Impact of sea surface temperature anomalies on the Atlantic tropical stormactivity and West African rainfall

The association between rainfall over the Sahel and Sudan region and tropic al storm activity in the Atlantic is examined using the NCEP-NCAR reanalysi s and sea surface temperature anomalies (SSTAs) from 1949 to 1998. Evidence indicates that both are influenced by global SSTAs. The SSTA modes generat ing favorable atmospheric conditions for tropical storms to develop are als o in favor of a wet rainfall season in the Sahel and Sudan region. The east erly waves over West Africa become tropical storms only if the atmospheric conditions over the Atlantic are favorable. These conditions are responses to SSTAs. In addition to ENSO, a multidecadal trend mode also plays a role. The posit ive phase of the trend mode features positive loadings in the North Pacific and the North Atlantic, and negative loadings over the three southern ocea ns. The positive (negative) phases of both modes are associated with increa sed (reduced) Atlantic tropical storm activity, and with wet (dry) West Afr ican monsoon seasons. The SSTAs over the tropical South Atlantic (S-ATL) ar e related to the rainfall dipole over West Africa, but the influence on tro pical storms is not large. Warm (cold) SSTAs over the tropical North Atlant ic enhance (suppress) the occurrence of tropical storms, but have little in fluence on rainfall over West Africa. The most prominent circulation features associated with the positive phases of SSTA modes are enhanced upper-level 200-hPa easterly winds and reduced vertical wind shear in the main development region of the tropical Atlantic , which are well-known features of active Atlantic tropical storm seasons. The associated low-level flow shows enhanced anomalous westerly winds acros s the Atlantic to Africa. That allows more moisture transport into Africa a nd, therefore, more rainfall.