Mechanism of observed North Atlantic multidecadal upper ocean heat content changes 1950-2020

We synthesize observational datasets and a state of the art forced global ocean model to construct a multidecadal upper ocean heat budget for the North Atlantic for the period 1950 to 2020. Using multiple independent estimates of the variables allows us to provide robust uncertainty estimates for each term. Time-varying ocean heat transport convergence dominates the budget on multidecadal timescales in all regions of the North Atlantic. In the subpolar region (north of 45N) we find that the heat transport convergence is dominated by geostrophic currents whereas in the subtropics (26-45N) advection by ageostrophic currents is also significant. The geostrophic advection is dominated (especially in the subpolar regions) by anomalous geostrophic currents acting on the mean temperature gradient. The timescale and spatial distribution of the anomalous geostrophic currents are consistent with basin scale ‘thermal’ Rossby waves propagating westwards/northwestwards in the subpolar gyre. Multidecadal changes in North Atlantic Changes in ocean heat storage directly affect the climate of the surrounding continents, and hence it is important to understanding the mechanism behind these.