In this study we investigate the interannual variability of the heat content of the upper North Atlantic and the main factors, which influence the observed variability: the ocean-atmosphere heat exchange and the ocean heat transport. The heat content and the ocean heat transport from the dataset ARMOR-3D, and the ocean reanalyzes ORAS5 and SODA3 (two versions) are cross-validated. All data sets show a similar decadal interannual variability of the heat content, as well as of oceanic heat transports the study regions, though the mean values may differ. The observed variations are linked to the North Atlantic Oscillation Index (NAOI).

The current velocity of the North Atlantic, East Greenland and Labrador currents of the Subpolar Gyre increases with the NAOI, but the velocity of the Irminger and West Greenland currents decreases. This forms a seesaw of heat advection by the North Atlantic Current between the Subpolar Gyre and Nordic Seas. In the Subpolar gyre during the periods of high NAOI, this negative anomaly of the ocean heat convergence adds to the intensified sea-surface heat release to the atmosphere, together effectively reducing the upper ocean heat content. The upper ocean heat content of the Norwegian Sea shows practically no link to the NAOI, in spite of a larger oceanic heat flux across its southern boundary during high NAOI.