As this year’s excruciating summer gave way to cooler temperatures and welcome rain, the country’s weather forecasters became involved in a serious debate. The 140-year-old Indian Meteorological Department predicted a deficient monsoon this year, saying it would be 88% of the season’s long-period average. Skymet, a three-year-old private forecaster, challenged this official forecast, saying that the subcontinent would have a normal monsoon of between 96% and 104% of the average.

The monsoon outdid expectations in June but entered a dry spell in early July. Both forecasters are sticking to their predictions. In a recent interview, the head of Skymet said that the IMD’s forecast was lower because it had given the El Niño more weightage.

The El Niño ocean effect originates in the Pacific Ocean. Warm currents heat the sea’s surface and the air above the ocean. This warm air rises and creates circulation patterns that disrupt atmospheric flows in other parts of the world. For example, moisture-laden air above the the Indian Ocean is drawn towards the Pacific, dissipating the rain that would otherwise have fallen over India. The stronger the El Niño, the more disruptive it is.

Since the late 19th century, six of India’s major droughts were caused by the El Niño phenomenon, including those in 2002 and 2009. Still, in some years an El Niño has developed but rainfall in India has been normal. Looking into these anomalies, scientists have found that other factors temper or boost the impact of El Niño, such as water temperatures, the cloud cover in the Indian Ocean, the formation of aerosols and the forest cover.

Although there’s no simple explanation for the monsoon’s vagaries, India’s climate scientists are slowly fitting pieces of the Indian monsoon puzzle together. “The Indian monsoon is probably the most complex [of global weather phenomena],” said Subimal Ghosh, associate professor of civil engineering at the Indian Institute of Technology, Bombay.

Yet the monsoon remains critical to India’s agricultural output and economic destiny, even though India’s rural economy may be changing and a bad monsoon might not send the countryside into distress: India Ratings, an affiliate of the global credit agency Fitch, calculates that non-agricultural incomes in rural India have risen to more than 70% from a little more than 60% a decade ago.

A shift in the El Niño

The El Niño itself has been changing in the last three decades. “All El Niño signals seem to be weakening,” said Ashok Karumuri, a meteorologist at the University of Hyderabad, referring to an apparent decline in the impact of the El Niño on the Indian southwest monsoon. “But a big El Niño with no neutralising factors, such as the Indian Ocean Dipole can create a dry year.” The Indian Ocean Dipole, IOD for short, is a temperature difference in different parts of the ocean that influences monsoon patterns.

The traditional El Niño phenomenon comes from a warming of the eastern Pacific Ocean. Climate scientists have observed a new flavour of the El Niño, called the El Niño Modoki, since the 1970s. In El Niño Modoki, the central Pacific heats up, flanked by cooler waters on either side, leading to circulation patterns that are even more detrimental to the Indian monsoon.

The warming of the Pacific might also be changing, adding to the complexity, according to Karumuri. In 2009, for instance, the Pacific became warmer than normal for more than five months at a stretch.

“There seem to be decadal changes in the Pacific,” agreed Sulochana Gadgil, honorary professor of atmospheric and ocean sciences at the Indian Institute of Science in Bangalore. But she thinks it is too early to say whether these changes are part of normal seasonal variations that are seen from year to year or if these changes are here to stay.

Sea temperatures

She points out that although the El Niño is the dominant factor in determining the fate of the monsoon, the IOD contributes almost as much to variability.

The IOD is the difference in the sea surface temperature between the western Indian Ocean near the Arabian Sea and the eastern part near the Bay of Bengal. A warmer western pole leads to a positive IOD, a condition that can neutralise an El Niño and lead to a good monsoon. If the IOD is in the opposite phase, with a warmer eastern pole and cooler western ocean, then it can reinforce the monsoon-destroying effect of an El Niño.

According to Gadgil, the apparent weakening of the El Niño has been more because of the neutralising effect of a favourable IOD condition.

Cloud cover

Gadgil has also been studying another atmospheric phenomenon close to home called the Equatorial Indian Ocean Oscillation, or EQUINOO. The EQUINOO is a see-saw in a cloud cover between the eastern and western parts of the Indian Ocean. In the four monsoon months there is above-average cloud formation over the eastern Indian Ocean in some years and more over the western Indian Ocean in others. Gadgil’s research shows dense clouds over the western Indian Ocean and fewer clouds in the east favours a healthy Indian monsoon.

Moreover, there’s a complex relationship between El Niño and 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 that might otherwise let the monsoon go about its business can create drought conditions if the EQUINOO phase favours the eastern Indian Ocean.

Forest cover

Then come the changes over land that influence the monsoon. Much of the great Indian monsoon is sustained by water evaporating from the subcontinent’s forests. Ghosh of IIT-Bombay has been studying this precipitation recycling.

In the pre-monsoon months, the soil of the subcontinent is mostly dry and there is little evaporation from the land surface into the atmosphere. When the monsoon begins and the soil soaks up moisture from the rains, it gets taken up by the forests and is recycled into the atmosphere by evapotranspiration, the process by with leaves release water into the air as a by-product of respiration. “In August and September, around a quarter of the precipitation in the Ganga basin and eastern India comes from the land surface,” said Ghosh of IIT-Bombay.

Ghosh’s work is supported by other research from IISc, which shows that large-scale deforestation around the world can lead to an 18% decrease in the Indian monsoon. Urbanisation and the creation of impervious land surfaces like roads and pavements that accompany it also clamp down on this rainfall recycling.

A counter-influence is seen from as far as West Asia. A study published last year found that dust over North Africa and the Arabian Peninsula absorbs sunlight, warms the air and strengthens winds carrying moisture eastwards. A week later heavier rainfall is seen in India.

Heavy rain in long term

As far as the year-to-year variability of the monsoon goes, changes in land use, urbanisation and aerosols are far less significant than the dominant El Niño and IOD. But they are important enough to influence the picture over the next century.

Kavirajan Rajendran, principal scientist at the CSIR Fourth Paradigm Institute in Bangalore, has run multi-model simulations to find out how the monsoon is likely to change under temperature scenarios predicted by the Intergovernmental Panel of Climate Change in its latest assessment. In an analysis published in the Geophysical Research Letters of the American Geophysical Union, Rajendran and his colleagues found a dramatic increase in monsoon rainfall between 2070 and 2099 and a reduction in the quantity of rain in other months of the year.

Between the myriad short and long-term factors pulling and pushing the Indian monsoon, there are inevitable hits and misses in forecasts. In such a dynamic system, no change is permanent or unidirectional. “Climate change means people think it is monotonically changing into something. That’s not how things work, ” Gadgil said.

This is the first part in a series on climate change.