Sensitivity of the heat budget in a midlatitude ocean model to variations in atmospheric forcing

Abstract

The rate of change of total heat storage in the ocean is determined primarily by the net heat fluxes across the air‐sea interface. In the context of a wind‐ and heat‐driven ocean model the long‐term mean heat budget was examined in a series of perturbation experiments in which atmospheric forcing parameters (surface winds and surface air temperature) were systematically varied. The thermodynamic effect of increasing surface wind speed is to take heat out of the mixed layer and thus cool the ocean. However, this wind‐induced cooling is overwhelmed by wind‐induced mechanical deepening of the mixed layer. A mixed layer deepened in this way is also cooler, so heat loss to the atmosphere is reduced. This ultimately leads to a warmer vertically integrated ocean. A change in wind stress forcing has little effect on total heat storage. Surface air temperatures are typically cooler than sea surface temperatures, so warmer air temperatures result in less heat flux out of the ocean, a warmer mixed layer, and more oceanic heat storage. The seasonal cycle is dominated by changes in radiative forcing to which the model responds exactly as it does to surface air temperature changes. As a result the model ocean stores less heat in winter (February–April) than in summer (August–October). The dependence of this seasonal change on mixed‐layer depth, mixed‐layer temperature and deep ocean temperature is quantified. Mixed‐layer depth is by far the most important controlling factor. Comparisons with climatological data from the North Atlantic and North Pacific Oceans confirm this general picture, although the comparisons suggest that the model mixed‐layer depth varies too much between seasons. The climatological data also indicate that the northern oceans store more heat in winter than in summer around 25°N. Possible explanations for this are discussed.