The India Meteorological Department (IMD) early this week said the southwest monsoon could start pulling back in next three or four days. The weather watchdog Tuesday said conditions were becoming favourable for withdrawal of the southwest monsoon. If the IMD predictions come true, it would mean the country is headed for another bout of deficit rainfall this year, too.
In April this year, in its pre-monsoon prediction, the Met office had forecast more-than-normal rainfall that might touch 106 per cent of the long period average (LPA). LPA is the average rainfall for the past 50 years.
Up till today the average rainfall in the country has been five per cent less. From June 1 to September 14, the country as a whole recorded 759.9 mm of rainfall as against 802 mm, which is the normal level. According to IMD sources, the monsoon deficiency has reached 13 per cent in east and northeast India while the deficiency has reached up to 12 per cent in southern peninsula.
The situation is better in northwest India where the shortfall is just two per cent. Central India is the only region that has received one per cent more rains than its normal limit. The local Met office has predicted a low pressure area on the east coast of India, which would cause good rains over Orissa, coastal Andhra Pradesh, east Madhya Pradesh and Telangana over the next three to four days.
If it happens, the monsoon can gain some lost steam and do a quick catch up. However, it may not be sufficient to bridge the shortfall leave alone meeting the IMD predictions of 106 per cent. This will presuppose rains in the remainder of September have to be 50 per cent more than its average. In terms of daily shower, it would mean 4mm of daily rainfall. However, in the first 15 days of the month rain has already been nearly 15 per cent below average.
At present, the LPA of seasonal rainfall over the country as a whole is 89 per cent. Rainfall below 90 per cent of the average is considered deficient. Rainfall between 96-104 per cent of LPA is normal and that between 105 and 104 per cent of LPA is above normal. Rainfall above 110 per cent of the average would mean an excessive monsoon. In both its April forecast and June update, the IMD had predicted above-normal rainfall for the country.
It had forecast rains in the region of 105 per cent of the long period average with a model error of plus and minus four per cent. To match IMD’s predictions, monsoon will have to make real good progress from here onwards. However, if rains do not make a strong recovery towards the fag end of the season, the Met might have to lower its forecast. If not the projections of normal to higher than normal rains will go wrong this year too — third time in a decade.
The Met office had faltered on its prediction twice in the last decade. The first one was in 2009 when it expected rains to be normal while in reality the country suffered one of its worst droughts in three decades and then in 2011.
The monsoon gets affected by the interplay of a variety of land and ocean phenomena. During the late 19th century, IMD predicted monsoon based only on an assessment of snow cover over the Eurasian region with lesser snow cover meaning a better monsoon.
In 1980s, the IMD used 16 predictors to make its forecast. Later, it was realised that 16 predictors was too many. The models were refined and for the April forecast this year, the IMD used five predictors and for the June update it used two more predictors.
Temperature difference between surface temperatures in the north Pacific and north Atlantic oceans, surface temperature of the Indian Ocean near the equator, mean sea level pressure in the Pacific Ocean near East Asia, land temperatures over northwest Europe are some of the predictors that the IMD uses to do a monsoon forecast. Any change in predictors may result in predictions going off the mark.
