Kerala Conundrum: How Does a Drier-than-Average Indian Monsoon Trigger Catastrophic Floods?

Torrential monsoon rains have inundated the picturesque, prosperous state of Kerala in far southwest India during August. As many as 350 people have died, and the flooding has forced at least 800,000 people from their homes, according to sources cited in a weather.com report. The flooding has been the worst in Kerala state in roughly a century, according to local officials.

The Kerala floods follow a pattern that’s all too common in this vast nation, which for millennia has swung between winters that are close to rainless and summers that are drenched by the South Asian monsoon. What’s a bit puzzling at first glance is that India’s 2018 monsoon has actually been a stingy one. Rainfall across India from June 1 through August 15 totaled just 91% of the season-to-date average, making for a deficit of 9%, according to the India Meteorological Department (IMD). The five worst Indian monsoons for rainfall deficit on record were:

1) 1877, 33%
2) 1899, 29% 
3) 1918, 25%
4) 1972, 24%
5) 2009, 22%

In the last 40 years, according to the IMD, the highest rainfall deficiency was recorded in drought years 2009 (21.8%), 2002 (19.2%), 1987 (19.4%), 1979 (19.0%) 1982 (14.5%), and 2015 (14%). The 2016 and 2017 monsoons were 3% and 5% deficient, respectively.

Out of the 36 climate districts where rainfall is tracked by the IMD, only one—Kerala—has seen a significantly wetter-than-average monsoon as of August 19. Every other region has either been near average or deficient (see Figure 1). East and northeast India was running about 27% below average as of mid-August.

Kerala itself was having a moderately wet monsoon—about 15-20% above average—until the second week of August, when a week of intense rainfall set in. From August 1 to 19, Kerala racked up 758.6 mm (29.87”), which is almost three times the normal for Aug. 1-19 of 287.6 mm (11.32”). Dozens of reservoirs across Kerala that were filled to near-capacity during the generous rains of June and July overflowed with the deluge of August, which led to flooding across nearly all of the state, according to the IMD.

Figure 1. Rainfall for the monsoon period to date (June 1-August 15) was average to below average for all of India’s precipitation monitoring districts except for Kerala (the blue strip at lower left). Image credit: India Meteorological Department.

A ”stuck” pattern gives Kerala its worst

The culprit behind Kerala’s flooding was a sprawling area of low pressure at the surface and aloft that parked itself across eastern India and the Bay of Bengal during the second week of August (see Figure 2 below). The area of low pressure was reinforced by two tropical depressions—BOB 04 on August 7-8 and BOB 05 on August 15-17—both of which moved from the Bay of Bengal into the state of Odisha in east-central India.

The winds circulating around the persistent zone of low pressure pushed moisture from the Arabian Sea directly against the higher terrain of the mountains known as the Western Ghats, which form the eastern border of Kerala. The result was an onslaught of orographic (terrain-induced) rainfall, the type of pattern that can pummel California’s Coast Range and Sierra Nevada during their wet season.

Across the rest of India, the "stuck" area of low pressure put a crimp in the southwest flow that normally delivers generous rains to central and northern India during August. This week, the persistent low has weakened and moved northward and inland, allowing a more typical southwest flow to become reestablished across India. This should give Kerala some relief, with the heaviest coastal rains shifting north toward Mumbai.

Figure 2. This model-generated analysis from 06Z (2 am EDT) August 14, 2018, shows the persistent pattern that led to heavy rainfall in the Kerala state of far southwest India. Shown is the circulation at 850 mb (about a mile above ground level) and the departure from average precipitable water (the amount of water vapor in a column above ground level). A sprawling cut-off low across northeast India kept precipitation relatively modest across most of the nation. However, strong southwest flow circulating around the low brought rich moisture into far southwest India, where upslope flow squeezed out huge amounts of rain across the narrow strip from the coastline to the mountain range known as the Western Ghats. Image credit: tropicaltidbits.com.

India’s flood toll has been stubbornly high for decades, and it’s still rising

On average, about 3% of India’s land mass gets flooded each year. That’s about 7.5 million hectares, or 29,000 square miles—roughly the size of Massachusetts, Vermont, and New Hampshire combined.

Sadly, the Kerala disaster is consistent with India’s typical death toll from flooding—1586 per year, on average, between 2006 and 2015—which has risen even as flooding deaths have tended to drop in other large developing countries. An October 2017 report in the Indian news site The Quint noted that India’s flood warnings are highly accurate but often aren’t adequately relayed to the people at risk. Moreover, fast-growing Indian cities are struggling to build and maintain the infrastructure needed to reduce their flood risk.

Figure 3. Volunteering members of the Kerala Adventure Sports Club carry flood relief material for marooned villages on the outskirt of Kozhikode district, north of Trivandrum, Kerala, on August 17, 2018. Troop reinforcements stepped up desperate rescue attempts in India's flood-stricken Kerala state on August 17 after more than 100 bodies were found in a day and a half. Image credit: Manjunath Kiran/AFP/Getty Images.

In the 2015 report Poverty and Death: Disaster Mortality 1996-2015, the United Nations Office for Disaster Risk Reduction concluded:

“Like China, India saw a sharp rise in the number of floods decade-on-decade, but unlike China, India’s flood mortality increased, with 90 floods claiming 15,860 lives in 2006-2015, up from 13,660 lives lost in 67 floods the previous decade. If India could emulate China in terms of reducing flood deaths, the declining global trend in flood mortality seen over the past decade could perhaps be extended.”

Even though the catastrophic Kerala floods could be viewed through a nationwide lens as part of a typical season of flood misery in what is actually a drier-than-average monsoon year, the regional picture is what’s most important in a nation like India, which is both large (nearly twice the size of Alaska) and populous (more than 1.3 billion people). For the still-suffering residents of Kerala, the last several weeks have been anything but typical.

Dr. Jeff Masters contributed to this post. We'll have a full update later Tuesday on formidable Category 4 Hurricane Lane and its potential impacts on Hawaii (see our post from Monday).