The Indian summer monsoon is not a single mechanism but a coupled system of ocean, atmosphere and orography. Examine the role of ENSO, the Indian Ocean Dipole and the Tibetan Plateau in shaping its variability.

Introduction:

The Indian southwest monsoon, from June to September, delivers about 75 per cent of the country's annual rainfall against an IMD Long Period Average of 87 cm. As P.R. Pisharoty and later T.N. Krishnamurthy argued, it is a coupled ocean-atmosphere-orography system, not a single mechanism — and forecasting must treat it as such.

Ocean-atmosphere coupling — ENSO:

El Niño warming in the central and eastern equatorial Pacific suppresses Indian rainfall by shifting the Walker circulation eastward; deficit monsoons of 2015 and 2023 illustrate the link. La Niña, the opposite cool Pacific phase, typically enhances rainfall — the 2020-22 "triple-dip" produced three consecutive surplus monsoons across India.

Indian Ocean Dipole:

Identified by N.H. Saji and colleagues in Nature (1999), a positive IOD — warm western and cool eastern Indian Ocean — can offset an El Niño's impact. The 1997 monsoon was normal despite a strong Pacific El Niño, and 2019 was a near-surplus year against a weak El Niño backdrop, both attributed to a strong positive IOD.

Orography and circulation:

Tibetan Plateau heating drives the Tibetan High and the Tropical Easterly Jet over peninsular India; the Mascarene High over the southern Indian Ocean and the Findlater (Somali) low-level jet supply moisture across the Arabian Sea. The Western Disturbances of the upper-level westerlies govern winter precipitation across northwest India.

Conclusion:

IMD's Monsoon Mission, operational since 2012, runs dynamical models incorporating these teleconnections to issue Long Range Forecasts. Treating the monsoon as a coupled system, not as a date on the calendar, is now the operational consensus.