The Decadal ENSO Variability in a Hybrid Coupled Model

Abstract

In this study, the characteristics of decadal ENSO variability in a long (100-yr) simulation of a hybrid coupled model (HCM) are investigated. To exclude the possibility that the decadal El Nino-Southern Oscillation (ENSO) variability is forced by midlatitude ocean variability, the atmospheric component model is coupled to an ocean model that is restricted to the tropical Pacific. The sea surface temperature anomaly (SSTA) variability from a 100-yr run of HCM compares favorably to the observations and shows fluctuations in the ENSO period and amplitude on decadal time scales. The spatial structure of the interannual ENSO variability in the HCM is similar to the observations, whereas on decadal time scales the spatial structure differs significantly from the observations suggesting the importance of extratropical oceanic processes or deficiencies in the model. The decadal mean of both the SSTA and the wind stress anomaly is too equatorially confined in the HCM compared to the observations. Simple coupled model experiments are performed to determine the source of decadal ENSO variability in the HCM. These experiments indicate that the slow time-scale variations in the mean state have little effect on the character of the ENSO variability. The decadal modulation of ENSO is primarily related to the details of atmospheric noise processes.