Decadal‐scale trends in mechanisms controlling meridional sea surface temperature gradients in the tropical Atlantic

Abstract

The meridional gradient of sea surface temperature (SST) in the tropical Atlantic hydrostatically controls the atmospheric pressure gradient, thereby steering the latitude position of the Intertropical Convergence Zone (ITCZ), which affects rainfall in adjacent land areas. This study investigates the mechanisms controlling long‐term changes in meridional SST gradients, using surface and subsurface ship observations from 1951–1990. SST forcing from surface heat fluxes and entrainment are scaled by monthly climatic‐mean mixed‐layer depth. Observed SST tendency and contributions from various forcing mechanisms are computed for the 1951–1990 mean and trend. A substantial warming trend in the surface waters of the tropical South Atlantic is concentrated in the austral summer months of February–March, while SST in the tropical North Atlantic increased in boreal summer (August–September) and decreased in winter. As a consequence of the strengthened interhemispheric SST gradient in February–March, the position of the ITCZ was displaced southward, and rainfall in northeast Brazil increased over 1951–1990. A southward displacement of the ITCZ during boreal summer, accompanying warming between the equator and the ITCZ contributed to the decrease of rainfall over the west African Sahel. Strongest warming trends in the respective summer hemisphere imply an amplification of both seasonal cooling during winter and warming during summer in each hemisphere. During September–February a trend toward intensified northeast trade winds results in increased seasonal cooling due to latent heat transfer, whereas weakened southeast trades reduce latent heat loss, intensifying seasonal warming. For March–August a dominant mechanism forcing enhanced seasonal cooling in the South Atlantic is associated with entrainment as mixed‐layer depth increases during the transition to winter; this process acts to dilute warm anomalies developed during summer.