The ocean's Meridional Overturning Circulation (MOC) transports large quantities of heat from the tropics toward the poles. How the MOC's heat transport may change in the future remains a first order question in climate science. However, our ability to address this question through future projections depends on how well small-scale diabatic processes such as mixing are represented in models.

The ocean is warmed in the tropics where the sea surface temperature (SST) is warm, and it is cooled poleward of 25° latitude where SSTs are cool. Therefore, the ocean's meridional heat transport is closely linked with heat transport in temperature space. Here, we introduce a closed budget for heat transport in the temperature-latitude plane, allowing us to isolate the dependence of the ocean's meridional heat transport on individual diabatic processes such as surface forcing and mixing. We demonstrate the utility of this framework using a 1⁄4° global ocean sea ice model. Mixing within the tropics and subtropics is found to be critical in allowing the MOC to span a large temperature range and therefore to transport more heat poleward. In particular, mixing allows the heat that enters the ocean in the tropics to penetrate to the cooler layers that outcrop at high-latitudes. These results have implications for the role of small-scale processes for ocean heat transport and the variability of ocean heat uptake over interannual and decadal time-scales.