The thermal bipolar ocean seesaw hypothesis was proposed 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. Following the Bergen ice2ice MIS3/Southern Ocean and Bipolar Seesaw Workshop a group of ice2ice researchers and some international collaborators were invited by Quaternary Science Reviews to write a review on the thermal seesaw. The resulting paper, “Beyond the bipolar seesaw: towards a process-understanding of inter-hemispheric coupling” by J. Pedro, M. Jochum, C. Buizert, F. He, S. Barker and S. Rasmussen, is now live and open access at QSR.
In the review we use theory, coupled model simulations and paleoclimate data to test the thermal seesaw. Our results lead to four main challenges to the hypothesis:
- Compensating heat transports: Changes in Atlantic heat transport invoked by the thermal seesaw are compensated by opposing changes in heat transport by the global atmosphere and the Pacific ocean. This compensation (which would come as no surprise to Vilhelm Bjerknes) has a decisive influence on the spatial pattern of climate anomalies. Changed ocean heat transport strongly affects sea ice, triggering ice-albedo feedbacks that dominate the high-latitude climate response. Changed atmospheric heat transport alters the position and intensity of the Hadley circulation and of atmospheric jets and eddies, dominating the response at low to mid-latitudes.
- A global ocean—not Southern Ocean—heat reservoir: According to the thermal seesaw hypothesis, the Southern Ocean acts as a heat reservoir during the DO/AIM events, integrating changes in South Atlantic temperature. We dispute this because South Atlantic temperature anomalies spread much more efficiently into the Indian and Pacific Ocean, by Kelvin and Rossby wave processes and by advection, than they do into the Southern Ocean. This is because the Southern Ocean has no zonal boundary to support waves and because the Antarctic Circumpolar Current (ACC) prohibits advection, since by definition the flow is along the current’s axis. In coupled model simulations the heat content of the Southern Ocean shows very little change during the DO/AIM events; instead, the global ocean north of the ACC better fits the description of heat reservoir.
- Wind-driven thermocline deepening: In the conventional thermal seesaw, a collapse of the AMOC causes heat that would otherwise be advected north to accumulate in the South Atlantic. In our coupled model experiments, a southward shift of the ITCZ accompanies northern high latitude cooling (as commonly seen in previous studies). The ITCZ shift and associated changes in wind stress over the South Atlantic gyre deepen the thermocline there, such that abrupt South Atlantic warming results from reduced northward advection coupled to the wind-driven deepening of the thermocline. The thermocline deepening is essential in enabling heat to be stored at depth rather than released to the atmosphere through the Ekman layer.
- Atmospheric heat transport warms Antarctica: Antarctica warms during AIM events due to an increase in poleward atmospheric heat and moisture transport following sea ice retreat and surface warming over the Southern Ocean. The sea-ice retreat is itself driven by gradual eddy-heat fluxes across the ACC that initiate sea ice melt and are amplified by sea-ice albedo feedbacks. The centennial lag of Antarctic warming after AMOC collapse (WAIS Divide Project Members, 2015) reflects the time required for heat to accumulate in the global ocean interior before it can be mixed across the ACC by eddies and trigger the sea-ice feedback. For the nitty gritty, including our full assessment of the Antarctic and Southern Ocean energy balance during AIM events, please do refer to the paper.
In the spirit of ice2ice the review was a collaboration between ice-core (Buizert, Rasmussen, Pedro), sediment core (Barker) and modelling (Jochum, He) specialists. I’m grateful to all these coauthors for their excellent (and diverse!) inputs and to ice2ice for the support throughout, including for the MIS3/Southern Ocean and Bipolar Seesaw Workshop that kicked this effort off.
Hobart, 26 June 2018
Stocker, T.F., Johnsen, S.J., 2003. A minimum thermodynamic model for the bipolar seesaw. Paleoceanography 18, 11-1.
Pedro, J. B., Jochum, M., Buizert, C., He, F., Barker, S., Rasmussen S.O., Beyond the bipolar seesaw: Toward a process understanding of inter-hemispheric coupling. Quaternary Science Reviews (in press), https://doi.org/10.1016/j.quascirev.2018.05.005, 2018.
WAIS Divide Project Members, 2015. Precise interpolar phasing of abrupt climate change during the last ice age. Nature 520, 661-665.