Southern India was hit by severe drought from 2016 to 2018 arising from low rainfall during the northeast monsoon, which occurs during the winter. So severe was the impact that a water crisis erupted in Chennai, India’s sixth-largest city of 1.1 crore inhabitants, as four of the city’s major reservoirs went bone-dry and groundwater levels plummeted. In the summer of 2019, a “Day Zero” was declared and residents scrambled to obtain water from tankers.

Now, after examining rainfall data over the past 150 years, researchers in India and the United States conclude that the 2016-2018 northeast monsoon drought was unprecedented with more than 40% deficit in northeast monsoonal rainfall during the three years.

The recent drought was worse than the Great Drought of 1874-1876 that led to crop failure, which in turn resulted in the Great Madras Famine of 1876 to 1878 that claimed millions of lives. The team demonstrates that cool phases in the equatorial Indian and Pacific Oceans are associated with the rainfall deficit.

“The consecutive failure of the northeast monsoon can result in a water crisis in South India,” lead author Vimal Mishra, associate professor at Indian Institute of Technology, Gandhinagar, told Mongabay-India, adding that “it has considerable implications to agricultural productivity”.

While India receives most of its annual rainfall during the Indian summer monsoon (June to September), southern India receives about 40% of its rainfall from October to December in what is known as the northeastern monsoon or the winter monsoon. It is crucial for drinking water and agriculture contributing to the livelihood of millions.

The South Indian states of Andhra Pradesh, Karnataka and Tamil Nadu continuously declared drought from 2016 to 2018 linked to low northeast monsoonal rainfall. Over 60% of the rural population in southern India is engaged in agriculture and relies on rainfall from the winter monsoon.

Northeast monsoon

How severe was the recent drought compared to those Southern India has experienced in the past? What are the causes of the deficit in the northeast monsoon? Mishra’s team sought to answer these questions.

To investigate the long-term history of northeastern monsoon droughts in the region, the team used rainfall observations from the India Meteorology Department from 1870 to 2018. Data on total water storage was obtained from NASA’s Gravity Recovery and Climate Experiment satellites for April 2002 to June 2017 while the Gravity Recovery and Climate Experiment Follow-On mission provided data for 2018 onwards.

Over the past 150 years, there were five main periods of drought with more than 29% deficit in rainfall (1876, 2016, 1938, 1988 and 1974 in order of severity). Looking at single year rainfalls, 1876 was the driest year with a precipitation deficit of 69% followed by 2016 with a deficit of 63%.

But when considering cumulative rainfall over three years, 2016 to 2018 was the worst northeastern monsoon drought with a precipitation deficit of 45% while the 1874 to 1876 drought, or the Great Drought as it is known, was the second-worst with a deficit of 37%.

The Gravity Recovery and Climate Experiment indicated that total water loss in Southern India in December 2016 was 79 cubic kilometres while the Gravity Recovery and Climate Experiment Follow-On data showed that the loss was 46.5 km3 in June 2017 and 41.7 km3 in June 2019. Loss in total water storage likely resulted in significant depletion of groundwater in the region, say the authors.

Three-year cumulative precipitation anomalies (mm) during the Northeast monsoon (October–December). Figure from Mishra et al. 2021

Factors causing deficits

The team examined sea surface temperatures, sea-level pressure and wind fields during the winter monsoon to understand how circulation patterns affect variability in northeast monsoonal rainfall.

Sea surface temperature over the equatorial Indian and Pacific Oceans affects year-to-year variability of the northeast monsoon, explained Mishra. “Sea surface temperature anomalies cooler than normal are linked to a weak northeast monsoon.”

In 2016 and 2017, cool sea surface temperature anomalies prevailed in the tropical Indo-Pacific Ocean and were associated with La Niña in the central Pacific, the researchers observed. La Niña is a climate pattern that occurs irregularly every two to seven years. During La Niña, the surface waters over the equatorial Pacific Ocean are cool and this affects global weather patterns.

At the same time, the researchers noted anomalous cooling was seen in the Indian Ocean. Such patterns along with those seen in sea-level pressure and surface-air temperatures gave rise to anomalous westerlies in the equatorial Indian Ocean, which weakened moisture transport from the Bay of Bengal during the northeast monsoon, explained the authors.

Interestingly, the study revealed that out of five of the major droughts that struck southern India over the past 150 years, four occurred during La Niña.

Deepti Singh, assistant professor at Washington State University, who was not connected with the study, notes that the paper “links the recent severe, multi-year drought primarily to La Niña conditions in the tropical Pacific Ocean in 2016-2017 and 2017-’18”.

This finding “implies that there is potential to predict them a few months in advance since La Niña events can be predicted with some skill in the summer,” said Singh, adding that “this means that stakeholders can prepare for and mitigate their impacts”.

While the study does not explain what made the 2016-2018 drought one of the strongest on record, “it demonstrates that natural climate variability can lead to extreme events”. She stresses that a better understanding of these drivers can inform our ability to predict severe droughts in the future.

“Timely predictions of such events can help better manage and potentially reduce their societal impacts,” Singh said. “This is particularly important since extreme La Niña conditions are projected to become more frequent with warming and if this link holds, it might mean increasing drought risks to the region, which will likely be worsened by hotter conditions.”

This article first appeared on Mongabay.