EXTRATROPICAL LARGE-SCALE AIR-SEA INTERACTION IN A COUPLED AND UNCOUPLED OCEAN-ATMOSPHERE MODEL

Spatial patterns of mid-latitude large-scale ocean-atmosphere interact ion on monthly to seasonal time scales have been observed to exhibit a similar structure in both the North Pacific and North Atlantic basins . These patterns have been interpreted as a generic oceanic response t o surface wind anomalies, whereby the anomalous winds give rise to cor responding anomalous regions of surface heat flux and consequent ocean ic cooling. This mechanistic concept is investigated in this study usi ng numerical models of a global atmosphere and a mid-latitude ocean ba sin (nominally the Atlantic). The models were run in both coupled and uncoupled mode. Model output was used to generate multi-year time seri es of monthly mean fields. Empirical orthogonal function (EOF) and sin gular value decomposition (SVD) analyses were then used to obtain the principal patterns of variability in heat flux, air temperature, wind speed, and sea surface temperature (SST), and to determine the relatio nships among these variables, SVD analysis indicates that the turbulen t heat flux from the ocean to the atmosphere is primarily controlled b y the surface scalar wind speed, and to a lesser extent by air tempera ture and SST. The principal patterns of air-sea interaction are closel y analogous to those found in observational data. In the atmosphere, t he pattern consists of a simultaneous strengthening (or weakening) of the mid-latitude westerlies and the easterly trades. In the ocean ther e is cooling (warming) under the anomalously strong (weak) westerlies and trade winds, with a weaker warming (cooling) in the region separat ing the westerly and easterly wind regimes. These patterns occur in bo th coupled and uncoupled models and the primary influence of the coupl ing is in localizing the interaction patterns. The oceanic patterns ca n be explained by the principal patterns of surface heat flux and the attendant warming or cooling of the ocean mixed layer.