A salty deep ocean as a prerequisite for glacial termination

Gregor Knorr,Xu Zhang,Paul Gierz,Xun Gong,Christian Stepanek,Lennert B Stap,Gerrit Lohmann,Stephen Barker

doi:10.1038/s41561-021-00857-3

Abstract

Deglacial transitions of the middle to late Pleistocene (terminations) are linked to gradual changes in insolation accompanied by abrupt shifts in ocean circulation. However, the reason these deglacial abrupt events are so special compared with their sub-glacial-maximum analogues, in particular with respect to the exaggerated warming observed across Antarctica, remains unclear. Here we show that an increase in the relative importance of salt versus temperature stratification in the glacial deep South Atlantic decreases the potential cooling effect of waters that may be upwelled in response to abrupt perturbations in ocean circulation, as compared with sub-glacial-maximum conditions. Using a comprehensive coupled atmosphere–ocean general circulation model, we then demonstrate that an increase in deep-ocean salinity stratification stabilizes relatively warm waters in the glacial deep ocean, which amplifies the high southern latitude surface ocean temperature response to an abrupt weakening of the Atlantic meridional overturning circulation during deglaciation. The mechanism can produce a doubling in the net rate of warming across Antarctica on a multicentennial timescale when starting from full glacial conditions (as compared with interglacial or subglacial conditions) and therefore helps to explain the large magnitude and rapidity of glacial terminations during the late Quaternary.

Highlights

Amultitude of ice-core and marine records show that the past 800,000 years of Earth’s history were dominated by global-scale alternations between glacial and interglacial conditions with a high degree of nonlinearity in the climatic response to gradual external forcing[1,2]
A possible mechanism to reach such a point is an abrupt weakening of the Atlantic meridional overturning circulation (AMOC) and a corresponding reduction of northward oceanic heat transport associated with Southern Ocean (SO) and Antarctic warming, according to the bipolar-seesaw concept[8,9,10], which has been detected in a wide spectrum of models[11,12]
In a hypothetical scenario with no change to the salinity distribution, we can see that such heterogeneous cooling of the glacial ocean could lead to an unstable water column (Extended Data Fig. 1), and yet proxy reconstructions suggest that the glacial version of AABW maintained its position as the deepest water mass at least in the South Atlantic region[24,25]

Summary

Introduction

Amultitude of ice-core and marine records show that the past 800,000 years of Earth’s history were dominated by global-scale alternations between glacial and interglacial conditions with a high degree of nonlinearity in the climatic response to gradual external forcing[1,2]. In stark contrast to the changes at intermediate depths, the weakest temperature and strongest salinity differences between the pre-industrial and glacial states are detected in the deep SO (Fig. 2c,d).

Results

Conclusion