Subseasonal Variability of the Southeast Pacific Stratus Cloud Deck

Abstract

Subseasonal variability of the stratus/stratocumulus cloud deck over the subtropical southeast Pacific is studied using satellite and buoy observations as well as the NCEP?NCAR reanalysis. It is found that subseasonal variability in the stratus cloud deck is closely related to variations in surface wind velocity, water vapor, sea level pressure, and 500-hPa geopotential height. An increase in cloud liquid water (CLW) over the subtropical southeast Pacific is found to be associated with the development of an anomalous anticyclonic circulation to the south off the west coast of Chile. The enhanced southerly to southeasterly winds advect cold and dry air into the stratus region against the mean sea surface temperature (SST) gradient. This cold and dry advection, together with increased wind speed, intensifies surface latent and sensible heat fluxes and destabilizes the boundary layer. Anomalous offshore easterlies north of the anomalous anticyclone cause a low-level divergence. The associated subsidence warming, together with the cold advection in the surface layer, strengthens the temperature inversion, conducive to the development of stratus clouds. Buoy observations confirm this subseasonal cloud variability and its relationship with surface meteorological variables. A lead/lag composite analysis indicates that circulation variables such as sea level pressure and surface wind lead cloud liquid water by 1?2 days while SST lags CLW by 1?2 days, suggesting that low-cloud variability is caused by atmospheric circulation changes rather than by the underlying ocean. The dynamic adjustment that leads to cloud fluctuations and possible orographic effects of the Andes are also discussed.