By late September and early October the circulation pattern near the Himalayas returns to its winter regime. The low pressure centre in the northwestern part of India appears to be broken up. Now, the low pressure centre becomes weaker and weaker, and ultimately shifts to the equatorial region. The monsoons begin to retreat.
Rainfall is the criterion adopted by the Meteorological Department of India for fixing the dates of onset and withdrawal of southwest monsoon over different parts of India. The Department chooses the middle dates of the five-day periods during which the characteristic rise or fall occurs in average rainfall.
By the end of September the rainfall starts decreasing in the northern plain of India until it ceases by the month of October. However, in the southern part of Indian Peninsula it continues to rain until the middle of December.
It may be pointed out that, unlike the sudden burst of monsoon, the retreat of monsoon is gradual. Now, the anticyclonic conditions take the place of cyclonic conditions in the northern plain. The monsoon retreats to the south giving place to cold and dry winds of winter monsoon.
By about September 15, the monsoon withdraws from the Punjab. The approximate dates of the retreat of monsoon from Uttar Pradesh and Bengal are October 1 and October 15 respectively. The earlier retreat of southwest monsoon is always viewed with great concern, for it dries up the standing crops which need a continuance of rainfall.
It is well to remember that the retreating southwest monsoons give rainfall to Karnataka in the months of October and November. Madras receives about 55 cm of rainfall in the months of October and November by these winds coming from the northern margin of cyclonic circulation.
At this period some depressions develop over the Bay of Bengal and the Arabian Sea. Sometimes they attain the hurricane intensity and produce occasional heavy rain and strong winds. Thus, winter precipitation of Southern India is due to the retreating southwest monsoon.
The northeast monsoon is yet to set in. During the period of the withdrawal of southwest monsoon, which lasts from about October to December, the winds gradually start blowing from westerly directions in the northern part of the subcontinent.
In the south the winds gradually change to northeasterly direction. In this way the winter monsoon develops gradually over the whole of the Indian sub-continent.