off-shelf waters is an order of magnitude larger than that in summer, setting up a cyclonic circulation on the shelf. At longer time scales, the calving of the Mertz Glacier Tongue (MGT) and its influence on the freshwater budget in the Adelie region, provides a unique window into potential influences of future climate change on AABW formation. Following the MGT calving, DSW formation dramatically decreased, with the DSW available for export as AABW decreasing by 100% in 2011. The reduced DSW export significantly hindered AABW production and suggests that continued changes in surface buoyancy fluxes and the observed freshening around Antarctic may drive dramatic changes in the strength and structure of the abyssal limb of the meridional overturning circulation in future decades.

In order to understand the role Antarctica and AABW will play in a changing climate, continued long-term monitoring on the Antarctic region and abyssal ocean is essential, as is continued development to improve climate model capability to represent AABW formation processes.