A preliminary study of the observed vertical mode structure of the summer circulation over tropical South America

In this paper we present a preliminary vertical modal decomposition of FGGE level IIIb data for the tropical sector over South America during the summer period 29 January-16 February. The vertical modes are defined by a linearized σ-coordinate primitive equation model as in Kasahara and Puri. The average flow during the analysed period is separated into 9 blocks covering tropical and subtropical South America and the average energetics is computed as a function of the vertical mode index. It is shown that the external mode is dominant at higher latitudes and, as the equator is approached, an internal mode with equivalent depth of the order of 250 m becomes dominant. The basic characteristics of the tropical upper circulation over South America, such as the Bolivian High and the deep trough off the NE coast of Brazil, are clearly depicted by the 4th internal mode. The flow at higher latitudes in the southern hemisphere is represented by a combination of both external and 4th internal modes with a dominant external mode at higher northern latitudes. The transient behaviour of the energetics over the continental blocks is correlated with the convective activity over the region.