- The Indian summer monsoon usually occurs from the end of May to September in India.
- Large-scale, global factors as well as short-term, local factors affect the summer monsoon.
- The amount and pattern of rainfall during the summer monsoon, impacts the livelihoods of farmers and the nation’s GDP.
The Indian summer monsoon is a vast weather event that is crucial to the 1.39 billion people that live in India, as most of the rainfall (70-90%) that the country receives, is during this period, May-end to September. The amount of rainfall that India receives during the summer monsoon varies every year. Even though this variation is only about 10% above or below the predicted average, it has a huge impact on the economy of the country. This is because the livelihoods of millions of farmers in India, and the nation’s GDP rests on the amount and pattern of rainfall during the summer monsoon. Therefore, predictions of the strength of the monsoon every year by the Indian Meteorological Department (IMD) are of great importance.
Ongoing research over the last 30-50 years has identified many factors that affect the Indian summer monsoon. These range from long-term, large-scale global effects such as the El Niño, La Niña, Indian Ocean Dipole, and more, to more short-term and local effects such as dust clouds and even patterns of irrigation.
What are El Niño and La Niña? How do they affect the Indian monsoon?
‘El Niño’, meaning ‘the boy child’ refers to a large-scale warming of the sea surface, along the central and east-central Pacific Ocean around the Equator. It is named in Spanish as a reference to the ‘Christ child’ as it appears around Christmas time in December, as a warm ocean current along the South American coast. The ‘La Niña’ meaning the ‘the girl child’ refers to the opposite conditions – a cooling of the sea surface in the areas where the El Niño occurs. The two phenomena occur cyclically to form the El Niño Southern Oscillation or the ENSO.
The warming in the tropical Pacific Ocean because of El Niño weakens the southeast trade winds flowing to the intertropical convergence zone over India. Since these winds are the main driving force of the Indian summer monsoon, El Niño events are associated with weak monsoons and lower than average rainfall. The La Niña has the opposite effect to the El Niño and is responsible for stronger monsoons and above-average rainfall.
Based on rainfall trends in India over 132 years, it is clear that severe droughts in India have always been in El Niño years, though the opposite is not true. The location of the El Niño event also influences its effects on the Indian monsoon – warming in the central Pacific Ocean affects the Indian monsoon more than if the warming is in the eastern Pacific Ocean.
Overall, it is estimated that about 30% of the yearly variability in India’s summer monsoon rainfall is attributable to the ENSO effect – the rest of the variability rests on other factors such as the IOD (Indian Ocean Dipole), EQUINOO (Equatorial Indian Ocean Oscillation), Atlantic sea surface temperature (SST) variability, and the Mascarene High.
Indian Ocean Dipole, Equatorial Indian Ocean Oscillation, Atlantic sea surface temperature variability, Mascarene High – what do these terms mean?
The Indian Ocean Dipole or IOD is caused by alternate warming and cooling in the Indian Ocean along the Equator from west to east, much like the ENSO. The Equatorial Indian Ocean Oscillation (EQUINOO) is thought to arise as an effect of the IOD and refers to increased and decreased cloud formation between the western and eastern equatorial Indian Ocean. Although the IOD was discovered only in 1999, and the EQUINOO in 2002, both have been recognised as important modulators of the Indian summer monsoon. Positive IOD and EQUINOO events are associated with more rainfall as these events increase moisture transport from the southeastern parts of the Indian Ocean.
The Atlantic SST variability affects the Indian summer monsoon in the same way that the ENSO does. A warming of the surface of the Atlantic Ocean weakens the monsoon, just as cooling of the ocean’s surface has the opposite effect. The effects of this phenomenon, also known as the Atlantic Niño, on the Indian summer monsoon, is thought to be mediated through perturbations in the jet streams above India.
The Mascarene High is a semi-permanent high-pressure zone in the south Indian Ocean, about 4,000 km from India, near the Mascarene Islands. The Mascarene High begins developing in mid-April and is a major factor in driving the circulation between the northern and southern hemispheres that powers the summer monsoon winds towards the Indian subcontinent from the Indian Ocean.
Recent work published in the journal Scientific Reports in 2021 shows that the yearly variations in the Indian monsoon rainfall are affected by these large-scale factors via fluxes in incoming and outgoing moisture through surface-level winds.
“The mechanism we have proposed provides a physical link between changes in high- and low-pressure patterns, wind circulation, and the moisture in the atmosphere. We also show that different tropical climate patterns such as the ENSO, IOD, EQUINOO, etc. modulate these factors, and hence the Indian summer monsoon,” says Arindam Chakraborty, a professor from the Centre for Atmospheric and Oceanic Sciences (CAOS) at the Indian Institute of Science, Bangalore and co-author of the study.
How does irrigation affect the Indian summer monsoon?
One of the rather surprising local factors that affects the Indian monsoon, is irrigation. According to a 2019 study in the journal Climate Dynamics, the trend of decreasing rainfall over the Indo-Gangetic plains could be due to the extensive irrigation in this area. The study finds that winter irrigation (November-March) actually strengthens the monsoon rains over the region for the following year and also reduces intra-seasonal variations in rainfall. However, with year-round irrigation, there is a noticeable decrease in the summer monsoon rainfall (June-September).
“This is likely because irrigation affects soil moisture levels and temperature simultaneously, which affects atmospheric stability. Since the irrigation is over vast tracts of land, these changes shift the moisture convergence zone to the south, during the active phase of the monsoon,” says Chakraborty, whose team investigated this phenomenon.
The link between irrigation and changes in the monsoonal rainfall patterns was also identified in another study in 2019 from the Department of Civil Engineering at IIT Bombay. This work shows that increased irrigation in northern India shifts monsoonal rainfall in September further towards the northwest of India, while also affecting rainfall patterns (increases in extreme rainfall events) in central India.
Aerosols and dust are other local factors that have been shown to affect the monsoon rainfall in India. In a recent study in 2022, researchers at IIT Bhubaneswar have shown that dust transported to the Arabian sea from the Middle-Eastern deserts (the Sahara and the Sinai) could increase rainfall in India and south Asia over short time scales of one or two weeks. In a similar study in 2014, scientists showed that dust aerosols likely heat up the atmosphere over north Africa and west Asia, which increases the flow of moisture over India. This results in rainfall, usually within a week of the event, over central India.
“This warming induced by dust (from the Middle-Eastern deserts) over the Arabian Sea, acts as a source of energy to speed up the monsoon circulation (winds and moisture) towards the Indian region,” says V. Vinoj, a Professor at the School of Earth, Ocean and Climate Science in IIT Bhubaneswar, who was involved in both studies. “Our work also shows that this relationship between the dust and the Indian monsoon is stronger during drought years associated with El Niño. In addition, we find that dust induced rainfall enhancement is widespread across the whole south Asian monsoon domain, often occurring as a pulse that triggers short term increased rainfall in an otherwise dry situation. We need to begin tracking dust storms to understand how they affect the Indian monsoons, because it’s very clear that are having a critical effect on the rainfall in certain regions like central India,” he adds.
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Banner image: Monsoon in Chennai. Dust induced rainfall enhancement is widespread across the whole south Asian monsoon domain. Photo by Vinoth Chandar/Wikimedia Commons.