Strengthened tropical Pacific influence on tropical Atlantic variability in global warming projections

Abstract

Projected changes in tropical Atlantic variability are examined by comparing a high-emission scenario (experiment ssp585) with a pre-industrial control simulation (experiment piControl) using an ensemble of 23 climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6). Consistent with previous studies, equatorial Atlantic sea-surface temperature (SST) variability (the Atlantic Zonal Mode or AZM) decreases, while variability off the equator tends to slightly increase. AZM composites show that in ssp585 the role of dynamic forcing by surface wind stress anomalies is diminished while thermodynamic forcing through latent heat flux and shortwave radiation becomes more prominent. This is related to a stronger influence of El Niño-Southern Oscillation (ENSO) on the AZM. As a consequence, a positive correlation between ENSO and AZM emerges, with the former preceding the latter by about half a year. A simple linear analysis indicates that, in some models, this association of the AZM with ENSO makes it more predictable, despite its weakening amplitude. The stronger ENSO influence on the AZM is partly explained by the stronger ENSO amplitude in ssp585. In addition, the weakening of coupled air-sea feedbacks in the equatorial Atlantic makes it more susceptible to ENSO forcing. In contrast to some previous studies, it is found that the mean-state equatorial Atlantic thermocline slightly shoals under radiative forcing, which should strengthen the AZM. Its decline is therefore more related to the weakening of mean upwelling and the weaker response of surface winds to SST anomalies, both of which weaken the Bjerknes feedback.