Maintenance of austral summertime upper-tropospheric circulation over tropical South America: The Bolivian high-Nordeste low system

Using the NASA/GEOS reanalysis data for 1980-95, the austral-summer station ary 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 fo rmed by a shortwave train across South America but modulated by the long-wa ve regime. The short-wave train exhibits a monsoonlike vertical phase rever sal in the midtroposphere and a quarter-wave phase shift relative to the di vergent circulation. As inferred from (a) the spatial relationship between the streamfunction and velocity potential and (b) the structure of the dive rgent circulation, the short-wave train forming the BH-NL system is maintai ned by South American local heating and remote African heating, while the l ong-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-potent ial maintenance equation (which links velocity potential and diabatic heati ng) and the streamfunction budget (which is the inverse Laplacian transform of the vorticity equation). Some simple relationships between streamfuncti on and velocity potential for both wave regimes are established to substant iate the links between diabatic heating and streamfunction; of particular i nterest is a Sverdrup balance in the short-wave regime. This simplified vor ticity equation explains the vertical structure of the short-wave train ass ociated with the BH-NL system and its spatial relationship with the diverge nt circulation. Based upon the diagnostic analysis of its maintenance a simple forced barot ropic model is adopted to simulate the BH-NL system with idealized forcings , which imitates the real 200-mb divergence centers over South America, Afr ica, and the tropical Pacific. Numerical simulations demonstrate that the f ormation of the BH-NL system is affected not only by the African remote for cing, but also by the tropical Pacific forcing.