Abstract
The thermal bipolar ocean seesaw hypothesis was advanced by Stocker and Johnsen (2003) as the ‘simplest possible thermodynamic model’ to explain the time relationship between Dansgaard–Oeschger (DO) and Antarctic Isotope Maxima (AIM) events. In this review we combine palaeoclimate observations, theory and general circulation model experiments to advance from the conceptual model toward a process understanding of interhemispheric coupling and the forcing of AIM events. We present four main results: (1) Changes in Atlantic heat transport invoked by the thermal seesaw are partially compensated by opposing changes in heat transport by the global atmosphere and Pacific Ocean. This compensation is an integral part of interhemispheric coupling, with a major influence on the global pattern of climate anomalies. (2) We support the role of a heat reservoir in interhemispheric coupling but argue that its location is the global interior ocean to the north of the Antarctic Circumpolar Current (ACC), not the commonly assumed Southern Ocean. (3) Energy budget analysis indicates that the process driving Antarctic warming during AIM events is an increase in poleward atmospheric heat and moisture transport following sea ice retreat and surface warming over the Southern Ocean. (4) The Antarctic sea ice retreat is itself driven by eddy-heat fluxes across the ACC, amplified by sea-ice–albedo feedbacks. The lag of Antarctic warming after AMOC collapse reflects the time required for heat to accumulate in the ocean interior north of the ACC (predominantly the upper 1500 m), before it can be mixed across this dynamic barrier by eddies.
Highlights
Stocker and Johnsen (2003) provide the thermodynamic basis for the hypothesis with their suggestion that the temperature anomalies in Greenland and Antarctica during these events could most be explained by changes in the rate of cross-equatorial ocean heat transport in the Atlantic, that are modulated at southern high latitudes by a large heat reservoir
To examine the Wunsch (2006) critique of the thermal seesaw model (Section 1.2) we consider how the meridional ocean and atmospheric heat fluxes adjust during the Antarctic Meridional Overturing Circulation (AMOC) perturbations
The meridional heat transport in the Indian and Pacific oceans adjusts in response to the AMOC perturbations
Summary
The thermal bipolar ocean seesaw hypothesis is the prevailing explanation for the coupling of DansgaardeOeschger (DO) and Antarctic Isotope Maxima (AIM) events. Stocker and Johnsen (2003) provide the thermodynamic basis for the hypothesis with their suggestion that the temperature anomalies in Greenland and Antarctica during these events could most be explained by changes in the rate of cross-equatorial ocean heat transport in the Atlantic, that are modulated at southern high latitudes by a large heat reservoir (commonly assumed to be the Southern Ocean). The thermal bipolar ocean seesaw hypothesis is the prevailing explanation for the coupling of DansgaardeOeschger (DO) and Antarctic Isotope Maxima (AIM) events. While the simplicity of the thermal seesaw hypothesis is attractive, the absence of details on the actual physical processes that connect north and south limits its application to the coupled climate system (Wunsch, 2006; Seager and Battisti, 2007; Clement and Peterson, 2008). The paper is structured as follows: The remainder of Section 1 outlines the development of the thermal seesaw hypothesis and several of its limitations; Section 2 introduces two transient global climate model (GCM) experiments that we use, along with palaeoclimate data, to explore these limitations; Section 3 presents our results on ocean, atmospheric and radiative processes responsible
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