Retreating Monsoon (North-Western Monsoon)
The months of October and November are well known for retreating monsoons. By the end of September, the southwest monsoon becomes weak.
• The low-pressure trough of the Ganga plain starts moving southward in response to the southward march of the sun and that attribute to the weakening of southwest monsoon.
• The monsoon retreats from the western Rajasthan by the first week of September. It withdraws from Rajasthan, Gujarat, Western Ganga plain and the Central Highlands by the end of the month.
• By the beginning of October, the low pressure covers northern parts of the Bay of Bengal and by early November, it moves over Karnataka and Tamil Nadu. By the middle of December, the centre of low pressure is completely removed from the Peninsula.
Key Points: the retreating southwest monsoon
The retreating southwest monsoon season is marked by clear skies and rise in temperature. The land is still moist.
Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’. In the second half of October, the mercury begins to fall rapidly, particularly in northern India.
The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula. Here, October and November are the rainiest months of the year.
The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula.
These tropical cyclones are very destructive. The thickly populated deltas of the Godavari, Krishna and Kaveri are their preferred targets. Every year cyclones bring disaster here.
A few cyclonic storms also strike the coast of West Bengal, Bangladesh and Myanmar. A bulk of the rainfall of the Coromondal coast is derived from these depressions and cyclones. Such cyclonic storms are less frequent in the Arabian Sea.
Cyclones during the Retreating monsoon phase
Most severe and devastating tropical cyclones originate in the Indian seas especially in the Bay of Bengal.
The highest frequency of the cyclones is in the month of October and the first half of November.
More cyclones are born in October and then in November and more cyclones originate in the Bay of Bengal than in the Arabian Sea.
In October, the Cyclones of the Bay of Bengal originate between 8°N and 14°N.
Initially they move in a west or northwesterly direction, but many of them later recurve and move towards the north-east.
Near 55 per cent of the Bay storms cross or affect the Indian coast.
The area’s most vulnerable to these storms include the coastal belts of Tamil Nadu, Andhra Pradesh, Odisha and West Bengal.
Many of the cyclones which strike the eastern coast of India, south of 15°N latitude cross the southern Peninsula and enter Arabian Sea.
During this process, they may weaken, but on re-entry over the Arabian sea they intensify into cyclonic storms.
The storms of Arabian sea originate between 12°N and 17°N latitudes in October and between 8° N and 13° N latitudes in November.
Generally, they move away from the coast in a north-westerly direction. But about 25% of them later recurve northeast and strike the Maharashtra or Gujarat coast.
In north-west India the western disturbances produce clouding and light rainfall in the otherwise fine weather.
The precipitation is in the form of snow in higher reaches of Jammu and Kashmir, Himachal Pradesh and in Kumaon Hills.