High-latitude obliquity as a dominant forcing in the Agulhas current system

T Caley,B Malaizé,I S Castañeda,S Schouten,K Charlier,N Caillon,T Laepple,J S Sinninghe Damsté,L Rossignol,H Rebaubier,J.-H Kim,J Giraudeau

doi:10.5194/cp-7-1285-2011

Abstract

Abstract. The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), can affect the Atlantic meridional overturning circulation (AMOC) and, thus, influence global climate. However, efforts to elucidate forcing mechanisms connecting the Agulhas leakage with the upstream dynamics of the current have been hampered by a lack of climate records extracted from the area where the Agulhas current originates. We determine 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the Agulhas current and show that these records contain strong 100-kyr and 41-kyr cycles. This latter obliquity-driven cycle is nearly in phase with changes in the annual mean insolation and air temperature at high southern latitudes. In contrast, our SST and SSS records did not reveal precession-driven cycles, which is surprising given the low-latitude location of the upstream Agulhas current. Together, this indicates that the dynamics of the Agulhas current system is mainly controlled by high latitude obliquity through its influence on the position of the Southern Hemisphere subtropical front (STF) and its associated westerlies. Our study demonstrates that obliquity may drive an important part of the 100 kyr cycles observed in the system rather than precession. Our results also suggest that a stronger Agulhas current, associated with a northward shift of the wind system during glacial periods, leads to reduced leakage, in accordance with the theory. We argue that during terminations, stronger Agulhas leakage of heat and salt was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term AMOC variations.

Highlights

The inter-ocean exchange of heat and salt (Agulhas leakage) from the Agulhas current is a key component of the global ocean “conveyor” circulation (Weijer et al, 2002; Gordon, 2003; Lutjeharms, 2006; Beal et al, 2011)
We argue that during terminations, stronger Agulhas leakage of heat and salt was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term Atlantic meridional overturning circulation (AMOC) variations
We propose that the Agulhas current system is mainly controlled by high latitude obliquity at orbital scale changes

Summary

Introduction

The inter-ocean exchange of heat and salt (Agulhas leakage) from the Agulhas current is a key component of the global ocean “conveyor” circulation (Weijer et al, 2002; Gordon, 2003; Lutjeharms, 2006; Beal et al, 2011). Modelling studies show that the Agulhas leakage alters the Atlantic Meridional Overturning Circulation (AMOC) on different time scales. Over a period of several hundred years, changes in the buoyancy of Atlantic thermocline waters can influence North Atlantic deep-water formation (Weijer et al, 2001, 2002), with implications for global climate. These later simulations suggest that a saltier Atlantic ocean, linked to a more efficient leakage of warm and saline Agulhas waters, leads to a stronger and more stable AMOC. Accurate knowledge of the mechanisms governing the Agulhas current system under different climatic conditions is essential for properly constraining the long-term AMOC response to the Agulhas leakage

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Conclusion