The Indian Monsoon system is one of the most important climatic phenomena affecting the Indian subcontinent. It is influenced by pressure differences, seasonal wind reversal, ocean-atmosphere interactions, jet streams, Tibetan Plateau heating, depressions, and oceanic phenomena such as the Indian Ocean Dipole (IOD). The monsoon system is highly dynamic and pulsating in nature and plays a major role in determining India’s rainfall distribution.
The term “Monsoon” has been derived from the Arabic word “Mousim” or the Malayan word “Monsin”, meaning season.The monsoon is characterized by a seasonal reversal of winds:
Unlike the Trade Winds, monsoon winds are not steady. They are pulsating in nature and are influenced by various atmospheric conditions encountered over warm tropical seas.The Indian monsoon system has two major branches:
| Branch | Region Covered |
|---|---|
| Arabian Sea Branch | Western Coast and Peninsular India |
| Bay of Bengal Branch | Eastern and Northeastern India |
The Arabian Sea branch reaches Mumbai around 10 June, while the Bay of Bengal branch advances rapidly and reaches Assam in the first week of June.
The South-West Monsoon Season extends from June to mid-September and is also known as the Hot-Wet Season.One of its most important features is the sudden onset of rainfall. With the arrival of monsoon:
The normal date of arrival of the monsoon:
| Region | Normal Date |
|---|---|
| Andaman & Nicobar Islands | 20 May |
| Kerala (mainland entry) | 1 June |
The advance of monsoon is faster in the Bay of Bengal branch compared to the Arabian Sea branch.
During summer, intense heating over Northwest India creates a low-pressure area.Atmospheric pressure increases southward, generating a pressure gradient that attracts moisture-laden winds from surrounding oceans.Over Peninsular India, winds blow from the southwest to northeast direction from both:
However, over the Indo-Gangetic Plain, the direction changes from east to west.
A major portion of South-West Monsoon rainfall is caused by depressions originating over:
About 3–4 depressions form every month between June and September.These depressions move inland through major river deltas because they require moisture to survive. Important river deltas associated with their movement include:
These systems cause heavy rainfall over adjoining regions.Some depressions also develop over land areas.
Trade Winds are also known as Easterlies because they predominantly blow from the east.They are the prevailing winds that blow from:
Trade Winds are:
Hence, unlike monsoon winds, they do not show pulsating behavior.
Trade winds blow from the north-east direction.
Trade winds blow from the south-east direction.This directional change occurs due to the Coriolis Effect.
Trade winds have historically been important for oceanic navigation. Sailing ships used them extensively for long-distance maritime travel.They are generally stronger:
Since they blow predominantly from the east, they are also called Tropical Easterlies.
Jet Streams are narrow bands of high-speed winds located in the upper troposphere at an altitude of approximately 9–16 km.They generally flow from west to east due to the Coriolis Effect.Jet streams originate due to:
They are characterized by:
| Jet Stream | Location | Characteristics |
|---|---|---|
| Polar Front Jet Stream (PFJ) | Around 60° latitude | Stronger during winter |
| Subtropical Westerly Jet Stream (STJ) | Around 27°–30° latitude | Influences Indian monsoon |
| Easterly Jet Stream | Southeast Asia, India, Africa during summer | Associated with warm air to north and cold air to south |
During winter, the Subtropical Westerly Jet Stream (STJ) blows south of the Himalayas.This:
The Polar Front Jet Stream guides Western Disturbances into India.These western disturbances originate in the Mediterranean region and bring:
to north and north-western India.
In summer, the Subtropical Westerly Jet Stream shifts north of the Himalayas due to the apparent movement of the Sun.Its northward shift allows moisture-bearing monsoon winds to enter India.The weakening and displacement of the STJ are therefore closely associated with the onset of the Indian Monsoon.
The South-West Monsoon is triggered by the formation of a low-pressure area over the Tibetan Plateau during summer.Intense summer heating over Tibet causes air to rise, creating a strong low-pressure zone.This low pressure draws moist south-western winds from the Indian Ocean towards the Indian subcontinent, resulting in monsoon rainfall.Thus, heating of the Tibetan Plateau is one of the key drivers of monsoon circulation.
Studies have shown that the strength of cloud bands plays an important role in determining:
Cloud bands are nearly continuous cloud formations associated with monsoon circulation.
The Indian Ocean Dipole (IOD) is an ocean-atmosphere coupled phenomenon characterized by differences in sea surface temperature (SST) between:
It significantly influences the Indian Monsoon along with:
IOD develops in the equatorial Indian Ocean from April–May and generally peaks in October.
During neutral conditions:
During Positive IOD:
This creates:
A Positive IOD is generally considered beneficial for the Indian Monsoon and strengthens monsoonal flow.
During Negative IOD:
This phase obstructs the progression of the Indian Monsoon over India.
The Polar Jet Stream can influence extreme weather events.When it dips southward, it can bring:
over North India.Jet stream shifts also influence:
Global warming is increasingly altering jet stream behavior, leading to erratic climatic conditions worldwide.
The Indian Monsoon is a highly complex climatic system influenced by atmospheric circulation, pressure gradients, jet streams, Tibetan Plateau heating, depressions, cloud bands, and ocean-atmosphere interactions like the Indian Ocean Dipole. The interaction of these factors determines the onset, intensity, progression, and variability of rainfall over the Indian subcontinent.