Weather: Reservoir levels comfortable in run up to S-W monsoon

A benevolent pre-monsoon thunderstorm season has delivered above normal rain amounting to 17 per cent till May 21 (April 1-May 21) for the country as a whole, and raised live reservoir storage to comfortable levels ahead of the monsoon expected to arrive late by four days over Kerala on June 5.

The Central Water Commission said in its latest weekly update on May 21 that the total live storage capacity of 123 reservoirs monitored by it is 171.090 billion cubic metre (BCM). This is about 66.36 per cent of the live storage capacity created in the country. The live storage available in these reservoirs is 60.73 BCM as on date, or 35 per cent of the total live storage capacity.

Circulation under watch

The corresponding level in the last year was 36.15 BCM and the average of last 10 years live storage was 37.58 BCM. Thus, the live storage available in 123 reservoirs as on May 21 is 168 per cent of the live storage of corresponding period of last year and 162 per cent of storage of average of last 10 years.

Meanwhile, weather watchers have been tracking a cyclonic circulation persisting over the South-East Arabian Sea (off Kerala coast and over Lakshadweep) for some time now. The India Meteorological Department (IMD) has already indicated that isolated scattered rain/thundershowers may scale up to heavy rain over the South Peninsula from May 28.

Likely onset window

Extended-range model guidance from the IMD suggests that the proceedings may lead to breaking of the monsoon over Kerala likely between May 29 and June 4 (within the predicted four-day window to either side of the June 5 median). Heavy rain may lash Kerala, Coastal Karnataka and Lakshadweep during this period.

Meanwhile, pre-monsoon showers have been consistently been deficient only in Tamil Nadu (-47 per cent); Andaman & Nicobar Islands (-33 per cent); and the North-Eastern States ranging from -21 per cent to -50 per cent. Many parts of North-West India and Central India, which normally experience top heat during this period, have instead received large excess rainfall and cooled as a result.

Lack of sufficient heating

The lack of heating and heat waves over this part of the region is being perceived as one factor that may have acted against the normal onset of the monsoon over the mainland. After all, it is the differential heating of the land vis-à-vis ocean that sets up the temperature-pressure gradient for the monsoon winds to rush in and set up the rains.

The Bay of Bengal arm of the monsoon announced its arrival over the South-East Bay of Bengal and the Andaman & Nicobar Islands after hitching a hike on super typhoon Amphan, but has been stalled there ever since. Amphan may have spirited away most of the monsoon moisture stored until then.

Super cyclone impact

The monsoon needs to recoup the moisture and the energy lost in hosting a massive and destructive cyclone that devastated West Bengal. The delay in the keenly awaited Arabian Sea arm of the monsoon is thus be attributed also the above normal pre-monsoon showers and resultant cooling of land.

After all, the monsoon rushes in taking advantage of the temperature-pressure gradient created by the differential heating of land and the ocean, a process compromised till now. It is in the wake of super cyclone Amphan that the land has started heating in right earnest to lower pressure levels to suitable levels. Winds can blow in only from an area of high pressure (ocean) to one of low pressure (land).

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