Influence of Mean State Changes on the Structure of ENSO in a Tropical Coupled GCM

Abstract

This study examines the response of the climate simulated by the Institut Pierre Simon Laplace tropical Pacific coupled general circulation model to two changes in parameterization: an improved coupling scheme at the coast, and the introduction of a saturation mixing ratio limiter in the water vapor advection scheme, which improves the rainfall distribution over and around orography. The main effect of these modifications is the suppression of spurious upwelling off the South American coast in Northern Hemisphere summer. Coupled feedbacks then extend this warming over the whole basin in an El Nino-like structure, with a maximum at the equator and in the eastern part of the basin. The mean precipitation pattern widens and moves equatorward as the trade winds weaken. This warmer mean state leads to a doubling of the standard deviation of interannual SST anomalies, and to a longer ENSO period. The structure of the ENSO cycle also shifts from westward propagation in the original simulation to a standing oscillation. The simulation of El Nino thus improves when compared to recent observed events. The study of ENSO spatial structure and lagged correlations shows that these changes of El Nino characteristics are caused by both the increase of amplitude and the modification of the spatial structure of the wind stress response to SST anomalies. These results show that both the mean state and variability of the tropical ocean can be very sensitive to biases or forcings, even geographically localized. They may also give some insight into the mechanisms re- sponsible for the changes in ENSO characteristics due to decadal variability or climate change.