What factors affect the Indian summer monsoon?

Subject: Geography

Section: Climatology

Context- Large-scale, global factors as well as short-term, local factors affect the summer monsoon.

Concept-

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.
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.

Indian Ocean Dipole: Equatorial Indian Ocean Oscillation

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.
Positive IOD and EQUINOO events are associated with more rainfall as these events increase moisture transport from the southeastern parts of the Indian Ocean.

Atlantic sea surface temperature variability:

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 effect of this phenomenon is known as the Atlantic Niño.

Mascarene High:

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.

How does irrigation affect the Indian summer monsoon?

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 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.

Other short term factors affecting Monsoon Rainfall:

Aerosols and dust are other local factors that have been shown to affect the monsoon rainfall in India.
- 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 scale.
- 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 towards the Indian region.