Examining the Origins of Ocean Heat Content Variability in the Eastern North Atlantic Subpolar Gyre

We analyze sources of ocean heat content (OHC) variability in the eastern North Atlantic subpolar gyre from both Eulerian and Lagrangian perspectives within two ocean simulations from 1990 to 2015. Heat budgets reveal that while the OHC seasonal cycle is driven by air-sea fluxes, interannual OHC variability is driven by both air-sea fluxes and the divergence of ocean heat transport, the latter of which is dominated by the oceanic flux through the southern face of the study area. Lagrangian trajectories initialized along the southern face and run backward in time indicate that interannual variability in the subtropical-origin volume flux (i.e., the upper limb of the overturning circulation) drives variability in the temperature flux through the southern face. As such, the heat carried by the imported subtropical waters is an important component of the eastern subpolar gyre heat budget on interannual time scales. Plain Language Summary The waters off northern Europe, or the eastern portion of the subpolar North Atlantic, have two characteristics that are of interest: (1) they affect the climate of northern Europe and (2) they are potentially predictable on monthly to decadal time scales. In this paper, we analyze the temperature variability of the eastern subpolar North Atlantic by bounding the region by four faces, three in the ocean plus the surface, and calculating the fluxes of heat across each face. We find that the heat fluxes through the surface explain the majority of the seasonal summer warming and winter cooling, but when the average seasonal cycle is removed, the remainder of the temperature variability is explained by a combination of the heat fluxes through the surface and from waters originating in the Gulf Stream. Thus, the classical view of Gulf Stream waters impacting the high-latitude North Atlantic temperature variability is confirmed, though with the important caveat that there are a number of other possible sources of variability (e.g., surface forcing, variability from the western subpolar North Atlantic) that combined exert a larger effect on the temperature variability of the eastern subpolar North Atlantic and cannot be ignored in seasonal to interannual predictions.