El Niño does hit the monsoon, but some links still missing

Fears of the impact of a strong El Niño – the warming of the central to eastern tropical Pacific Ocean waters – on South Asia’s monsoon system seem to have been confirmed by deficit rains in the region this year.

More than halfway into the South Asian monsoon, a crucial lifeline for agriculture in the subcontinent, the India Meteorological Department (IMD) has recorded 10 per cent deficit rain.

“El Niño has adversely affected the monsoon so far, as per the model forecasts. It is also expected to affect the monsoon in the coming months,” India’s Minister for Science and Technology Harsh Vardhan confirmed in a written reply to the Indian Parliament recently.

“Given the strong El Niño conditions that we see since the last two months or so, to me, it looks that the current El Niño has played a major role in the 2015 monsoon rainfall deficit,” Ashok Karumuri, associate professor, University Centre for Earth and Space Sciences at the Hyderabad University, told thethirdpole.net.

Adding to the impact is the absence of a rain-inducing event that scientists refer to as the ‘Indian Ocean Dipole’ – the difference between in sea surface temperatures between India’s west and east coasts. It is this difference which leads to a lot of the rain in South Asia during the monsoon.

Krishna Achuta Rao, associate professor at the Centre for Atmospheric Sciences(CAOS), Indian Institute of Technology (IIT), Delhi, is a little more cautious in his conclusion. “The developing El Niño does appear to be one of the strongest ever, on par or stronger than the 1997-98 event,” he said. “However, it is too early to conclusively say that the reason for the monsoon deficit is the El Niño. We have to remember that while 2014 wasn’t officially an El Niño year, we still had a slight deficit.”

“This year the monsoon is not yet over yet, but the rainfall received so far is found to be below normal. El Niño conditions are prevailing in the Pacific, but we can’t say the last word because the strongest phase of El Niño is yet to come, in December,” added P.K. Dinesh Kumar, scientist at the National Institute of Oceanography’s regional centre in the coastal town of Kochi in Kerala.

Changing El Niño

Of late, scientists have been reporting changes in El Niño trends in the Pacific, and parallel weaker monsoon rains over South Asia.

At a session on climate modelling at a workshop on climate change held at the Tata Institute of Social Sciences (TISS), Mumbai, on July 31-Aug 1, Karumuri reported how El Niño, “one of the strongest drivers” of monsoon rainfall over South Asia, has been changing in recent years.

More warming has been observed in the central tropical Pacific Ocean, rather than the eastern tropical Pacific Ocean, as was seen earlier. The impact of a Central Pacific El Niño would be greater over the South Asian monsoon system, with more dry spells, he said.

In 2007, Karumuri, who was then with the Indian Institute of Tropical Meteorology(IITM), Pune, had used the term ‘El Niño Modoki’ (Modoki is Japanese for ‘same but different’) for the central Pacific El Niño. “El Niño Modokis have a maximum anomalous warming in central tropical Pacific for more than a few seasons, generally also flanked by cooling on both sides in summer,” he explained. These events influence the South Asian summer rains, particularly in the Indian peninsula, he added.

“It turns out that these central Pacific warming events affect the monsoon adversely compared to the eastern Pacific ones,” agreed Achuta Rao. “The central Pacific warming appear to have been more frequent in recent decades compared to historically typical east Pacific warming events.”

Karumuri was also part of an international team from France, India and the US who in June this year reported in Nature about the drying of the Indian subcontinent, with rapid Indian Ocean warming and fewer temperature differences between the land and sea in the region.

Their analysis, using multiple data sets, showed a “significant weakening trend” in summer rainfall during 1901-2012 over the central-east and northern regions of India, along the Ganga-Brahmaputra-Meghna basins and the Himalayan foothills.

They reported that temperature differences between land and sea in South Asia were declining due to rapid warming in the Indian Ocean, and a relatively subdued warming over the subcontinent. They concluded that higher warming in the Indian Ocean dampened summer monsoon circulation and reduced rainfall over parts of South Asia.

At the TISS workshop, Achuta Rao observed changes in the climate in South Asia – there were inter-decadal variations in monsoon rains, with a noticeable declining trend in monsoon rainfall, more frequent deficit monsoon rains, an increase in monsoon breaks and rise in sea surface temperatures. “The frequency of heavy precipitation events is increasing, while light rain events are decreasing,” he said at the workshop.

The observations add to recent research published by the Kochi and Visakhapatnam regional centres of the Council of Scientific and Industrial Research’s National Institute of Oceanography in the journal Regional Studies in Marine Science.

Warming sea

Scientists from the two centres analysed historical weather data sets from the Arabian Sea and Bay of Bengal from 1880 to 2010, and noted higher sea surface temperatures, especially from 1960 to 2010, in both the basins.

They found sea temperatures were rising faster in the Bay of Bengal than in the Arabian Sea.

The team next studied whether the rise in sea temperature was linked to three climatic events – El Niño, La Niña (the reverse of El Nino, cooling of Pacific waters) and the Indian Ocean Dipole.

They found that climatic events caused sea temperatures to peak in April and May, as well as cool in September in both the basins. These anomalies over the study period 1880-2011 were linked to occurrences of El Niño, La Niña and the Indian Ocean Dipole.

They reported that warmer sea surface temperatures were linked to El Niño.

Although research does not elaborate on the El Niño-monsoon relationship directly, it shows that Arabian Sea and Bay of Bengal sea surface temperatures are increasing at a constant pace, said Dinesh Kumar, lead author of the paper. This, in turn, will result in less differential heating of the land and sea, which reduces the strength of the monsoon.

Trouble brewing

While the financial implications of a weak monsoon will be computed only after the season gets over in September, smallholder farmers across South Asia will clearly be the worst hit.

In India, agriculture now contributes less than 12 per cent of the GDP, so the impact on the economy may be manageable. But over half the 1.25 billion Indians still depend on farming for their livelihoods. And of the farmers, an estimated 61 per cent are in areas without irrigation. Mostly with holdings of less than acre, these farmers are always the worst sufferers whenever the monsoon is poor.

Prices of a few staple food items like onions have started rising already, and the government has raised its minimum export price in an effort to check the rise in the domestic market. Apart from that, there is not yet any sign that the authorities are preparing to tackle the economic fallout of a deficit monsoon, either this year or into the future.

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