M5 Heat and Salt Transport

Topic

The meridional transport of heat and salt by the ocean is a fundamental element of the climate system: of the total heat that must be transported from the equator toward the poles to maintain Earth's energy balance, the ocean accounts for approximately half, while the atmosphere transports the remaining half.

In the North Atlantic, this poleward oceanic heat transport reaches a magnitude of nearly 1.3 petawatts (1,300 trillion watts) at 26° North latitude, driven primarily by the aforementioned Atlantic Meridional Overturning Circulation; it is precisely this heat—released back into the atmosphere in the subpolar Atlantic and the Arctic—that keeps Europe between five and ten degrees Celsius warmer than other continents at the same latitude.

This heat transport can be broken down into three distinct mechanisms: transport by the mean circulation, which accounts for the majority of transport in the Atlantic and is directly linked to the meridional overturning circulation; transport by large wind-driven gyres, which feature a significant zonal component in the tropics; and transport by mesoscale eddies, which dominate both the Antarctic Circumpolar Current and the rest of the Southern Ocean.

It is precisely within the Antarctic Circumpolar Current—where closed-gyre circulation is absent and the Atlantic Meridional Overturning Circulation does not penetrate—that these mesoscale eddies become the primary mechanism for transporting heat and other water properties across the region. The expected behavior of this entire system includes a net poleward heat transport in both hemispheres—with a minimum right at the equator—and a marked asymmetry between the North and South: the Atlantic transports heat northward across both hemispheres, unlike the other oceans. Furthermore, a trend toward the weakening of this meridional heat transport is observed under climate change—particularly in the North Atlantic—linked to the weakening of the meridional overturning circulation itself, as described earlier in this document.

Thus, from the half of the planet's heat that the ocean transports toward the poles to the weakening of that very transport under climate change—and encompassing the mean circulation, wind-driven gyres, and mesoscale eddies that share the task depending on the region—the meridional transport of heat and salt enables the ocean to function as a second climate engine, as vital as the atmosphere itself, in distributing energy across the planet.

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