Changes in ENSO in Response to Greenhouse Warming as Simulated by the CCSR/NIES/FRCGC Coupled GCM

A coupled general circulation model, MIROC3.2, was used to investigate the impacts of global warming on the El Nino-Southern Oscillation (ENSO) variability. The model has relatively fine resolution which captures the atmosphere and ocean dynamics well. The model shows an overall warm trend in the tropical Pacific under the greenhouse warming, particularly in the central equatorial Pacific. Superimposed on such trend is the increase in amplitude of interannual ENSO variability. A sensitivity analysis shows that ocean, rather than the atmosphere, is responsible for the strengthening. The greenhouse warming tends to increase the surface waters more, leading to strengthened subsurface vertical stratification in the equatorial ocean. The strengthening of the subsurface stratification is greater in the western half of the equatorial Pacific where the unit amount of surface wind stress change tends to accompany in-situ subsurface temperature anomalies greater under the warmed climate. This as well as the change of the mean meridional temperature gradient appears to be responsible for the increase in ENSO amplitude.