The withdrawal of monsoon marks the retreat or end of the Southwest Monsoon season in India. Unlike the abrupt onset, the withdrawal is gradual and systematic, starting from northwestern India and ending over the southern peninsula. The Indian Meteorological Department (IMD) uses specific meteorological indicators to define and declare the withdrawal phase.
Recognizing these indicators is essential for agriculture planning, water management, and disaster preparedness, especially in a country where monsoon rains govern economic and ecological well-being.
Key Indicators of Monsoon Withdrawal:
a) Cessation of Rainfall:
- A key sign is the significant and sustained reduction or complete cessation of rainfall over a region.
- Typically, when no substantial rain is recorded for 5 or more consecutive days, it indicates the retreat of monsoon from that area.
- The process starts from Rajasthan and northwest India, where dry conditions first appear.
b) Establishment of Anticyclonic Circulation:
- With the retreat of moist winds, dry northwesterly winds begin to dominate.
- An anticyclone (high-pressure area) forms in the lower troposphere (850 hPa level), especially over Rajasthan and adjoining regions.
- This shift in circulation patterns signifies subsiding air, which suppresses cloud formation and rainfall.
c) Decline in Humidity Levels:
- A sharp drop in relative humidity, especially in the afternoon (below 50–60%), is observed.
- The atmosphere becomes drier, reducing the chances of rain formation.
d) Clear Skies and Rising Temperatures:
- Clear or cloudless skies become common.
- Day temperatures rise, especially in north and northwest India, due to increased solar radiation.
- The contrast between day and night temperatures also increases.
e) Change in Wind Patterns:
- Winds shift from moist southwesterlies to dry north or northwesterlies.
- This reversal in wind direction is a major meteorological marker for the withdrawal phase.
f) Reduction in Cyclonic Activity:
- The frequency of monsoon depressions and lows from the Bay of Bengal declines.
- This indicates the easterly trough weakening, a key driver of monsoon rains.
g) Change in Upper-Air Features:
- The monsoon trough in the upper atmosphere becomes less defined or disintegrates.
- Wind shear zones weaken, and the tropical easterly jet stream shifts southward.
Sequence of Monsoon Withdrawal in India:
| Region | Typical Withdrawal Time |
|---|---|
| Rajasthan, NW India | Late September |
| Central India | Early October |
| Southern Peninsula & East | Mid to Late October |
| Entire Country (end phase) | By Mid-November |
The withdrawal phase of the monsoon is marked by a combination of meteorological indicators, such as cessation of rainfall, anticyclonic formation, falling humidity, shifting wind patterns, and clear skies. These indicators help meteorologists track the retreat, while farmers, planners, and policymakers use them to prepare for post-monsoon agricultural activities, water storage, and drought mitigation. Timely recognition of these signs is critical to managing the seasonal transitions in a climate-sensitive region like India.