| 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. While these atmospheric processes are understood from previous work, the present study investigates the mechanisms controlling long-term changes in meridional SST gradients, using surface and subsurface marine observations. SST forcing associated with various components of the mixed-layer heat budget are computed for each month of 1951-90, with the goal of examining decadal-scale variability. Forcing from surface fluxes and entrainment is scaled to the average annual cycle of mixed-layer depth. Trends over 1951-90 are computed for SST forcing and compared to trends in observed SST tendency.; A substantial warming trend in the tropical South Atlantic is concentrated in austral summer, while in the tropical North Atlantic SST increased in boreal summer 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 1951-90. During boreal summer, significant warming at 10-0{dollar}spcircrm N{dollar} relative to 20-10{dollar}spcircrm N{dollar} is associated with a southward displacement of the ITCZ, contributing to the decrease of rainfall over the West African Sahel.; Strongest warming trends in the respective summer hemisphere imply enhanced seasonal warming during the transition from winter to summer. During September-February, decreased latent heat transfer represents the dominant forcing mechanism contributing to enhanced warming in the South Atlantic. During this season, the mixed-layer depth decreases, concentrating heat gain within a relatively thin layer. Similar processes take place during the March-August transition to boreal summer in the North Atlantic. SST trends diminish from the summer to the winter, tantamount to an accentuation of the seasonal cooling rate. A dominant mechanism contributing to enhanced seasonal cooling is associated with entrainment as mixed-layer depth increases during the transition to winter, which acts to dilute warm anomalies developed during summer. While mechanisms of decadal-scale change specific to the tropical Atlantic are identified here, similar mechanisms may influence SST trends in the global oceans. |
