Current trends show India does look set for a timely onset of the crucial southwest monsoons on its shores this year, the country’s top weatherman said, citing key meteorological parameters.
An aspect that is particularly heartening is the recent ample pre-monsoon rains over southern Kerala coast and the north-east — “a good sign” for timely onset of the southwest monsoon, Ajit Tyagi, director general of the India Meteorological Department (IMD) said in an interview.
“We are expecting good showers in the northeast. Pre-monsoon rains in Kerala and the northeast region have started,” Tyagi said adding that pre-monsoon showers in the northeast often merge with the monsoon.
Various other parameters studied also seem favourable, he said. The southwest monsoon, which feeds India’s largely agrarian economy, sets over Kerala coast around June 1 and covers the entire country by end July.
Last year’s forecasts gone awry seem to have made the apex weather body tread with caution this time around, and though most dynamical models are predicting “above normal” rains, the IMD is sticking too its “normal” monsoon forecast.
The Met department had ended up red-faced last year as its initial forecast was 96 per cent of long period average rain, while India got only 78 per cent, with almost two-third of the country suffering a drought situation.
“Generally things are okay. Since our last forecast in April of a normal monsoon with rain at 98 per cent of long period average, things have not changed much,” Tyagi said. On April 23, the department, in its first long-range forecast, said, rainfall during the June-September season as a whole is likely to be “normal” this year.
Also Read
Quantitatively, monsoon season rainfall is likely to be 98 per cent of long period average, with a model error of 5 per cent, it said.Incidentally, the weather department has never ever forecast a deficient monsoon.
When factor
Tyagi said the Met department hopes to come out with its forecast of the date of monsoon onset in the country around May 15. Some reports had said monsoon is likely to hit Indian shores 7-10 days ahead of schedule, as temperatures in the Bay of Bengal — an indicator of monsoon arrival—had started rising in the first half of April.
Temperatures in Bay of Bengal usually start rising about 40 days before onset of the southwest monsoon rain in India.
Tyagi said Met would not like to hazard a guess on this and would rather wait for other parameters that become evident in May, such as clouding and rain over the Indian Ocean and Arabian Sea, to give a definitive view on the timing.
“It is too early to be certain on when it will come.”
He also ruled out high temperatures over northern India as one of the causes of early onset on monsoon rains.
There was speculation that the prevailing heat wave over northern and northwestern India would create a gradient that is conducive to bringing monsoon clouds to the subcontinent ahead of schedule.
Tyagi dismissed this presumption, as “a very simplistic” reasoning.
He said monsoon is a larger phenomenon, and factors such as cross equatorial flow, pressure and temperature gradient in southern India, temperature and velocity of upper atmospheric winds in northern India, and clouding and rainfall over Arabian Sea, Bay of Bengal and Indian Ocean play a much more important role in movement of monsoon currents over northern India than just temperatures.
He said high temperature over northern India has not been found to be a useful consistent statistical parameter over the years to gauge monsoon onset.
“We are waiting for all the relevant data,” Tyagi said.
He, however, cautioned that merely a timely onset does not dictate the spread and distribution of rains in the four-month monsoon season.
“Last year, monsoon onset was early. Then it fizzled out and we had a drought-like situation in many states,” said Tyagi, who holds a doctorate in meteorology from the Indian Institute of Technology, and had headed the forecast division of the Indian Air Force before joining the IMD.
Last year, the country had faced the worst drought in 37 years, as monsoon distribution was very poor despite an early onset.
Erratic rainfall had hit kharif sowing and lowered output of key summer sown or kharif crops such as rice, pulses and oilseeds.
Current swap
Tyagi said this year, monsoon rains are expected to be normal, as El Nino is expected to neutralise and La Nina conditions, which are usually conducive to good monsoon rain in the country, may develop towards the end of the June-September season.
“El Nino (waning) can be predicted with some certainty,” he said.
Many meteorological agencies blame the development of El Nino—warm water currents in the equatorial Pacific that sap the monsoon current of momentum and moisture—for last year’s drought.
Australian Met agencies have forecast that El Nino would fade after July, but are divided on when and whether La Nina would develop.
A Canadian weather forecaster, World Weather Inc, however, sees a rapid shift from El Nino to La Nina this year, which may lead to excess rains and heavy floods in many parts of India.
Tyagi said both of these are “extreme forecasts” and stuck to IMD’s earlier forecast of El Nino conditions maintaining till early part of the monsoon season, neutralising in the subsequent months and La Nina developing by July-August.
“About 80 per cent of models are showing this is how El Nino will develop,” he said.
Tyagi said though El Nino is one of the parameters for rain forecast, other parameters play an important role, and “there is no one-to-one relationship between El Nino and Indian monsoon... other parameters too have a large role.”
He said in the past, even during a full-blown El Nino year, India has received good monsoon rains. “Last year, El Nino was weak and just developing, but we had a drought.”
“There are many non-linear correlations in time and space that have an impact on monsoon rains,” he said, adding that snow cover, ocean temperatures, Atlantic Sea winds and even the monsoon’s own internal dynamics determine the quantum, intra-seasonal variability and spatial distribution of monsoon rain.