Importance of the non-linearity of the convective response to surface temperature for eastern Pacific El Niños

Understanding the key physical processes involved in the development and diversity of El Niño events is essential to anticipate their multiple impacts. Current El Niño theories generally assume that wind stress responds linearly to El Niño Sea Surface Temperature (SST) anomalies. Yet, the deep atmospheric convection that energizes this wind stress response has obvious nonlinear features. Observations indeed indicate that rainfall (a proxy of the tropospheric heating) increases slowly with SST up to 26.5^OC, followed by a sharp increase of rainfall at higher SSTs. In this study, we use that mean observed relation to derive a nonlinear relation between SST and rainfall anomalies, that depends on the background climatological SST. This relation performs much better to explain rainfall anomalies in the eastern Pacific (Niño3 region) than a linear relation, which underestimates rainfall during most extreme El Niños events. On the other hand, it underestimates rainfall anomalies in the western Pacific (Nino4), because it only considers the local SST forcing and neglects the atmospheric convergence feedback. Our observational results are in line with previous modeling studies, who have also underlined the importance of the nonlinearity of the convective response to SST anomalies for large El Niño events in coupled models. We end by discussing other possible sources of nonlinearity in the wind stress and heat flux responses to SST, which play a strong role in the most essential El Niño feedbacks.