Upper air mechanisms of the Southern Oscillation in the tropical Atlantic sector

Anomalies of upper air circulation in January of Pacific warm and cold years are investigated from the 1958–1997 National Center for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) Reanalysis. The focus is on mechanisms conducive to slack meridional pressure gradient on the equatorwardside of the North Atlantic high (NAH) during Pacific warm years, because earlier research has shown how the concomitantly weak North Atlantic tradewinds, through reduced evaporation and wind stirring, in the subsequent months lead to warm anomalies in the tropical North Atlantic and thus anomalously strong interhemispheric gradient of sea surface temperature (SST); this being a major factor for the failure of the March–June rainy season of northeast Brazil. The present analysis revealed for January of Pacific warm years a train of centers with alternatingly positive and negative departures of upper tropospheric topography extending from the equatorial Pacific across North America to the North Atlantic, and these topography anomalies are accompanied by anomalies in the fields of upper tropospheric divergence and divergent flow. In particular, anomalous upper tropospheric convergence and midtropospheric subsidence overly the equatorward flank of the NAH, thus favoring slack meridional gradient of surface pressure. While the origin of the upper tropospheric wave train from the Pacific to the Atlantic merits further study, the present analysis thus identified the critical Atlantic upper air mechanism through which the Southern Oscillation affects surface pressure and SST fields in the tropical Atlantic and rainfall in northeast Brazil.