Prof R K Ganjoo

The Indian Summer Monsoon (ISM), also known as Southwest Indian Monsoon or South Asia Monsoon, covers the countries of Bangladesh, Bhutan, India, Pakistan, Nepal and Sri Lanka of South Asia. The name ‘monsoon’ comes from the Arabic word ‘mausim’, meaning season. In India, the Indian Summer Monsoon envelops the Peninsular part, Indo-Gangetic Plains and part of northeast.

Besides South Asia, prominent monsoon also occurs in Africa, Australia and Central America. Commonly known as a rainy phase to a common man, after the long hot and dry season, is full of pack of surprises. The rainy phase may sometimes be a short time heavy or a dry phase. Any variability in Indian Summer Monsoon is likely to affect the availability of fresh water, agriculture, and livelihood of more than a billion people across South Asia.

The ISM (June-September) accounts for about 75% of rainfall in the subcontinent driving the agriculture and economy. The Council on Energy, Environment and Water (CEEW), New Delhi in one of its reports highlighted the erratic behaviour of ISM to the extent that adjacent tehsils are experiencing different trends in rainfall.

The arid states of India, namely Rajasthan, Gujarat and central Maharashtra, experienced 10%-30% more ISM rains in the 2012- 2022 decade than the preceding three decade of 1982-2011. About 11% of tehsils lying in the agriculturally important Indo-Gangetic plains, the fragile Indian Himalayan region and the northeastern states have received a decrease of more than 10% in ISM rainfall.

Geological studies infer the uplift of the Himalaya-Tibet Plateau at around 50 million years ago after India Plate collided with the Eurasia Plate. The scientific proxy evidence further add to the onset of monsoon around 20 million years ago when a large part of Himalaya and Tibet Plateau gained sufficient elevation to approximately 3500 meters above sea level.

The intensification of ISM becomes evident at around 7 million years ago when most of the Himalaya- Tibet Plateau gained the present elevation and served as major driver in governing the variability in ISM rainfall. The variability in ISM rainfall is geologically well documented for past about 12,000 years (Holocene period) in the marine sediments of Bay of Bengal and Arabian Sea, and continental deposits of lakes and caves.

The ISM strengthened during the Greenlandian age (11,700-8,300 years before the present), followed by decrease in precipitation during the Northgrippian age (8,300 – 4,200 years before the present). The Indian subcontinent witnessed a protracted dry event during the Meghalayan age beginning at ~4,200 years before the present and ending at ~3,400 years before the present.

Other significant events of the Meghalayan age include the Medieval Climate Anomaly (MCA) and the Current Warm Period (CWP) showing that ISM is interrupted with low precipitation in the Indian subcontinent coinciding the Little Ice Age (LIA), a cold phase across Northern Hemisphere (European Alps, New Zealand, Alaska, and the southern Andes) from early 14th century to mid-19th century.

Sharp cooling in the North Atlantic resulted in abrupt reduction in ISM intensity at ?8,200 years before the present, indicating a strong connection between ISM and North Atlantic climate.

The El Niño phase is shown to weaken the ISM while a La Niña phase strengthens the ISM. The positive phase of IOD strengthens the ISM and the negative phase weakens it, showing a relationship with the ISM opposite to that of ENSO. Signatures of Bond Events (BE) are registered in the North Atlantic and Mediterranean regions.

However, a close connection and interplay of North Atlantic Oscillation (NAO) and ENSO in modulating the ISM intensities is well studied with evidence of ISM weakening with increased winter precipitation during BE-0 and possibly during BE-2.

The climate change over the Arctic region and North Atlantic shows a mechanistic link with the ISM during the Holocene. The millennial-scale variability in the ISM is associated with the Heinrich and Bond events.

The cooling in the Arctic Sea, ice expansion in the North Atlantic, and weakening of the Atlantic overturning meridional oscillations due to high freshwater flux and ice rafting in the North Atlantic caused weak ISM precipitation over the south and southeast Asia.

Further, Indian Summer Monsoon describes a seasonal change in the atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ). The ITCZ, earlier known as Intertropical Front, also commonly known as Doldrums or Calms represents the area of the convergence of northeast and southeast trade winds.

The ITCZ encircles the planet Earth near geographic equator, though the specific position varies seasonally corresponding with the location of thermal equator.

Large scale atmospheric, oceanic and coupled climate phenomena act as drivers in influencing the spatio-temporal complexity of ISM rainfall variability.

The interaction of monsoon depressions, Madden-Julian Oscillation, El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Indian Ocean capacitor, Eurasian snow cover changes, the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation, and anthropogenic are considered some of the major drivers in causing the spatio-temporal complexities in ISM.

The ISM rainfall undergoes variability in different time scales including sub-monthly, intra-seasonal, inter-annual, and inter-decadal that is tele-connected to the Eurasian winter snow cover, land surface temperature, equatorial Pacific Sea Surface Temperature (SST), North Atlantic SST, equatorial Indian Ocean SST, winds and convection, equatorial Atlantic SST, Himalaya-Tibet snow cover, etc. I

nter-annual time scale variability in rainfall is also linked to many climate modes, such as El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Indian Ocean Dipole (IOD), equatorial Indian Ocean Oscillation (EQUINOO), etc.

Despite increased scholarship on climate change in India indicating the growing understanding and interest in the subject, especially the behavioural pattern of Indian Summer Monsoon, prediction of the intensity of ISM is a challenge to the meteorologists.

The predominance of climate related uncertainties across different temporal and spatial scales interferes with the validation of the prediction of ISM. The intersection of ISM with other drivers of change, thus, present a radical uncertainty in drawing accurate prediction of the intensity of ISM.