How ocean circulation affects the pattern of warming in the North Atlantic Ocean

What is it about?

There has been a pronounced warming of the North Atlantic Ocean over the last 60 years. The warming pattern varies on a decadal timescale, often warming occurring in the subtropics coinciding with cooling occurring in the subpolar latitudes or vice versa. In this study, we explore whether these decadal changes in ocean heat storage are controlled by changes in how the ocean redistributes heat, rather than by the direct effect of air–sea heat fluxes. We reconstruct the changes in ocean heat content from 1965 to 2000. We find that the subtropical changes in heat content are mainly controlled by a wind-induced redistribution of heat, while the subpolar changes in heat content are mainly controlled by the heat redistribution involving the ocean overturning. In reaching this view, the heat transport is separated into contributions from a wind-driven vertical cell, an overturning vertical cell and a horizontal cell. Over the North Atlantic, the heat is carried northward like a baton being passed in a relay race: the wind-induced transfer dominates in low latitudes near the equator, the overturning dominates at mid and high latitudes, and the horizontal cell augments the overturning contribution at high latitudes.

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Why is it important?

It is important to understand the reason for different ocean warming patterns. While the global ocean is warming from the increased radiative forcing from the increase in atmospheric CO2, there are regional patterns of warming that can reverse in sign and are a much larger magnitude than the global-mean response. The North Atlantic Ocean is a particular region of rapid heat gain, but the response is not uniform with latitude. Instead we find that there are different regional warming responses, connected to the direct effect of the winds in redistributing heat and the effect of changes in the ocean overturning.

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