Modification of surface winds near ocean fronts: Effects of Gulf Stream rings on scatterometer (QuikSCAT, NSCAT) wind observations

Abstract

[1] Modifications to surface winds by currents and sea surface temperature (SST) gradients near frontal boundaries of Gulf Stream rings are analyzed using satellite SST and scatterometer (NASA's Quick Scatterometer (QuikSCAT), NASA scatterometer (NSCAT)) wind observations. A component of scatterometer wind approximately equal and opposite to the surface current vector is observed and attributed to the fact that scatterometers detect relative motion of water and air. Warm-core ring (WCR) SSTs act to destabilize the marine atmospheric boundary layer (MABL), increasing surface wind magnitude by 10-15% and decreasing veering angle by 5-15°relative to large-scale mean winds. Cold-core ring (CCR) SSTs cause impacts of similar magnitude and opposite sense. Magnitudes and directions of modifications are accounted for by MABL dynamics of a nonlinear planetary boundary layer model forced by air-sea temperature differences. Wind modifications occur within tens of kilometers of SST fronts, implying a wind response timescale of order 1 hour. By contrast, uniformity of modified winds across the larger area within rings suggests the response time for the MABL to return to equilibrium downstream from a front exceeds 10 hours. Over WCRs, strong divergence (convergence) occurs on the upwind (downwind) side; curl is strongly negative (positive) to the right (left) side facing downwind. Opposite patterns are generally seen over CCRs. Divergence (curl) peaks where winds blow perpendicular (parallel) to SST fronts. SST image analysis indicates enhanced cloudiness occurs with downwind convergence over WCRs. Wind stress curl due to ring modifications causes dipolar Ekman pumping sufficient to influence ring translation and decay processes.

Imprint of Southern Ocean eddies on chlorophyll

“…• S. Anticyclones exhibit a higher percentage of data gaps than cyclones (47% versus 42%), which can be explained by the impact of their sea surface 30 temperature anomalies on cloud cover (Park et al, 2006;Small et al, 2008;Frenger et al, 2013). We account for the issue Figure S1) for both anticyclonic (warm-core, SLA lifting and thermocline deepening) and cyclonic (cold-core, SLA deepening and thermocline lifting) eddies.…”

Importance of the sea surface curvature to interpret the normalized radar cross section

“…Consistent inversion schemes merging these two sources of information are promising. In particular, this will help to better decipher between wind effects and current impacts on the apparent surface roughness [Quilfen et al, 2001;Kelly et al, 2001;Park et al, 2006].…”

Acknowledging contributions

“…This is likely to overestimate the air-sea temperature difference as in reality the air will partially adjust to the eddy SST.) Park et al (2006) found that near-surface wind speeds increase by about 10% over warm core rings and reduced by around 15% over cold core rings (Fig. 8).…”

“…The opposite patterns are seen over cold core rings. An important effect of the enhanced convergence on the downwind side of warm core rings is seen in cloudiness fraction data from SST images, which reveals substantially enhanced cloud probabilities on the downwind side (Park et al 2006). …”