Active Ocean Dynamical Adjustment Plays No Role in Southern Ocean Heat Storage
The world’s ocean takes up more than 90% of the excess heat and a third of the additional carbon dioxide generated by burning fossil fuels. How the changes in ocean circulation, especially the overturning component in the Southern Ocean (SO), contribute to ocean heat uptake and storage is still unclear. Using an innovative, partial coupling technique, a group of climate researchers strive to isolate the ocean circulation change, or active ocean dynamical component, from the passive component in the total ocean heat uptake and associated redistribution of ocean heat storage.
Estimates show that the SO, south of 30°S, stores about 75% of the global ocean heat uptake. However, among those, there is almost no contribution from the ocean's dynamical adjustment to the forcing of increasing carbon dioxide concentrations. The ocean dynamically induced heat convergence/divergence is nearly perfectly balanced by the surface fluxes. The perfect balance epitomizes the “redistributive” effect of the ocean dynamical feedback dubbed in the climate change research community, where the ocean circulation change acts only to redistribute the heat in the ocean. If this result holds for the real climate, it simplifies the problem of ocean heat uptake and allows researchers to focus on passive ocean heat uptake processes.
In response to a fourfold increase in carbon dioxide, the SO absorbs excess heat around 60°S and releases some back to the atmosphere at around 45°S. The northward ocean heat transport (OHT) plays a major role in this redistribution. However, the relative roles of the mean ocean circulation and ocean circulation changes in the uptake and redistribution of heat in the SO remain controversial. Using a novel partial coupling technique, researchers are able to separate the roles of mean ocean circulation (passive component) and ocean circulation change (active component). The mean ocean circulation and ocean circulation change are of equal importance for the ocean heat uptake: the divergence of the passive OHT acts to balance the passive surface heat gain to the south of ~50°S, while the convergence of the active OHT acts to balance the active surface heat loss to the north of ~50°S. Interestingly, the active OHT plays a purely redistributive role, resulting in no net change in ocean heat storage. Both the active and passive ocean heat transports in the SO are overcompensated by the reverse atmospheric heat transport via the Bjerknes compensation mechanism.