Subpolar Atlantic Ocean mixed layer heat content variability is increasingly driven by an active ocean

Cold conditions in the upper layer of the subpolar North Atlantic Ocean, at a time of pervasive warming elsewhere, have provoked significant debate. Uncertainty arises both from potential causes (surface heat loss and ocean circulation changes) and characteristic timescales (interannual to multidecadal). Resolution of these uncertainties is important as cold conditions have been linked to recent European weather extremes and a decline in the Atlantic overturning circulation. Using observations, supported by high resolution climate model analysis, we show that a surprisingly active ocean regularly generates both cold and warm interannual anomalies in addition to those generated by surface heat exchange. Furthermore, we identify distinct sea surface temperature patterns that characterise whether the ocean or atmosphere has the strongest influence in a particular year. Applying these new insights to observations, we find an increasing role for the ocean in setting North Atlantic mixed layer heat content variability since 1960.