The role of ocean circulation in transforming surface forcing into interannual-to-multidecadal oceanic variability is an area of ongoing debate. Here, a novel method, establishing exact causal links, is used to quantitatively determine the role of ocean active and passive processes in transforming stochastic surface forcing into heat content variability. To this end, we use a global ocean model in which the dynamical response to forcing can be switched on (fully active) or off (purely passive) and consider the resulting effect on heat content variance. While the ocean passive processes mainly control the surface variance (over 92%) in all basins, most regions show the importance of active processes at depth. This role is particularly important for full-depth North Atlantic heat content, which we investigate further, highlighting signatures of the meridional overturning circulation in delaying the variance growth.

Plain Language Summary

The ocean’s role in climate is fundamental due to its ability to absorb significant amounts of heat relative to the other components of the Earth system. However, changes in heat can modify the ocean currents which transport it. The importance of this feedback effect remains uncertain, and so our study aims to determine how important this process is. We achieve this by alternately switching on and off the ability of simulated ocean currents to respond to changes in heat and salt driven by the atmosphere in a state-of-the art numerical simulation of the ocean. We then compare how variable the heat content of the ocean is in both “on” and “off” cases. We show that ocean circulation changes are unimportant near the surface, but in most regions they play a key role at depth. We look in detail at the North Atlantic, the region where circulation changes have the most important effect.