Interbasin coupling between the tropical Indian and Pacific Ocean on interannual timescale: observation and CMIP5 reproduction
The interaction of El Nino–Southern Oscillation (ENSO) with tropical Indian Ocean’s two major modes, i.e. Indian Ocean Dipole (IOD) and Indian Ocean basinwide mode (IOBM), is of great importance to understanding global climate variability. Using observational data for the last 50 years and the phase five of Coupled Model Intercomparison Project (CMIP5) historical simulation for the last 100 years, this study investigates the role of interbasin coupling between the Indian and Pacific Ocean on El Nino evolution. Analyses suggest that the combined effect of the IOD during the developing El Nino phase and the IOBM during the decaying phase plays a critical role in leading a fast transition from El Nino to La Nina. In particular, a faster IOD termination and predominant IOBM in the El Nino winter result in prevailing easterly wind anomalies through the eastern Indian Ocean to the western Pacific, countervailing the IOD-related westerly wind anomalies over the western Pacific. The significant easterly wind anomalies then contribute to the maintenance of the western North Pacific subtropical high anomalies until the El Nino decaying summer, consequently facilitating rapid termination of El Nino and transition to La Nina. Meanwhile, the sole effect of either IOD or IOBM causes a slow decay of El Nino. The 20 CMIP5 models generally capture the role of interbasin coupling on the El Nino evolution, in spite of models’ common deficiencies in simulating the easterly wind anomalies after decay of IOD. The late IOD demise might cause weaker El Nino phase transition in models due to the longer-lasting destructive interference between IOD- and IOBM-related western Pacific wind anomalies. This study indicates that challenges still remain in better simulations of the various aspects of interbasin Indo-Pacific coupling and then a diversity of the ENSO life cycle.