Tropical Indian Ocean and ENSO relationships in a changed climate

We explore the current (1958-2005 period) and near future (2006-2050 period) teleconnections between El Nino Southern Oscillation (ENSO), Indian Ocean Basin Mode (IOBM), and Indian Ocean Dipole (IOD) as simulated in historical and Representative Concentration Pathway (RCP8.5) simulations of 32 coupled models that participated in the phase five of Coupled Model Intercomparison Project (CMIP5). A set of 16 CMIP5 models out of 32 models, which perform best to simulate tropical climate variability in recent decades, is first selected using a robust method based on the Empirical Orthogonal Function analysis for detailed analysis. Most of these models show modest capability in reproducing the seasonal cycle of ENSO types in the current period. Further, amplitude of Indian Ocean (IO) modes is overestimated by the 16 models along with large inter-model spread. Based on these results, a subset of 9 models is formed, which simulate a realistic seasonal phase-locking of ENSO for a robust assessment of future teleconnections. No significant change in El Nino amplitude is detected in near future. However, the IOBM is projected to be weaker during late spring and early summer. The IOD is projected to be stronger during boreal summer in the future relative to the current period. We also investigate if there are any changes from historical to RCP 8.5 simulations in the strength of the IO negative feedback on ENSO with a multiple linear regression approach. The IO negative feedback strengthens significantly in the RCP8.5 scenario due to the increasing role of IOBM in speeding the transition from El Nino to La Nina, despite its reduction of amplitude. In contrast, IOD loses its predictive value in the future projections.

Automated summary

This study explores teleconnections between ENSO, IOBM, and IOD in historical and future climate simulations from 32 CMIP5 models. Findings suggest that most models have moderate capability in reproducing the seasonal cycle of ENSO, with some overestimating the amplitude of IO modes and a projected increase in IOD strength during boreal summer in the future.