Weakening of the equatorial Atlantic cold tongue over the past six decades

Seasonal and interannual variations of the equatorial cold tongue are defining features of the tropical Atlantic Ocean, with significant climatic and biogeochemical effects. However, its long-term changes are poorly understood owing to biases in observations and climate models. Here we use a suite of bias-corrected observations, and find that cold-tongue variability has weakened during the past six decades. We find that sea surface temperature has increased across the basin, with a local enhancement over the eastern equatorial Atlantic. This warming pattern of the sea surface is most pronounced during boreal summer, reducing the annual cycle through a positive ocean-atmosphere feedback. Specifically, the eastward-intensified warming leads to enhanced atmospheric convection in the equatorial eastern Atlantic region, as well as to less vigorous trade winds. These in turn deepen the thermocline in the east, and reinforce the sea surface warming pattern. The flattened thermocline and reduced thermocline feedback weaken interannual variability of equatorial sea surface temperatures and Guinea coast precipitation associated with the Atlantic Niño. We suggest that the observed changes could be associated with cooling by anthropogenic aerosols, an effect that is stronger in the Northern than in the Southern Hemisphere. If the aerosol emissions decrease in the next decades, the tropical Atlantic may experience yet another shift as the greenhouse gas forcing increases.