All eyes on monsoon as cooler summer delays crucial pattern

An uncharacteristically cooler start to the summer, which is likely to persist for a few more weeks in several parts of the country, may hurt the arrival of the crucial monsoon season, weather scientists have said, at a time when the rainy season is expected to anyway be sapped by the Pacific warming phenomenon El Nino.

Large parts of the country, especially the north and the northwest that by now usually begin recording peaks of over 40°C, have logged temperatures that are up to 10°C lower than normal due to rains.

And this has implications for what happens when the monsoon begins to arrive from the southwest of the country. “When heating is low during the pre-monsoon season, especially during April and May, it can impact the monsoon in some ways. More heating over northwest India helps with the onset of monsoon,” said M Rajeevan, former secretary, ministry of earth sciences.

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Rajeevan explained that when the ground heats up, especially in heatwave prone parts of northwest India (which includes Rajasthan, Haryana and Delhi), it creates an area of low pressure. “There is something called the heat low which forms over northwest India. It’s a very important feature for the monsoon. If it doesn’t form, monsoon circulation will not be strengthened. While these features may not affect the total seasonal monsoon, they are definitely likely to impact onset of monsoon,” he added.

A low-pressure area is one where the air has thinned out and to balance it, air from other directions fills in. In the case of the monsoon, a low-pressure is created in the run-up months of April to June as the sun’s rays move northwards, heating up the Arabian sea and the land in the same latitudes.

The heat the land absorbs, and feeds back into the atmosphere, is crucial in creating this intense heat-low pressure band, which is known as the monsoon trough. It is here that the moisture laden winds that blow in from the Arabian sea mostly turn into rain.

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But this is just one of several factors that create a careful meteorological choreography to bring India the rains that are lifeblood to its agriculture. “There are other features that are important to consider like, say, El Nino, La Nina etc. Monsoon usually arrives late during El Nino years. The lack of temperature gradient (temperature difference between land and ocean) does impact the monsoon but not the total seasonal rainfall. I would say we shouldn’t worry but prepare ourselves,” Rajeevan added.

El Nino is the warming of waters in the Pacific Ocean, which creates a cascade of weather effects across the world – in India, this leads to the monsoon being drier than usual. The La Nina manifests in an exactly opposite phenomenon, leading to heavier rains in India.

The India Meteorological Department’s (IMD) extended range forecast indicates that maximum temperatures are expected to be 5-6 degrees below normal over most parts of the country till May 4 followed by a rise in heating over isolated parts of east India -- but conditions are predicted to still be significantly below normal over rest of the country till May 11.

Some studies on land temperature and monsoon conclude that temperature is not the main determant for monsoon. “The Indian summer monsoon rainfall does depend to some extent on the land surface temperature in May. However, the correlation is poorer than that with El Nino Southern Oscillation (ENSO) and Equatorial Indian Ocean Oscillation (EQUINOO), which are known to play important roles in the interannual variation of the monsoon. The variance of summer monsoon explained by the May land surface temperature is only about 10%,” said a study led by Sulochana Gadgil and others at Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science.

The IMD’s latest assessments issued on Friday showed El Nino conditions will develop by June, with a 72% probability of it happening during May, June and July, period and an over 80% probability of it developing during June, July and August period. “During monsoon it may be a moderate El Nino but not a strong one,” M Mohapatra, director general, IMD said on Friday.

“We are seeing normal and below normal temperatures over some parts of the country but not eastern India. We do not expect these temperatures to impact monsoon onset or seasonal rainfall,” Mohapatra said on Sunday.

A third expert said the conditions during the monsoon arrival will play a greater role. “The large difference or variation of temperatures will only be limited till onset of monsoon. Once onset is made it doesn’t matter much,” said OP Sreejith, head, climate monitoring and prediction group at IMD, Pune.

“Pre-monsoon temperatures over Indian landmass can be one among several of the factors affecting the monsoon onset dynamics. I’ll be more concerned about the teleconnections with El Nino during the monsoon season. An El Nino can enhance the tropospheric temperature gradient by increasing the temperatures over Eurasia and thereby weaken the monsoon winds.

Even though land surface temperatures cool with rains, the (latent) heat is released to the troposphere/atmosphere over the land. So land surface temperatures alone do not drive the monsoon,” said Roxy Mathew Koll, climate scientist at Indian Institute of Tropical Meteorology.

A fifth expert too said much will depend on how things pan out later. “It’s very difficult to say. Monsoon doesn’t only depend on temperatures but mainly on global parameters such as El Nino, La Nina, Indian Ocean Dipole etc. Some models are indicating that most of NW India will become dry from May 4 or 5 and temperatures will rise again thereafter over central India and adjoining areas and cross 40 degree C. Weather activity due to upcoming western disturbances may continue over northwest India but once the heat low forms during the remaining 20 days of May, there is not much to be worried about,” said Mahesh Palawat, vice president, climate and meteorology, Skymet Weather.

A western disturbance as a cyclonic circulation lies over central Pakistan in lower and upper tropospheric levels. At present, there are multiple cyclonic circulations and low-pressure belts that is leading to rain across large parts of the country: a cyclonic circulation induced by a western disturbance is lying over south Pakistan and adjoining West Rajasthan, a cyclonic circulation is lying over southwest Uttar Pradesh and another over south Chhattisgarh, and a trough/wind discontinuity is running from east Vidarbha to north-interior Tamil Nadu.

Another active western disturbance is again likely to affect northwest India from the night of May 1 and IMD has issued an ‘orange’ alert for West Bengal, Sikkim, Jharkhand, Uttar Pradesh, Punjab, Haryana, Himachal Pradesh, Uttarakhand on May 1.

The IMD, in early April, forecast a “normal” monsoon at 96% (with an error margin of +/-5%) of the long period average.