Antarctic density stratification and the strength of the circumpolar current during the Last Glacial Maximum

Jean Lynch-Stieglitz,Takamitsu Ito,Elisabeth Michel

doi:10.1002/2015pa002915

Abstract

The interaction between ocean circulation and biological processes in the Southern Ocean is thought to be a major control on atmospheric carbon dioxide content over glacial cycles. A better understanding of stratification and circulation in the Southern Ocean during the Last Glacial Maximum (LGM) provides information that will help us to assess these scenarios. First, we evaluate the link between Southern Ocean stratification and circulation states in a suite of climate model simulations. While simulated Antarctic Circumpolar Current (ACC) transport varies widely (80–350 Sverdrup (Sv)), it co-varies with horizontal and vertical stratification and the formation of the southern deep water. We then test the LGM simulations against available data from paleoceanographic proxies, which can be used to assess the density stratification and ACC transport south of Australia. The paleoceanographic data suggest a moderate increase in the Southern Ocean stratification and the ACC strength during the LGM. Even with the relatively large uncertainty in the proxy-based estimates, extreme scenarios exhibited by some climate models with ACC transports of greater than 250 Sv and highly saline Antarctic Bottom Water are highly unlikely.

Highlights

The Antarctic Circumpolar Current (ACC) carries approximately 150 Sv (1 Sv = 106 m3 sÀ1) of water eastward around the Antarctic, vertically extending from the surface to the bottom of the Southern Ocean
A vertical profile of benthic foraminiferal δ18O from south of Australia (Figure 3) shows that for both recent (Holocene) and Last Glacial Maximum (LGM) sediments, the oxygen isotope ratio increases with depth as seawater potential density increases, with little overall change in the vertical gradient
The denser waters south of the ACC are accompanied by higher δ18O of benthic foraminifera, but the gradient in foraminiferal δ18O across the ACC is slightly greater between 1 and 2.5 km during the LGM than for the recent sediments

Summary

Introduction

The ACC is in thermal wind balance: the buoyancy force acting on the tilted density surfaces across the ACC (Figure 1) is balanced by the Coriolis force acting on the horizontal current. Today, this baroclinic transport (integrated thermal-wind transport referenced to zero bottom velocity) dominates the total ACC transport. The mean flow of the ACC is set not by the strength of the winds over the Southern Ocean, but rather by surface buoyancy fluxes and physical processes that control density stratification in this region [Howard et al, 2015; Mazloff et al, 2013; Munday et al, 2011]

Methods

Results

Conclusion