Large-Scale Patterns and Long-Term Trends of Circulation Variability Associated with Sahel Rainfall Anomalies

[摘要] Trends in the large-scale tropical circulation during 1948-1883 are studied in relation to the persis-tent drought in Subsaharan Africa, based on ship observations over the tropical oceans and surface records of stations in Western Africa, and using cluster, correlation, and trend analyses. Deficient Sahel rainy seasons are characterized in the Atlantic sector by an equatorward expansion of the North Atlantic subtropical high, enhanced Northeast trades and reduced southerly cross-equatorial flow, and anomalously cold surface waters across the tropical North Atlantic, and in the Indian Ocean by relatively high sea surface temperature. These associations with Sahel rainfall variations are typical both of year-to-year and trend-like changes.Trends of the large-scale circulation setting in the Atlantic sector include a pressure rise over the tropical North Atlantic; southward migration of the near-equatorial trough; acceleration of both the North and South Atlantic trades along with increasing cloudiness in the equatorial zone; increasing cloudiness over the tropical North Atlantic and the Central American Seas; and cooling of surface waters in a band across the tropical North Atlantic contrasting with warming in the South Atlantic. Records at land stations in Western Africa show near-coastal temperature and pressure trends consistent with the adjacent ocean areas. In the interior of Subsaharan Africa warming trends prevail, presumably as direct consequence of reduced inflow of cool maritime monsoon air, precipitation, and in-situ evaporation. Downward pressure trends at continental stations in Subsaharan Africa may reflect the hydrostatic effect of increasing temperature.

[学科分类] 大气科学