Impact of the Indian Ocean on ENSO variability in a hybrid coupled model

This study examines the impact of the Indian Ocean on El Nino and the Southern Oscillation (ENSO) variability through a series of numerical experiments with a hybrid coupled model. In the control run, an atmospheric general circulation model (AGCM) is coupled to the Zebiak-Cane simple ocean model in the tropical Pacific. Outside the tropical Pacific climatological sea surface temperatures are prescribed in the control simulation. In the first experiment, sea surface temperature anomalies (SSTAs) in the Indian Ocean are statistically predicted based on the state of the Pacific, and used to force the atmosphere. In the second experiment, a slab thermodynamic mixed layer model is coupled to the AGCM in the Indian Ocean. The Indian Ocean modifies the ENSO frequency via interactions with the Indian monsoon, but only when air-sea interactions in the Indian Ocean are included in the experimental design (i.e. the second experiment). The inclusion of the Indian Ocean, however, has little impact on the ENSO amplitude, which is at variance with other coupled simulations, suggesting that some missing dynamics or physics (i.e. Indian Ocean dynamics, Indonesian Throughtlow, etc.) may play an important role. The Indian summer monsoon is more tightly coupled to ENSO in the second experiment than in the control run and the first experiment. The power spectrum of the Indian monsoon rainfall has a significant biennial timescale of around 20-30 months in the second experiment, which may enhance the biennial time-scale of ENSO variability through a shift of the horizontal structure of zonal wind stress variability in the central equatorial Pacific. Copyright (c) 2007 Royal Meteorological Society.