A Preliminary Study of Summertime Tropospheric Circulation Patterns over South America Estimated from Cloud Winds

Abstract

Cloud winds obtained from SMS-I geostationary satellite images for the Southern Hemisphere summer-time are used to describe the tropospheric circulation over tropical and subtropical South America. These data provide good spatial and temporal, coverage over South America for at least two levels in the troposphere. The summertime circulation derived from cloud winds is similar in pattern to that obtained from sparse conventional winds but is more intense. The, major features of the time-averaged circulation at the upper level (300–100 mb) are an intense, quasi-stationary anticyclone-trough system extending over most of South America north of 35°S; a band of easterly flow near 10°S; and strong cross-equatorial northward flow between 60 and 80°W longitudes. At the lower level (900–700 mb) the major features are the northerly and northwesterly jet-type flow along the eastern slopes of the central Andes and northeasterly trades over much of the remainder of the continental region to the north and cast. Generally, divergent flow at the upper levels and convergent flow at the lower levels leads to the estimate, of ascending motion over most of the Amazon River Basin. Evidence of sinking motion is found over northeastern Brazil. At upper levels, the cloud wind estimate of the northward transport of westerly momentum across the equator is almost an order of magnitude larger during the observation period (1975–77) than estimates obtained from conventional data.