Impact of Gyre-Specific Overturning Changes on North Atlantic Heat Content

Abstract:

North Atlantic climate variability on decadal time scales is often characterised by basin-scale changes in sea surface temperature that are generally attributed to coherent changes in the meridional overturning circulation (MOC). However, this view is inconsistent with striking gyre contrasts in ocean heat content over the basin: the subtropics are often warm when the subpolar gyre is cool, and vice versa. Based on dynamical assimilations of historical data over the last 60 years, we explore how these gyre-scale changes in heat content are affected by the meridional overturning. The overturning anomalies often have opposing signs in each gyre and are excited by surface winds and associated air-sea heat fluxes. Stronger Trade winds warm the subtropical gyre through enhanced northward wind-driven heat transport. Simultaneous increases in mid-latitude westerly winds cool the Labrador Sea leading to denser waters spreading along the western boundary, which enhance the MOC at the subtropical/subpolar boundary and drive a warming of the subpolar gyre. Hence, the surface winds and buoyancy forcing imprint themselves on thermal and overturning anomalies in a different manner for each gyre.