March 16, 2016
The great Indian monsoon hit mainland India in early June and, as feared, hasn’t quite delivered. Meteorologists, farmers and economists, who have been waiting with trepidation for what is expected to be a below par rainfall year, will continue to watch the clouds closely. As the El Niño Southern Oscillation (ENSO) unfolds in the Pacific Ocean, there is a 70% probability that it could snatch the rain away from South Asia and produce a drought.
This is bad news, since South Asia gets between 70 and 80% of its annual rainfall in the four monsoon months – June to September. Over 60% of the farmers in India depend on this rain to grow their major crops for the year.
Sulochana Gadgil’s eyes are on the satellite pictures from the Indian Ocean. An honorary professor of atmospheric and ocean sciences at the Indian Institute of Science, Bangalore, Gadgil is watching another phenomenon closer home called the Equatorial Indian Ocean Oscillation (EQUINOO). This oscillation, Gadgil says, has a major impact on the Indian monsoons and the potential to alleviate the impact of El Niño.
EQUINOO is a see-saw of atmospheric conditions between the eastern and western parts of the Indian Ocean. Normally during the Indian summer the eastern equatorial Indian Ocean, which is warmer than the western part, has abundant cloud cover. In the four months of the monsoon, there is more than average cloud formation over the eastern Indian Ocean in some years, and more over the western Indian Ocean in others. The condition that favours a healthy Indian monsoon is one where there are dense clouds over the western Indian Ocean and fewer clouds in the east.
The El Niño location factor
The threat to the Indian monsoon from El Niño depends on the location of the current. An El Niño in the central Pacific has a profound influence on rain-bearing winds over the subcontinent. High sea surface temperatures (SST) brought on by the warm bubble of water heats the air above the ocean. Rising warm air forms circulation cells in the upper atmosphere that draws air currents toward the region. The Indian Ocean is located in this downdraft and monsoon air currents here are disrupted. The stronger the El Niño, the more disruptive it is.
Six of the major droughts in India since the late 19th century have been El Niño droughts, including those in 2002 and 2009. But not all El Niño years have been drought or even sub-normal rainfall years. This has led to some spectacularly wrong predictions of the Indian monsoon.
Gadgil and her colleagues discovered the EQUINOO in 2003 while looking for explanations for monsoon prediction failures. “What happened in 1997 was that everybody thought it would be a drought (year) because it was a very strong El Niño. So people went ahead and predicted a drought, and what we got was above average rain. In fact, there were papers written that the monsoon-ENSO link is now weaker. But then came the El Niño of 2002, which was much weaker than the El Niño of 1997, and we got a severe drought. This led many people into wondering what exactly is going on. There must be one more thing,” said Gadgil.
Gadgil and her collaborators propose that the South Asian monsoon depends on two main factors – the strength of El Niño, and the phase of EQUINOO. A strong El Niño portends a drought in India but can be countered by an EQUINOO phase that brings rain in the western Indian Ocean. A weak El Niño, which might otherwise let the monsoon go about its business, can create drought conditions if the EQUINOO phase favours the eastern Indian Ocean.
In addition, an El Niño formation in the central Pacific suppresses the cloud formation in the eastern Indian Ocean, creating conditions favourable to the Indian monsoon. According to Gadgil, in 1997 there was a “tug-of-war” between El Niño and EQUINOO that ultimately resulted in a normal monsoon. Heavy clouds in the western equatorial Indian Ocean prevailed and brought rain to the subcontinent. In 2002, the El Niño and EQUINOO colluded. The clouds in the Indian Ocean formed mostly over the eastern Indian Ocean. The result was a drought.
EQUINOO conditions positive this year
Gadgil, who has been monitoring atmospherics in the Indian Ocean, says EQUINOO conditions in the summer months were positive for a good monsoon this year. But an atmospheric system is dynamic and liable to change.
With EQUINOO being a relatively new discovery, climate models are not able to predict EQUINOO or simulate monsoon-EQUINOO links. In an editorial in the journal Current Science, Gadgil says that while El Niño explains 29% of the monsoon variation in India, EQUINOO explains another 19%. Taking into account the influence of El Niño on cloud formation in the Indian Ocean the two phenomena together can explain more than 50% of the variation, which could result in far higher accuracy in monsoon forecasting than we have today.
Gadgil hopes that observation and modelling studies for EQUINOO will be taken up in a big way for better accuracy in predicting the Indian climate. “I personally expect that in the next five to ten years we will definitely be able to do it,” she said.