Direct Effects of Radiative Forcing on Ocean Heat Uptake

The ocean plays a critical role in determining the transient global warming from increased greenhouse gases by taking up heat. A growing body of literature makes use of low-order models that capture the behavior of coupled climate models by assuming that the global-mean ocean heat uptake is a temperature-dependent process (that is, it is proportional to surface temperature anomalies). However, there are also "direct", temperature-independent changes from increased greenhouse gases in the atmospheric circulation and hydrological cycle that have the potential to alter the ocean circulation and heat content.

To evaluate how these responses affect ocean heat uptake, we use the output of the comprehensive climate model simulations to force a comprehensively configured ocean general circulation model (OGCM). Perturbation experiments apply a change in wind stress, freshwater fluxes, and downwelling shortwave radiation to this forcing according to the fields' direct response to quadrupling atmospheric CO2. The results of the perturbation OGCM simulations show that the direct changes in surface winds, which are a significant fraction of the total changes in Southern Ocean westerlies, alters the overturning circulation in both the Pacific and Atlantic Oceans. This in turn, has a cooling effect on the ocean at shallow depths. In contrast, the changes in freshwater fluxes and shortwave radiation have a modest influence on the global circulation with a correspondingly small effect on the global surface energy balance. Guided by the OGCM simulations, we formulate a revised two-box (surface and deep ocean) energy balance model to account for the ocean heat content's direct response to surface wind changes.