Reduced Passive Ocean Heat Uptake in an Evolving Ocean

Abstract:

The absorption of heat by the global ocean mitigates anthropogenically-driven surface warming for centuries. Observations and modelling studies suggest that the majority of present-day ocean heat uptake occurs with the Southern Ocean, predominantly through the passive subduction of anomalously warm surface waters. At the same time, significant emphasis has been placed on how changes in the Atlantic Meridional Overturning Circulation, and associated changes in North Atlantic heat uptake, might influence global heat uptake and surface warming over the coming century. Relatively less work regards how circulation changes might modulate heat uptake, and particularly passive heat uptake, within the Southern Ocean, despite its modern preeminence.

Here, we use an idealized framework to quantify how anthropogenically-driven changes to the global ocean state might impact the ocean’s capacity to (passively) absorb heat. To do so, we calculate Green’s Function snapshots of the evolving ocean circulation (including advection and mixing) in a fully-coupled climate model as it adjusts to an RCP 8.5 forcing scenario. Using this framework, we find that the efficiency of global ocean heat uptake ---the global-mean heat uptake per degree surface warming --- declines by approximately 25% between snapshots of the preindustrial ocean and its state in 2100. Reduced Southern Ocean subduction explains over 75% of this global decline; reduced deep water formation in the North Atlantic accounts for only ~10%. These geographically remote differences in the Southern and North Atlantic Oceans may not, however, be dynamically independent. Reduced Southern Ocean heat uptake stems from adjustments in the global thermocline, linked in part to an inter-basin response to overturning changes in the North Atlantic. These results suggest that the ocean’s evolving capacity to mitigate surface warming may depend sensitively on the remote connections between localised air-sea coupling.