Weakened interannual Tropical Atlantic variability in CMIP6 historical simulations

Climate variability in the Tropical Atlantic is complex with strong ocean-atmosphere coupling, where the sea surface temperature variability impacts the hydroclimate of the surrounding continents. Most of the Tropical Atlantic interannual variability is explained by its equatorial (Atlantic Zonal Mode, AZM) and meridional (Atlantic Meridional Mode, AMM) modes of variability. Our results using an ensemble of 37 models from the Coupled Model Intercomparison Project Phase 6 historical simulations show multidecadal changes in the Tropical Atlantic variability from 1900 to 2014. Within it, most models (at least 60%) show a decrease in variability after the 1970 s, in accordance with observational data sets. The agreement among simulations points out the role of the external forcing. After 1970, an anthropogenically induced warming trend is observed in the equatorial Atlantic accompanied by a weakening of the winds. This drives a weakening in the Bjerknes Feedback by deepening the thermocline in the eastern equatorial Atlantic, reduced SST sensitivity to thermocline anomalies, thus decreasing AZM variability. The interplay of the meridional asymmetric warming in the Tropical Atlantic related to the anthropogenic forcing, the relaxed northeast trade winds, and the background state of the negative Atlantic Multidecadal Variability, decreases AMM variability despite the individual increase in variability of the North and South Tropical Atlantic. Associated with these factors the African Sahel shows a positive precipitation trend and the Intertropical Convergence Zone tends to shift northward, which reinforces the increased precipitation.