Surface winds off Peru-Chile: Observing closer to the coast from radar altimetry

Abstract

The near-shore surface mesoscale atmospheric circulation in the upwelling systems off Peru and Chile is influen-tial on the Sea Surface Temperature through Ekman transport and pumping. There has been a debate whether ornot the so-called“wind drop-off”, that isa shoreward decrease of the surface wind speed nearthe coast, can act asan effective forcing of upwelling through Ekman pumping. Although the wind drop-off has been simulated byhigh-resolution atmospheric models, it has not been well documented due to uncertainties in thescatterometry-derived wind estimates associated with land contamination. Here we use the along-track altime-try-derived surface wind speed data from ENVISAT, Jason-1, Jason-2, and SARAL satellites, to document the spa-tial variability of the mean wind drop-off near the coast as estimated from the inversion of the radarbackscattering coefficient. The data arefirst calibrated so as tofit with the scatterometer observations of previousand current satellite missions (QuikSCAT, ASCAT). The calibrated data are then analyzed near the coast and awind drop-offscaleisestimated. The results indicate that the wind drop-off takes placeall alongthe coast, thoughwith a significant alongshore variability in its magnitude. Differences between products are shown to be relatedboth to the differences in repeat cycle between the different altimetry missions and to the peculiarities of thecoastline shape at the coastal latitudes of the incident tracks. The relative contribution of Ekman pumping andEkman transport to the total transport is also estimated indicating a comparable contribution off Chile whiletransport associated to Ekman pumping is on average ~1.4 larger than Ekman transport off Peru. Despite thealiasing effect associated with the weak repetitivity of the satellite orbit and the high frequency variability ofthe winds in this region, the analysis suggests that the seasonal cycle of the surface winds near the coast couldbe resolved at least off Peru.