El Nino-Southern Oscillation sensitivity to cumulus entrainment in a coupled general circulation model

A series of 200 year long integrations is performed using the Geophysical Fluid Dynamics Laboratory CM2.1 by varying the Tokioka parameter, a minimum entrainment rate threshold in the cumulus parameterization. Changing the threshold alters both the tropical Pacific mean state and the El Nino-Southern Oscillation (ENSO) variability. Increasing the Tokioka parameter causes a basin-wide cooling in the tropical Pacific with the reduction of high clouds. The degree of cooling in the western part of the basin is bigger than that in the east. As a result, the east-west asymmetry in the tropical Pacific sea surface temperature (SST) decreases with increasing the Tokioka parameter. Accompanied with the reduced east-west SST asymmetry are the increase of mean precipitation over the eastern Pacific and the eastward shift of the atmospheric responses to the ENSO-related SST forcing. The eastward shifted wind stress anomaly associated with ENSO leads to the stronger ENSO variability. In this way the magnitude of ENSO simulated in this model increases with the Tokioka parameter. Implication of our results on the relationship between the tropical Pacific mean state and ENSO is discussed.