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Amanda Plagge, James B. Edson, and Douglas Vandemark

Abstract

Observations of ocean–atmosphere coupling across persistent mesoscale sea surface temperature (SST) gradients are used to examine the controls of atmospheric stability, pressure gradient force, and heat flux that are considered central to oft-observed coupling between wind and SST. Moored air–sea flux measurements near the Gulf Stream are combined with QuikSCAT satellite scatterometer equivalent neutral wind (ENW) data to assess correlations between SST, air–sea fluxes, pressure, and wind perturbations at scales of 10–100 days. The net effect of ocean fronts meandering past the site enabled buoy observation of SST impacts on wind, with coupling coefficients of 0.3–0.5 similar to past studies. Wind stress–SST and ENW–SST correlation coefficients are slightly higher, and roughly 20% of the ENW perturbation is attributed to stratification impacts predicted by Monin–Obukhov (MO) similarity theory. Significantly higher correlation is observed when relating wind or stress perturbations to buoyant heat flux variation. Atmospheric pressure perturbation with SST of order 0.5 hPa °C−1 is observed, as well as high negative correlation between wind and pressure variations. Length and time scales associated with the coupling indicate that peak correlations occur at 50–70 days and 300–500 km, consistent with mesoscale meander scales. Coupling coefficient values vary significantly depending on analysis time scale and exhibit a range near to recently observed interbasin variability. This variability is attributed to the extent of oceanic length scales permitted in the analysis. Together, results affirm the central role of SST-induced turbulent heat flux in controlling pressure field adjustments and thereby the wind perturbations over SST fronts.

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Amanda M. Plagge, Douglas Vandemark, and Bertrand Chapron

Abstract

A 5-yr dataset collected over two surface current and meteorological moorings allows rigorous evaluation of questions surrounding wave–current interaction and the scatterometer. Results demonstrate that scatterometer winds represent winds relative to the moving sea surface, affirming previous observational efforts that inferred the phenomenon using climatological approaches over larger time and space scales in equatorial and western boundary currents. Comparisons of wind residuals between Ku-band Quick Scatterometer (QuikSCAT) and buoy measurements show nearly one-to-one correlations with ocean surface velocity for 5-, 12.5-, and 25-km resolution wind speed products, especially under conditions of moderate wind speed and near-neutral atmospheric stability. Scatterometer and buoy wind direction differences due to currents were observed to be negligible for the range of surface velocities encountered and the length scales observed by QuikSCAT. Similar analyses are applied to C-band Advanced Scatterometer (ASCAT) satellite wind measurements at the same sites, as well as to satellite altimeter winds, and overall confirm the results seen with QuikSCAT; differences are likely the combined result of sampling, satellite wind algorithms, and geophysical wind–wave coupling in the presence of currents. On the whole, this study affirms that at length scales of 10 km and longer the scatterometer wind can be considered to be current relative. Observed differences between earth-relative and current-relative winds of order 10%–20% of the wind velocity are not uncommon in this and other ocean regions and this study more fully validates that microwave remote sensing winds appear to respond to wind stress even in the presence of larger-scale currents.

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