Observations of wind stress response to sea surface temperature perturbations with synthetic aperture radar

Our purpose is to detect ocean surface features, specifically oceanic thermal fronts, through analysis of SAR (synthetic aperture radar)-derived wind stress fields. Fine-resolution measurements of near-surface wind speeds over the Gulf Stream region of the Northwest Atlantic were made using SAR images collected by RADARSAT-2. Linear statistical relationships between the wind stress curl and divergence to the crosswind and downwind components of the SST gradient field were used to derive a new method for detecting Gulf Stream thermal fronts from Synthetic Aperture Radar (SAR) imagery. In particular, sea surface temperature front features, as suggested by corresponding AVHRR and MODIS images, are evident in both of the wind stress curl and divergence fields. The Gulf Stream provides energy, which is transported from the tropics to middle and high latitudes, to maintain the heat flux exchange between the ocean and atmosphere, thus affecting the entire troposphere. In terms of climatic tendencies, Minobe et al. [2008] suggest that surface wind convergence associated with low pressure and enhanced rain occur on the offshore flank of the Gulf stream SST (sea surface temperature) front, whereas surface wind divergence associated with high pressure occurs on the onshore flank of the front. Wind convergence and divergence is closely associated with surface winds that occur across an SST front, ultimately affecting middle and upper tropospheric dynamics. Therefore, the curl and divergence of the wind speed (or wind stress) are important factors related to SST variations of the Gulf Stream [Small et al, 2008]. The curl and divergence of wind stress are linearly related to the crosswind and downwind components of the SST gradient, respectively [Chelton et al., 2001; 2004]. O’Neill et al. [2003] suggest that different responses to the crosswind and downwind SST gradients between curl and divergence may result from secondary circulations which produce significant perturbations in the surface wind near SST fronts. The magnitudes of the curl and divergence of wind stress vary in both temporal and spatial domains [O’Neill et al., 2005]. The SST induced wind stress perturbations are larger than associated wind speed perturbations. They also found that two sources of SST induced wind stress curl and divergence perturbations are the nonlinear stresswind relationship and spatial gradients in SST induced wind speed, respectively [O’Neill et