An Air-Sea Interaction Model for Cold-Air Outbreaks

Abstract

A two-dimensional dry model for the atmosphere is coupled with a two-dimensional primitive equation model for the ocean to investigate how cold fronts interact with the Gulf Stream and its adjacent waters during cold-air outbreaks. The development of a cross-stream frontal circulation under the influence of the ocean surface heating and its impact on the ocean circulation are determined for various frontal and synoptic conditions. The diabatic heating under an offshore wind is shown to generate a convective boundary layer over the ocean, which deepens seaward. The interaction of the convective boundary layer and the seaward-moving cold front decreases the speed of the frontal propagation and induces a downdraft behind the front and an updraft near the nose of the front. This process is largely independent of the vertical shear of the offshore wind. The postfrontal downdraft over the coastal ocean intensifies the low-level southward wind, which in turn weakens the Gulf Stream transport. To lowest order, this sequence of events is the primary effect of coastal air?sea interaction on a weekly time scale identified by the model. Conventional oceanography models that treat the ocean cooling as a forcing independent of the wind stress may grossly underestimate its effects during cold-air outbreaks. The present model also suggests that the conventional meteorological practice of fixing the sea surface temperature is a reasonable approximation on time scales less than one week, as far as the dry two-dimensional model is concerned. It is speculated that the accuracy of this meteorological approximation will deteriorate if moisture effects and alongshore variations are incorporated; this requires future model verification.