The ocean’s global overturning circulation and climate: from observations to process understanding

The ocean’s prominent partnership in climate has been evident for more than a century. Its sea surface temperature (SST) is its principal physical component that affects the atmosphere. Processes setting the observed SST distribution are myriad, and include not just air-sea heat exchange but also the ocean’s lateral and vertical circulation, turbulence distribution, and stratification. And also for more than a century, it has been understood that the ocean has a global scale overturning circulation, in which dense cold waters sink to depth at higher latitudes and rise to the surface wherever they can be forced to do so. This circulation exerts strong controls on ocean stratification, and hence affects large-scale SST variations. This in turn affects where the ocean is best poised to absorb and store excess (anthropogenic) heat.

Here I summarize the global overturning circulation, the underlying global-scale pressure gradients that drive it, and its external forcing, which is a combination of buoyancy (heat and freshwater), wind, and turbulence. Critical asymmetries arising from geography exist between each of the major ocean basins, and between the northern and southern hemispheres. In the Southern Ocean, because of the open ocean that encircles Antarctica, basic dynamics dictates no flow of surface warm waters to the continent, and instead only Deep Waters from all three northern oceans are pulled southward, and then rise to the sea surface. This rise occurs in a southward and upwards spiral, with turbulence at mid-ocean ridge eddy hotspots mitigating its upward progression.