Quasi-asymmetric response of the Indian summer monsoon rainfall to opposite phases of the IOD

Swadhin K Behera,J V Ratnam

doi:10.1038/s41598-017-18396-6

Swadhin K Behera, J V Ratnam

Open Access

https://doi.org/10.1038/s41598-017-18396-6

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Abstract

The El Niño/Southern Oscillation has been traditionally linked to the extremes in the Indian summer monsoon rainfall (ISMR) affecting more than a billion people in the region. This trans-oceanic influence is seen to be moderated by the Indian Ocean Dipole (IOD) phenomenon in recent decades. In the presence of a positive IOD (pIOD), the otherwise subdued ISMR in an El Niño year remains close to normal even in the face of record breaking El Niños. While this general influence of pIOD on ISMR is understood, the influence of negative IOD (nIOD) on ISMR is not yet recognized. In this study, it is revealed that those opposite phases of IOD are associated with distinct regional asymmetries in rainfall anomalies. The pIOD is associated with a tripolar pattern in rainfall anomalies with above normal rainfall in central parts of India and below normal rainfall to north and south of it. Conversely, the nIOD is associated with a zonal dipole having above (below) normal rainfall on the western (eastern) half of the country. This spatial quasi-asymmetry arises from the differences in the atmospheric responses and the associated differences in moisture transports to the region during contrasting phases of the IOD.

Highlights

The summer monsoon variability over the Indian subcontinent is extensively studied owing to its strong impacts on the socio-economic conditions involving a large human population in the region
The Indian summer monsoon rainfall (ISMR) variability related to opposite phases of the Indian Ocean Dipole (IOD) is investigated by selecting positive IOD (pIOD) and negative IOD events based on dipole mode index (DMI) of June-September (Fig. 1)
This is counter intuitive, as one would expect large-scale rainfall deficiencies in response to negative IOD (nIOD) (Fig. 2b), in a symmetric sense, since previous studies have documented a positive rainfall response arising from the pIOD

Summary

Introduction

The summer monsoon variability over the Indian subcontinent is extensively studied owing to its strong impacts on the socio-economic conditions involving a large human population in the region. The IOD phenomenon modulates the meridional circulation in the region by inducing anomalous convergence patterns over the Bay of Bengal and strengthening of the monsoon trough over central India as seen in the typical IOD year of 199411. This relationship between IOD and Indian summer monsoon rainfall (ISMR) was firmly established[12,13] with long time-series of observational data and atmospheric model experiments.

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