On the time variability of the net ocean-to-atmosphere heat flux in midlatitudes, with application to the North Atlantic basin

摘要

A new diagnostic to investigate the role of ocean dynamics in midlatitude air-sea interactions is presented and tested against observations. It is based on the analysis of the time variability of the net ocean-to-atmosphere heat flux (F_s). A hierarchy of air-sea interaction models indicate that, in the absence of ocean dynamics, the power spectrum of F_s should be blue at time-scales longer than a threshold set by the late-winter ocean mixed-layer thickness and the sensitivity of F_s to sea surface temperature anomalies. Comparison of the predicted F_s spectrum with observations over the North Atlantic shows a good agreement over the subpolar gyre where the deep ocean mixed layer combined with strong stochastic forcing allows large fluctuations in F_s at decadal and longer time-scales. Discrepancies, however, arise over the Gulf Stream extension region. Here it is suggested that the observed variability of F_s at time-scales longer than a decade is controlled by geostrophic ocean dynamics ratherthan local atmospheric forcing. The diagnostic appears to be a useful and simple tool to investigate the role of ocean dynamics in the upperocean heat budget. It is particularly well suited to the analysis of long simulations of coupled ocean-atmospheremodels. One implication of the study for ocean-only numerical simulations is that one cannot specify externally the low-frequency variability of F_s. The latter should only arise as a consequence of ocean dynamics.