Gyre-specific Ocean Heat Content Changes Controlled by the Meridional Overturning in the North Atlantic

In the North Atlantic, there are pronounced gyre-scale changes in ocean heat content on interannual to decadal time scales. This climate variability is investigated using a semi-diagnostic dynamical analysis of historical temperature and salinity data from 1962 to 2011 together with idealised isopycnic model experiments. On timescales of typically five years, the tendencies in upper ocean heat content are not simply explained by the area-averaged atmospheric forcing for each gyre, but instead dominated by heat convergences associated with the meridional overturning circulation (MOC). In the subtropics, the most pronounced warming events are associated with an increased influx of tropical heat driven by stronger Trade winds. In the subpolar gyre, the warming and cooling events are associated with changes in western boundary density, where increasing boundary density in the Labrador Sea leads to an enhanced overturning and an influx of subtropical heat. The different effects of the meridional overturning are a consequence of how the poleward heat transport is achieved in a different manner over the basin: the heat carried mainly by the MOC over the upper 100m at low latitudes and instead by the MOC from 100m to 1300m at mid and high latitudes, augmented by the gyre transport at high latitudes. In summary, upper ocean heat content anomalies are formed in a different manner in the subtropical and subpolar gyres, with different components of the meridional overturning circulation probably excited by the local imprint of atmospheric forcing.