The Hindu Business Line : Rains may have ended, but there can be surprises

THE northeast monsoon may have officially drawn to a close but that does not mean the rain activity in the southern peninsula will come to a complete stop.

"You will still have to contend with rogue easterly waves travelling straight west from the western Pacific/South China Sea and loaded with enough moisture to trigger a weather event or two over Tamil Nadu and Kerala," says Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF).

But Dr Gupta hastened to add that mid-January to February-end happens to be the leanest period for the southern peninsula. Moisture availability is at a premium, with the seasonal anti-cyclone sitting in heavy with attendant subsidence of air. This suppresses convective activity that is so crucial to creating a weather event.

"As of now, the skies over the peninsula are clear, with a bright sun beating down on cities such as Bangalore. The mercury is scaling up gradually in the region," Dr Gupta said. Data over the last 10 years is in agreement with the inference that frequency of cyclonic circulations has been least or even nil during the one-and-a-half month period in the beginning of the year.

This scenario results from the southward shift of the inter-tropical convergence zone (ITCZ) shifting south in tandem with the sun's movement south of the Equator. The sea surface temperatures (SSTs), a crucial factor deciding convective activity, fall appreciably below the threshold 26.5 deg C as a result.

Unlike western disturbances (westerly waves that affect north and northwest India), easterly waves are not a planetary scale phenomenon. They are prone to sudden death as they travel west, and only those with the right amplitude grows big to impact the ITCZ and knock the massive anti-cyclone off its perch to cause rainfall in the southern peninsula.

From March onwards, the sun reverts to moving north again, dragging the ITCZ along. Rising SSTs bring convective activity over the oceans back to life and more often than not set up storms. This is how the pre-monsoon rains set in over parts of the country.

Meanwhile in the north, the existing western disturbance as an upper air system over Jammu and Kashmir and adjoining Himachal Pradesh on Thursday has been showing a tendency to move east and may progressively become unimportant.

With this eastward movement, there have been changes in the wind pattern over the northwest leading to fall in night temperatures by two to three deg C. The trend may continue through Friday but will be reversed in view of the approach of a fresh western disturbance.

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