Weak La Nina, negative IOD will together underwrite normal monsoon

The normal rainfall outlook for India and South Asia this year is predicated on a weak La Nina emerging in the Equatorial East Pacific and an associated weak negative Indian Ocean Dipole (IOD) during the latter part of the season (August-September).

“We are looking at a situation where we will have a weak La Nina in the Equatorial Pacific, which can be favourable for the monsoon, and a weak negative IOD closer home, which is not so good. Each tends to cancel out the other’s overbearing influence on the monsoon, and hence the overall normal rainfall outlook,” says DS Pai, Head, Climate Research and Services, India Meteorological Department (IMD), Pune. Pai has authored many of the Indian monsoon long-range forecasts.

Weak trends

Pai said this in the context of both the IMD and the South Asian Climate Outlook forum agreeing to a normal rainfall forecast for this year’s monsoon for India and the larger South Asian region. Apart from the weak La Nina (cooling in the East Equatorial Pacific and warming in the West) and the weak negative IOD (warmer in the East Indian Ocean relative to the West), other parameters examined have also tended to indicate conditions ideal for a normal season this year.

“Normally, during a season when neither the Pacific nor the Indian Ocean is unable to exert its influence on the monsoon, factors deciding intra-seasonal variability assume more importance,” explains Pai. For instance, the number of low-pressure areas or depressions, or the movement of the rain-driving Madden-Julian Oscillation (MJO) waves across the Indian Ocean.

“These are not predictable at this point in time. Neither can we say with confidence whether a low-pressure area will form on June 1 or July 15, or that it would become a depression. But these are the main factors deciding the rainfall pattern,” said Pai.

Spatial rainfall

This is why a spatial spread of rainfall cannot be forecast at this time, Pai said responding to a question on how Kerala and coastal Karnataka, which bore the brunt of the monsoon of 2018 and to a lesser extent in 2019, will fare.

“It is difficult to take a call because the uncertainties with regard to what is happening in the Pacific Ocean. Currently, the ocean temperatures in the eastern part are warmer by 0.5oC. A few models are predicting that during the latter part of the season, these temperatures may cool and go into a weak La Nina, with ocean temperatures getting lower to minus 0.5oC,” Pai said.

“It is likely that the La Nina is likely causing the IOD to go into a weaker negative mode during the latter part of the season. This has happened many times in the past. When you have an El Nino in the Pacific, you have a positive IOD, and during a La Nina, a negative IOD,” Pai explained.

“As for coastal Kerala and Karnataka, even in normal times, we can expect periods of heavy rainfall, given the presence of the Western Ghats. A low-pressure area can trigger high winds and heavy rainfall here, and these States should keep themselves ready to deal with such possibilities even during normal times.”

Increasing rainfall Central India

Significantly, Pai said that researchers have found an increasing trend in rainfall over Central India, but not along Kerala and Karnataka. “If at all, we’ve picked a slight decreasing trend in rainfall in the latter. But these things can change year after year. So these States always have to be prepared for heavy rainfall events.”

The disasters witnessed here in the past few years have more to do with the urbanisation, deforestation, land use pattern and overall development, than heavy rainfall events.

“Maybe absence of such events as witnessed more recently in 2002, 2004, 2009, 2012, 2014, and 2015 — all below-normal or drought years — encouraged people to encroach floodplains and settle in large numbers. During a heavy rainfall event, it is normal for the river to claim its original space, causing floods and claiming lives and property. If we take the 30-year average, we are still in the below-rainfall period with 2019 being an exception. This will not change overnight. But if we’re lucky, there’s a chance that this will change to favour us. It might happen over a 5-10 year period marked by more number of above-normal monsoon years.”

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