ENSO sensitivity to convective triggers in a coupled GCM

Abstract

A series of 100-year long simulations is performed using GFDL CM2.1, by varying the Tokioka parameter, which is the minimum entrainment rate threshold in the cumulus parameterization of a GCM. It is found that the magnitude of ENSO simulated in this model increases with the Tokioka parameter. Changing the Tokioka parameter alters both the Pacific-Ocean mean state and ENSO variability, which are tightly coupled. When a large Tokioka parameter is used, deep convection is highly suppressed; thereby the amount of high cloud is reduced. With less high cloud in the western Pacific under large Tokioka parameter, the east-west asymmetry in the Pacific-basin mean state is reduced. The atmospheric responses to ENSO-related SST forcing are shifted eastward, which leads to stronger ENSO variability. It is suggested that eastward shift of the atmospheric response is associated with east-west asymmetry and resultant wetness over the eastern Pacific. Our results suggest that the east-west asymmetry is one of the metrics for ENSO amplitude in coupled climate models.