Surface wind modification near mid-latitude ocean fronts : observational and dynamical analysis

Graduation date: 2008Interactions between surface winds and meanders in mid-latitude sea surface temperature\ud (SST) fronts with horizontal length scales of 100-1000 km are investigated\ud from satellite observations and numerical simulations. Observations from the Sea-\ud Winds scatterometer on the QuikSCAT satellite show that the magnitude, direction,\ud curl, and divergence of the surface wind stress and 10-m winds are well correlated with\ud small-scale SST structures associated with large-scale ocean currents. Detailed analysis\ud of the response of the surface winds to SST fronts from these satellite observations\ud exposed shortcomings in previous conceptual hypotheses governing the relationships\ud between surface winds and SST. To gain understanding of the physical mechanisms\ud needed to explain the satellite wind observations, we performed a numerical experiment\ud simulating the atmospheric flow over meandering SST fronts. Based on these\ud results, a new conceptual model is constructed to explain the dynamical response of\ud the surface winds consistent with the satellite observations and numerical simulation\ud analysis.\ud Of particular importance was the finding that the wind stress curl and divergence\ud fields observed from QuikSCAT are linearly related to the crosswind and downwind\ud components of the SST gradient, respectively. This relationship was generally thought\ud to result from modification of the vertical turbulent mixing of momentum within the\ud atmospheric boundary layer (ABL). We show that this mechanism is overly simplistic;\ud nearly all of the terms in the momentum budget are needed to explain these\ud observed statistical relationships, consistent with recent work. SST-induced surface\ud wind changes are a manifestation of more complicated changes to the vertical structure\ud of the dynamic forces within the ABL.\ud Among the most significant of several new findings presented here concerns the\ud influence of SST on the meridional wind field. Since winds are generally westerly at\ud mid-latitudes, SST-induced changes in meridional wind cause changes in the surface\ud wind direction that significantly influence the wind stress curl and divergence fields\ud through modification of streamline curvature and diffluence. From numerical and\ud analytical results, these meridional wind perturbations are shown to result from a\ud baroclinic Ekman adjustment mechanism modified by horizontal advection