The effect of oceanic eddies on marine stratocumulus over the southeast Pacific

Air-sea interaction in the southeast Pacific (SEP) plays an important role in the regional and global climate system. The combination of cool sea surface temperature (SST) and subsiding warm, dry air provide an ideal environment for the formation of marine stratocumulus clouds. In this study, the modulation of clouds due to ocean meso-scale fluctuations of SST was investigated by use of multi-year satellite data sets. Monthly averaged and 0.25-degree gridded sea surface temperature (SST), column water vapor (CWV), column liquid water (CLW), surface wind velocity, and sea surface height (SSH) were analyzed to obtain correlations among monthly anomalies of the variables, including wind divergence. Monthly anomalies, constructed by removing 6-year (1998-2003) monthly means, were averaged into 0.25C bins, over a 10x10 degree box centered at 20S, 85W. The analysis indicated that the anomaly of SST was correlated with anomalies of SSH and CWV. Wind divergence at a height of 10 m was negatively correlated with SST anomaly. The relationship between CLW anomaly and SST anomaly showed different phases of cloud formation. Preliminary interpretation suggests that high values of wind divergence were associated with negative SST anomalies and relatively high CLW, consistent with descending air and relatively dense stratus clouds. For SST anomalies near zero and slightly positive, wind divergence was intermediate and CLW exhibited a broad minimum, consistent with weak subsidence and relatively less dense stratus cloud. For SST anomalies in excess of 1C, there was evidence of the break-up of stratus into cumulus cloud, driven by convection. Cold SST and negative SSH anomalies (anticyclonic ocean eddies) are regions of high biological productivity, which may lead to the generation of aerosols in the marine atmosphere. Therefore clouds may be modulated by upper ocean biological productivity in addition to SST and wind.