Combined Impact of the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation on Indian summer monsoon rainfall

This study provides a relevant ground for acquiring deeper insight about the low-frequency variability of precipitation over India and its homogeneous monsoon regions (HMRs) under the combined interplay of both the Atlantic Multidecadal Oscillation (AMO) and the Interdecadal Pacific Oscillation (IPO). The spatial correlation analysis conveys that the influence of AMO and IPO on decadal-to-multidecadal variability of rainfall over India is not uniform, which is also supported by the results of regional correlation analysis. The percent of variance in the rainfall on decadal-to-multidecadal timescales that can be attributed to these oceanic indices is 88.3% for west central and 84.4% for northeast regions, which implies that the recent changes in rainfall over respective regions is mainly caused by internal natural variability. The opposite phases of AMO and IPO together modulates the rainfall over west central and northeast regions in an asymmetric manner. Based on the projected conditions of these oceanic indices the outlook for west central/northeast regions is fairly good/bad and these regions will experience above-/below-normal precipitation in the upcoming decade or two. The rainfall over northwest region is mainly influenced by the IPO and this region will also likely to receive above-normal precipitation in the upcoming decade or so due to the present cold phase of IPO. Wind circulation pattern reveals that during the warm phase of AMO the southwesterlies over Indian region are strengthened by the equatorial Atlantic winds coming through the equatorial Africa, while in the cold phase of IPO it gets strengthen by the easterlies from the equatorial Pacific. This study proposes that the natural variability on decadal-to-multidecadal timescales in the Pacific as well as in the extratropical North Atlantic Oceans extremely affects the climate of India. This raises the possibility that the regional SST variability of the Pacific and the extratropical North Atlantic may provide supplementary information that will improve monsoon predictions over India. Furthermore, the results presented here have important ramifications for the development of skillful decadal predictions at regional scales, which depends on better understanding of the linked mechanisms and in particular of the identification of the climate patterns that offer some degree of decadal predictability (for e.g., AMO and IPO/PDO). This will in turn help the society in improving the plans to mitigate the adverse effect of monsoon.