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Vinson Kurian

Thiruvananthapuram, July 10 The India Meteorological Department has officially notified the prospects of a new low-pressure area taking shape over the North Bay of Bengal by Saturday, which will help scale up rainfall in Orissa and Gangetic West Bengal early next week.

But the ‘low’ is seen as brewing right into the midst of an area buffeted by strong vertical wind shear (change in wind direction with height), which will prevent it from intensifying appreciably, says Dr Akhilesh Gupta, Advisor to the Department of Science and Technology.

All these are being attributed to a significant shift in pattern of monsoon flows over the Bay. The flows are being pushed into a much southern latitude (10 deg N to 15 deg N) than the normal (17 deg N to 20 deg N) in what is seen as a ‘repositioning manoeuvre’ in which they will stream into Typhoon Man-Yi in the west Pacific, currently the dominant system in the larger monsoon ensemble.

Flows thus getting re-directed will affect the strength of the monsoon over mainland India at least till such time as Man-Yi makes a landfall over the Japanese coast expectedly by Saturday. But the anomalous southerly shift of the core of flows will have triggered a renewed wet session in peninsular India, even kicking up a cyclonic circulation over land.

This is normal, and happens based on the simple premise (barotropic instability or horizontal wind shear) that unsettled weather conditions will be created to the north of the prevailing strong monsoon flows. Such type of weather does not need the patronage of a monsoon ‘low’ or depression, either for initiation or for sustenance, Dr Gupta said.

The expected wet session in the peninsula will benefit interior Karnataka, Andhra Pradesh and northern Tamil Nadu. Meanwhile, Monday’s depression over northwest Madhya Pradesh had weakened into a well marked low pressure area by the evening and lay over northwest Madhya Pradesh and adjoining east Rajasthan. On Tuesday, it weakened further to a ‘low’ and shifted perch to over southeast Rajasthan and neighbourhood.

The current meteorological analysis suggests that fairly widespread rainfall with isolated heavy to very heavy falls are likely over northeast India, sub-Himalayan West Bengal and Sikkim and Bihar during the next three days. This can be attributed to the interaction of a western disturbance and monsoon flows in the region.

An Ohio State University update said the axis of the monsoon trough passed through the centre of the ‘low’ in northwest India and extended into the northeast Bay. The offshore trough running south from Gujarat coast to the Kerala coast was also persisting.

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