Exiting monsoon sets up showers over peninsula

The southwest monsoon, now on its leg over the peninsula, has been active over south interior Karnataka during the 24 hours ending Tuesday morning, an update from the Chennai Regional Met Centre said.

Forecast for the next two days said that rain or thundershowers are likely to occur at a few places over Kerala, Tamil Nadu, Puducherry, coastal and south interior Karnataka.

Isolated rain or thundershowers have been forecast over Lakshadweep, coastal Andhra Pradesh, Rayalaseema and north interior Karnataka.

COMPLETE REVERSAL

The circulation features during the northeast monsoon season develop as a complete reversal of the southwest monsoon pattern, facilitating low-level air flow from the north towards the Indian Ocean.

Associated northeasterlies traversing the Bay of Bengal and entering the southeast peninsular coast India and neighbouring Sri Lanka give rise to copious winter rains.

The southwesterlies across the peninsula are on the ebb in what is now a transition phase to the northeast monsoon. Helpful northerly flows are discernible but their easterly component has not evolved to the desired pattern.

These flows have to strengthen to set up the northeast monsoon onset. This requires a 'booster' circulation in the Bay of Bengal helping the flows to converge and be redirected towards Sri Lanka and the southeast coast of India.

Between 1901 and 2000, the onset of easterly winds took place between September 23 and November 1. The normal date for this change in wind direction was October 15.

During the same period of 100 years, the onset happened between October 4 and November 11, with the median normal being October 20.

WESTERN DISTURBANCE

International models have been predicting the formation of a Bay circulation during the course of the week but it would have to contend with opposite flows generated by an approaching western disturbance.

No big traction in the Bay is being forecast for the moment until October 20, though the US Naval Fleet Numerical Meteorology and Oceanography sees south China, the South China Sea and Indo-China progressively being brought under seasonal rains from the weekend.

What the nearly contiguous Bay of Bengal would need is a trigger mechanism in the form of a cyclonic circulation. The US Naval forecaster, however, sees some rain being dropped over Sri Lanka and along the southwest coast during this phase.

According to the US National Centres for Environmental Prediction (NCEP), the peninsular showers presumably sustained by residual southwest flows, may sustain until October 20.

Sri Lanka and the southeast coast are forecast to get rains sourced to what are identified as purely easterly flows only after October 20, the NCEP said.

Abnormal warming of the equatorial and east Pacific associated with an El Nino, the like of which is active concurrently, has been known to trigger the failure of the southwest monsoon.

But studies show that an El Nino may have the opposite effect on the northeast monsoon and may bring in reasonably good rain.