Wintertime air‐sea interaction processes across the Gulf Stream

Abstract

Aircraft, buoy and satellite measurements have been used to study the wintertime air‐sea interaction processes across the Gulf Stream during January 25–30, 1986. The turbulent flux regime in the marine atmospheric boundary layer exhibited considerable spatial and temporal variability during this 6‐day period, which was related to both the evolution of the synoptic scale atmospheric conditions and the sea surface temperature (SST) field. During the pre‐storm conditions prior to January 25, the spatial structure of the SST field played an important role in generating a shallow atmospheric frontal zone along the Gulf Stream front by causing differential heating of the marine atmospheric boundary layer over the stream versus over the cooler shelf waters. As this front moved shoreward on January 25, the warm, moist, maritime air flowing northwestward behind the front induced moderate ocean‐to‐atmosphere heat fluxes (∼300 W m−2 total heat flux measured over the core of the Gulf Stream). The subsequent outbreak of eastward flowing cold, dry, continental air over the ocean on January 27 and 28 generated high total heat fluxes (∼1060 W m−2 over the core of the Stream), as did a second, somewhat weaker outbreak which followed on January 30 (∼680 W2 over the core of the Stream). During each of these outbreaks, with air flowing from land out over the continental shelf, Gulf Stream and Sargasso Sea waters, the SST field again affected the spatial structure of the flux fields. The near‐surface fluxes of both sensible and latent heat were found to be relatively low over the cool continental shelf waters, while higher fluxes were seen over the Gulf Stream and Sargasso Sea. Similar spatial structure was seen in the near‐surface momentum flux values, but relative changes were typically smaller from one location to another on a particular day. The most noticeable responses of the Gulf Stream to these surface fluxes were the deepening of its mixed layer and a loss of upper layer heat; however, no direct current observations were made in the stream, so velocity changes may not be assessed. An average mixed layer deepening of about 35 m was observed in the stream, and the upper layer heat loss was estimated to be 3.2×1013 J m−1 alongstream, an amount sufficient to decrease the average mixed layer temperature by 0.62°C. No path changes in the stream could be attributed to the atmospheric forcing of this period, since there was a large offshore movement of the stream in the region of the Charleston bump at this time due to other processes. Any path changes that may have been associated with the atmospheric forcing would have been masked by that offshore movement.