Thermally driven wind variability in the planetary boundary layer above Lima, Peru

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Abstract
Land‐sea thermal forcing of the coastal wind near Lima, Peru, is examined as a mechanism to explain the observed wind variability there, in particular the similarity between monthly anomalies of sea surface temperature and wind speeds on the interannual time scales typical of El Niño occurrences. Aerological and surface meteorological observations from Peru coastal sites are analyzed for two time scales: over 15–20 year periods (1958–1977) for monthly averaged data and over a year (1976–1977) for daily data. Wind profile characteristics and annual cycles, and their relation to the cross‐coastal temperature gradient are qualitatively and quantitatively consistent with the thermally forced boundary‐layer model proposed by Lettau (1978) to explain the Peru coastal wind. The cross‐coastal air temperature difference between Lima and Callao explained 55% of the variance in the surface wind speeds at Lima during 1976–1977. The mean annual cycles of the surface wind at Lima and other Peru coastal locations are in phase with the annual solar heating cycle where the seasonality of low cloudiness is large but with the annual cycle of the southeast trade circulation where it is not. This suggests that insolation over the desert is the principal source of thermal forcing variability. During El Niño periods the alongshore wind and the crosscoastal circulation above Lima intensify, in conjunction with anomalously warm sea surface temperatures. The planetary boundary layer is destabilized, and the base of the winter inversion is eroded upward. The observations are consistent with a reduction of desert cloudiness during El Niño events owing to air‐sea exchange and a consequent increase in the thermal forcing of the wind.