Dynamical analysis of the coupling between sea surface temperature and the surface vorticity and divergence fields associated with the Agulhas Return Current

The dynamical origins of the coupling between sea surface temperature (SST) and the surface vorticity and divergence fields is investigated using the Weather Research and Forecasting (WRF) mesoscale model over a section of the Agulhas Return Current in the Southern Ocean between approximately 50-80ºE and 40-50ºS. This study analyzes the surface momentum budget to determine the dominant forces responsible for the relationship between the vorticity and divergence fields and the crosswind and downwind components of the SST gradient observed in QuikSCAT scatterometer observations. A non-evolving SST field for the one-month period July 2002 from the Advanced Microwave Scanning Radiometer on the EOS-Aqua (AMSR-E) satellite was used as the SST boundary condition. The WRF surface vorticity and divergence dependencies on the crosswind and downwind SST gradients agree well with the dependencies computed from the QuikSCAT wind fields over the same time period, engendering confidence in the WRF model's ability to accurately simulate the ocean-atmosphere interaction of interest here.

From analysis of the surface momentum budget, the separate roles of hydrostatic pressure gradients, vertical turbulent stress divergence, and horizontal advection are demonstrated. Coriolis forces are considered only in their role in deflecting these accelerations. Pressure gradients play the dominant role in the vorticity response to small-scale SST gradients, with turbulent friction and horizontal advection playing comparatively smaller roles. Horizontal advection and turbulent friction comprise the divergence response to SST gradients, with pressure gradients playing little or no role. Our results are broadly consistent with previous studies promoting the importance of both the vertical turbulent mixing of momentum and hydrostatic pressure gradient mechanisms. However, few past studies have considered the importance of horizontal advection, which plays a surprisingly strong role in the SST-induced surface wind response due to the strong mean wind speed in this region, whose one-month scalar-average is between 12 and 16 m/s throughout the model domain. We conclude that elements of the vertical turbulent mixing, pressure gradient and horizontal advection mechanisms act together to produce the surface vorticity and divergence responses to SST gradients.

Through analysis of the vorticity and divergence fields decomposed into curvilinear natural coordinates, these three mechanisms produce vorticity and divergence perturbations through changes in both wind speed and direction near SST fronts. Previous conceptual models have generally only considered SST effects on the wind speed gradient component of the vorticity and divergence. Wind direction gradients are shown to substantially alter the vorticity and divergence responses to SST gradients, leading to vorticity responses that are about 40% weaker than the divergence responses over the region considered here.