Indo-Pacific Control on Ocean Overturning Across Climates

Deep water formation in polar regions is traditionally assumed to determine the strength and structure of the deep overturning circulation. Here, we argue that surface heating in the tropical Indo-Pacific Ocean has a leading order impact on the global circulation of sub-thermocline waters. A suite of coupled climate simulations, equilibrated to a wide range of CO₂ levels, shows that as global mean temperature increases, the net surface uptake of heat into (and the export from) the Indo-Pacific basin and the sum of global overturning both increase monotonically. However, the strengthening of global overturning in progressively warmer climates results from the complex evolution of the AMOC and abyssal overturning branches, individually. Similarly, heat transport to high latitudes is hemispherically asymmetric with respect to a warming climate: Southern Ocean warming is delayed compared to the North Atlantic. The phasing of high latitude changes is explained using scaling arguments that highlight mechanistic differences between North Atlantic (advective) and Southern Ocean (eddy diffusive) heat transport. These results emphasize the climatic importance of low latitude heat uptake and export pathways across a wide range of climate states.