INDIA’S monsoon is one of the world’s most important weather events. About half of the country's population—that is, 600m people—depend directly on the rain it bears. The monsoon sweeps northward across the subcontinent, bringing moist air from the south and south-west Indian Ocean. As it hits the land, and especially as it rises towards the Himalayas, it dumps its cargo of water, producing about three quarters of India’s total rainfall between June and September. Two-thirds of Indian agriculture is still fed by this rain, rather than by irrigation, which means India’s harvest depends on it. When the monsoon fails, as it has done this year, millions suffer. Crops wilt or fail altogether, farm land dries up, reservoirs, already too-small, run low, and winter crops (which are mostly irrigated) are imperilled. In some places this year, a lack of rain has led to shortages of drinking water. Like all weather patterns, the monsoon is erratic. Four years in ten count as abnormal. But this year—in which total rainfall is 14% below the 50-year-average between June and September—is exceptional. Droughts of this sort happens about once every 18 years. There is also extreme variation within the variation. Some parts of the country, the western state of Gujarat for example, have seen higher-than-normal rainfall. Others, especially in the north and the eastern coast, have had precipitation that is 40% below average.

Climate change seems to be making the variations more extreme. The Intergovernmental Panel on Climate Change, an international group of scientists who advise governments on global warming, has warned that because of climate change monsoon rainfall extremes are likely to increase. But exactly why this should so be is up for debate. No one yet fully understands the link between the monsoon and El Niño, a warming of the waters in the central and eastern Pacific Ocean. Over the past century, most climate scientists have argued that a strong El Niño is associated with a weak monsoon because, as the Pacific warms, the air rises and comes down again over the subcontinent, driven by prevailing wind patterns. This descending warmer air is associated with higher pressure, less moisture and a weaker monsoon. The current El Niño is the strongest since 1997 and 1998, according to Australia’s Bureau of Meteorology, and will be at its most powerful at the end of the year.

During the 1980s and 1990s, however, this link seemed to be broken. The year 1997 saw one of the strongest El Niños on record, but a normal monsoon. Balaji Rajagopalan of the University of Colorado, Boulder, argues that the puzzle can be explained by looking at which part of the Pacific warms up during an El Niño. If the eastern waters warm, the air comes down again over Indonesia and South East Asia, which tend to be drier than normal. But this may not affect India. If the central Pacific warms, the high pressure tends to form over India and the monsoon fails. If Professor Rajagopalan is right, this year’s El Niño is getting stronger in the central Pacific than in the east. The Indian Meteorological Department is hoping to incorporate this information into its monsoon forecasting system.

Dig deeper:

Storing wild seeds will save harvests—and lives (September 2015)

Why central bankers fixate on the monsoon as much as farmers (June 2015)