The role of the Atlantic freshwater balance in the hysteresis of the meridional overturning circulation

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Article  

  Climate Dynamics
Publisher: Springer-Verlag Heidelberg
ISSN: 0930-7575 (Paper) 1432-0894 (Online)
DOI: 10.1007/s00382-003-0359-8
Issue: Volume 21, Numbers 7-8

Date:  December 2003
Pages: 707 - 717  
The role of the Atlantic freshwater balance in the hysteresis of the meridional overturning circulation
J. M. Gregory1, 2 , O. A. Saenko3 and A. J. Weaver3

(1)  Hadley Centre Meteorological Office, Bracknell, UK,  
(2)  Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK,  
(3)  School of Earth and Ocean Sciences, University of Victoria, Canada,  

Received: 31 March 2003  Accepted: 22 August 2003  Published online: 4 November 2003

Abstract   We have studied the response of the Atlantic meridional overturning circulation to surface freshwater forcing using an ocean GCM coupled to an energy-moisture-balance atmosphere model. The overturning collapses rapidly when a slowly increasing forcing applied to the North Atlantic passes a positive threshold, and spins up equally quickly when the forcing falls below a negative threshold. This well-known behaviour is referred to as hysteresis because the thresholds in forcing are different for the transitions in opposite directions. However, we argue that the behaviour of the Atlantic salinity is more fundamental than the forcing. Hysteresis as a function of freshwater forcing occurs because the states with North Atlantic overturning on and off each tend to reinforce their associated salinity distributions and inhibit the transition to the other state. During the collapse, the Atlantic becomes less saline because of the import of 80 Sv year of freshwater by ocean transports across 30°S; during the spin-up this freshwater is exported again. We show that qualitatively similar hysteresis behaviour can be produced by perturbing the system without any net freshwater forcing. The salinity flip-flop is associated with the appearance and disappearance of a shallow reverse overturning circulation south of the Equator, which is present while the northern overturning is absent, and may provide the mechanism for the ocean freshwater influx during collapse.