Surface Wind Response to oceanic Fronts

Spaceborn scatterometer (NSCAT and QuikSCAT) wind data are combined with Gulf Stream north wall positions, manually digitized from two-day composite sea surface temperature (SST) fields obtained from Advanced Very High Resolution Radiometer (AVHRR) data. These data reveal the response of the surface wind field to large open ocean fronts characterized by sharp gradients in both SST and near surface currents. Each scatterometer pass was paired with the Gulf Stream path closest in time. The period studied, determined by the availability of AVHRR data, was from 16 September 1996 to 29 June 1997 for NSCAT and from 24 July 1999 to 31 December 2000 for QuikSCAT. All match-ups were then visually examined and only those for which the Gulf Stream presented a reasonably straight segment over which the wind field was free of atmospheric fronts or large curvature were selected. Ten match-ups met these criteria.

The response of the scatterometer wind field to the SST/current front was analyzed in detail for these ten cases using the Pennsylvania State University (PSU)-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). This includes an analysis of both the dynamical forcing of the wind (by the currents) and the thermal forcing due to the step in SST. To do so the planetary boundary layer (PBL) model Medium-Range Forecast (MRF)used in MM5 was modified to allow the incorporation of the Gulf Stream current as part of the bottom boundary condition. Changes in the modeled surface wind field across the front in each of the ten cases agrees well with changes in the observed winds. The importance of pressure gradients induced by changes in air temperature, moisture, and ver tical mixing across oceanic front is studied in the momentum budget analysis. Our findings suggest that the perturbation pressure resulting from the thermal forcing by the front accounts for the decrease in wind speed when moving from warm to cold water and the increase observed in the converse. The frontal adjustment of the surface wind to the front occurs as a result of the vertical motion induced by horizontal divergence/convergence and advection in the marine atmospheric boundary layer (MABL). Finally the numerical simulations suggest that the dynamical and thermal effects are very nearly additive.

Corresponding author address: Qingtao Song, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882. E-mail: qsong@gso.uri.edu