The Hindu Business Line : Stage being set for monsoon exit from West

Thiruvananthapuram, Sept. 4 There are early indications of a large anti-cyclonic anomaly developing in the lower troposphere over west Rajasthan and adjoining north Gujarat from September 8 to trigger the withdrawal of monsoon.

Anti-cyclonic (high pressure area with clear skies) circulations do not allow for ‘lifting’ motion of air, which is the hallmark of cyclonic circulations (low pressure, aiding ascending motion and cloud development).

The anti-cyclone promotes ‘subsidence’ (sit-in) of air, and therefore, suppression of rainfall in the lower and middle tropospheric levels. This is what forces the withdrawal, first from west Rajasthan and adjoining Gujarat.

Dr Akhilesh Gupta, Senior Advisor to Department of Science and Technology and an expert meteorologist, told Business Line that these need to be watched and analysed to arrive at a final conclusion.

DELAYED WITHDRAWAL

The withdrawal process normally starts around September 1, but has been delayed in view of the arrival of a mid-latitude westerly trough (western disturbance) from across the border signalling unsettled weather for northwest India, of which Rajasthan is a part.

This trough would fetch scattered rains especially over Punjab, Haryana (including Delhi), Jammu and Kashmir, and Himachal Pradesh on Friday and Saturday to the accompaniment of isolated thunder activity, Dr Gupta said, quoting numerical prediction models.

The monsoon is currently in weak phase with the seasonal trough running close to the foothills of the Himalayas for the past one week or so. As a result, fairly widespread rainfall is forecast for Arunachal Pradesh, Assam, Meghalaya, Sub-Himalayan West Bengal and Sikkim and scattered to fairly widespread rainfall activity over south peninsular India, especially Tamil Nadu.

REVIVAL IN SOUTH

Beginning Friday, a temporary revival of monsoon is likely in peninsular India. The revival is associated with strengthening of the westerly flows over the Arabian Sea and the Bay of Bengal. A weak ‘low’ is indicated for the Bay off the north Andhra coast, but would be part of the larger shear zone of monsoon turbulence stretching across the peninsula.

This low would not have the strength to penetrate the hinterland during this phase of the monsoon. But its very presence ensures that the shear zone is kept in place to drive rains into peninsular neighbourhood.

According to Dr Gupta, heavy to very heavy rains are indicated northward from Kerala. There is a possibility that Konkan-Mumbai and South Gujarat may also receive good precipitation during this phase.

The rain belt may cover south peninsular India comprising Kerala, Karnataka, Andhra Pradesh and many parts of Maharashtra, including Konkan, Madhya Maharashtra and Marathawada, Goa, south Gujarat and parts of Orissa, Chhattisgarh and South Madhya Pradesh.

MAY NOT LAST

The current Madden Julian Oscillation (MJO) wave signals and experimental prediction suggest that this productive phase may not last beyond middle of this month.

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