Joint effect of the Indian Ocean Dipole and the Silk Road Pattern on Indian rainfall during summer to autumn transition

Abstract

Variations in Indian summer rainfall pose a threat to water security that can cause widespread socio-economic impacts. Summer rainfall is affected by both tropical and extratropical circulation anomalies owing to India's particular geographical location. Using observational, reanalysis, and CMIP6 model datasets, this study reveals the individual and joint effects of the Indian Ocean Dipole (IOD) and Silk Road Pattern (SRP) on the interannual variability of Indian rainfall during the summer to autumn transition (i.e., September). The results indicate that there is a consistent positive correlation between the IOD and SRP and the second mode of the Indian rainfall anomaly, which features a north–south inverse dipole pattern. During the positive phase of the IOD, the zonal sea surface temperature anomaly (SSTA) gradient of warm west and cold east in the equatorial Indian Ocean drives a Gill-type response, with an anticyclonic anomaly over southern India. This suppresses local convection and results in precipitation deficit across southern India. The SRP exhibits a baroclinic structure over west-central Asia, with an upper (lower) tropospheric anticyclonic (cyclonic) anomaly over west-central Asia (northern India) during its positive phase. This lower-level cyclonic anomaly is drastically amplified under the easterly vertical shear condition, favoring stronger moisture convergence and increased precipitation over northern India. The joint effect of the IOD and SRP explains nearly 29% of the Indian rainfall anomaly, higher than the individual contributions of the IOD (17%) or SRP (12%). These results highlight the need to consider the joint effects of tropical and mid-latitude anomalous drivers on precipitation patterns and provide the basis for more accurate simulation and prediction of Indian rainfall.