Abstract
El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) tend to exert an offsetting impact on Indian summer monsoon rainfall (ISMR), with an El Niño event tending to lower, whereas a positive IOD tending to increase ISMR. Simulation of these relationships in Phase Five of the Coupled Model Intercomparison Project has not been fully assessed, nor is their impact on the response of ISMR to greenhouse warming. Here we show that the majority of models simulate an unrealistic present-day IOD-ISMR correlation due to an overly strong control by ENSO. As such, a positive IOD is associated with an ISMR reduction in the simulated present-day climate. This unrealistic present-day correlation is relevant to future ISMR projection, inducing an underestimation in the projected ISMR increase. Thus uncertainties in ISMR projection can be in part induced by present-day simulation of ENSO, the IOD, their relationship and their rainfall correlations.
Highlights
Indian summer monsoon rainfall (ISMR) from June to September (JJAS) sources its water from vapour transport by southwesterlies, which supports convection over the Indian subcontinent[1, 2]
The 1997 El Niño event was strongest of the 20th century, which alone would lead to an ISMR reduction, but a moderate ISMR increase occurred; this is because El Niño-Southern Oscillation (ENSO)’s influence on ISMR was overwhelmed by the concurrent 1997 extreme positive Indian Ocean Dipole (IOD) event[21], usually considered as a secondary influence on the ISMR13
We assessed the ISMR correlation with ENSO and the IOD in climate models and found that there is an overly strong IOD-ISMR negative correlation compared to observations
Summary
Indian summer monsoon rainfall (ISMR) from June to September (JJAS) sources its water from vapour transport by southwesterlies, which supports convection over the Indian subcontinent[1, 2]. 85% of the annual rainfall total is contributed by ISMR, and year-to-year variations of ISMR often manifest as floods and droughts, causing damage to rain-fed agriculture and affecting the livelihood of more than 1/6th of the world’s population[3,4,5] These fluctuations are mainly attributed to slow-varying sea surface temperature (SST) in the tropics[6]. CMIP5 models project an increase in ISMR in a warmer climate in terms of a multi-model ensemble mean (MMEM) with a reasonably strong inter-model consensus[16, 17]. These models suffer from many biases and the projections have a large inter-model spread, undermining the confidence in the projections[17]. We assess whether future ISMR projections are contingent upon the realism of the simulated IOD correlation with ISMR in the present-day climate
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