Annual cycle of upper air circulation and convective activity over the tropical Americas


  • Stefan Hastenrath


A combination of novel data sets, including satellite-derived outgoing longwave radiation and highly reflective clouds, rain gauge records, and upper air analyses of the European Centre for Medium-Range Weather Forecasts have been evaluated in context, to reappraise the multiyear mean convective activity and circulation and their annual cycle variations over the tropical Americas. In austral summer a maximum of 200 mbar geopotential height, anticyclonic vortex in the wind field, and extremum of stream function are located over the western part of the Amazon basin, well to the west of the extremum of the velocity potential. To the north of that and the nucleus of the most intense convection and rainfall, the major upper tropospheric divergence is found over northern South America. From this, outflow is directed primarily northward into an area of strongest convergence situated over the equatorward side of the North Atlantic surface high. In the evolution from austral to boreal summer, the high over western Amazonia vanishes, and the centers of strongest convection, upper tropospheric velocity potential, and divergence establish themselves in the realm of the Intertropical Convergence Zone over the eastern equatorial Pacific, with outflow directed mainly into a broad zone of upper tropospheric convergence in the southern hemisphere. After the height of the boreal summer, the centers of convection and velocity potential develop again over southeastern Amazonia, and the upper tropospheric high and weak anticyclone separately form again over the western portion of the Amazon basin.