Maintenance of Austral Summertime Upper-Tropospheric Circulation over Tropical South America: The Bolivian High–Nordeste Low System

Abstract

Using the NASA/GEOS reanalysis data for 1980–95, the austral-summer stationary eddies in the tropical–subtropical Southern Hemisphere are examined in two wave regimes: long and short wave (wave 1 and waves 2–6, respectively). The basic structure of the Bolivian high–Nordeste low (BH–NL) system is formed by a short-wave train across South America but modulated by the long-wave regime. The short-wave train exhibits a monsoonlike vertical phase reversal in the midtroposphere and a quarter-wave phase shift relative to the divergent circulation. As inferred from (a) the spatial relationship between the streamfunction and velocity potential and (b) the structure of the divergent circulation, the short-wave train forming the BH–NL system is maintained by South American local heating and remote African heating, while the long-wave regime is maintained by western tropical Pacific heating. The maintenance of the stationary waves in the two wave regimes is further illustrated by a simple diagnostic scheme that includes the velocity-potential maintenance equation (which links velocity potential and diabatic heating) and the streamfunction budget (which is the inverse Laplacian transform of the vorticity equation). Some simple relationships between streamfunction and velocity potential for both wave regimes are established to substantiate the links between diabatic heating and streamfunction; of particular interest is a Sverdrup balance in the short-wave regime. This simplified vorticity equation explains the vertical structure of the short-wave train associated with the BH–NL system and its spatial relationship with the divergent circulation. Based upon the diagnostic analysis of its maintenance a simple forced barotropic model is adopted to simulate the BH–NL system with idealized forcings, which imitates the real 200-mb divergence centers over South America, Africa, and the tropical Pacific. Numerical simulations demonstrate that the formation of the BH–NL system is affected not only by the African remote forcing, but also by the tropical Pacific forcing.