Weakened impact of the Atlantic Nino on the future equatorial Atlantic and Guinea Coast rainfall

The Guinea Coast is the southern part of the West African continent. Its summer rainfall variability mostly occurs on interannual timescales and is highly influenced by the sea surface temperature (SST) variability in the eastern equatorial Atlantic, which is the centre of action of the Atlantic Nino mode. Using both historical and scenario (SSP5-8.5) simulations from 31 general circulation models (GCMs) participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we first show that these models present a wet bias during boreal summer. This bias is associated with overly high mean boreal summer SSTs in the eastern equatorial and south Atlantic regions. Next, we analyse the near-term, mid-term and long-term changes of the Atlantic Nino relative to the present-day situation, in a climate with a high anthropogenic emission of greenhouse gases. We find a gradual decrease in the equatorial Atlantic SST anomalies associated with the Atlantic Nino in the future. This result reflects a possible reduction of the Atlantic Nino variability in the future due to a weakening of the Bjerknes feedback over the equatorial Atlantic. In a warmer climate, an anomalous higher sea level pressure in the western equatorial Atlantic relative to the eastern equatorial Atlantic weakens the climatological trade winds over the equatorial Atlantic. As a result, the eastern equatorial Atlantic thermocline is deeper and responds less to the Atlantic Nino events. Among the models that simulate a realistic rainfall pattern associated with the Atlantic Nino in the present-day climate, there are 12 GCMs which project a long-term decrease in the Guinea Coast rainfall response related to the Atlantic Nino. In these models, the zonal 850 hPa wind response to the Atlantic Nino over the equatorial Atlantic is strongly attenuated in the future climate. We also find that 12 other GCMs show no robust change in the patterns associated with the Atlantic Nino. There is a higher confidence in the mid-term and long-term reduction of the rainfall associated with the Atlantic Nino over the Atlantic Ocean than over the Guinea Coast. We also found a projected decrease in the convection associated with the Atlantic Nino in the majority of the models.

Automated summary

The Guinea Coast is influenced by the sea surface temperature variability in the eastern equatorial Atlantic, and the models suggest a possible weakening of the Atlantic Nino variability in the future, which may lead to a reduction in rainfall on the Guinea Coast.