Why India’s northeastern states are witnessing frequent floods

India’s northeastern states are suffering from floods and landslides once again in mid-June. Devastating floods had also wreaked havoc in mid-May. More than 10 lakh people have been affected and well over 1 lakh houses are damaged.

Reports of devastating floods in the North East have been making headlines for nearly a decade. Much attention has been paid to the processes that cause large-scale extreme rainfall events over the rest of India.

Seasonal total rainfall has been decreasing over the core monsoon zone as well the North East over many decades. Decreasing rainfall has been attributed to the pollution-driven solar dimming which has prevented the subcontinent from warming and thus weakened the monsoon circulation.

In the meantime, the low-pressure systems that originate in the Bay of Bengal and propagate onto land have reduced in number at the active/break periods but have increased at weather timescales of a few days. Low-pressure systems contribute over 50% of the seasonal total rainfall.

A warmer Arabian Sea is pumping more moisture onto land and combining with the low-pressure systems to produce large-scale rainfall events which have increased from two per year to six per year in recent decades.

Rising temperatures

What then is driving the river overflows, floods and landslides over the North East? Clearly, the mountainous regions not only extract rain from moist winds but also combine with land-use changes to convert rainfall into floods and landslides.

The low-level jet or the southwesterly winds over the Arabian Sea have drifted northward over the last few decades and their variability at active/break timescales has increased. This has reduced the total rainfall over Kerala and increased the rainfall over the northern reaches of the Western Ghats into Gujarat.

Whether the rainfall has increased or decreased in a particular region, temperatures have warmed everywhere and the warmer atmosphere is a thirsty atmosphere demanding more moisture. The northward tilt of the low-level jet trucks additional moisture all the way to the Northeast picking up additional moisture along the way from the Ganga basin with its vast and irrigated agricultural fields.

Over 20% of the moisture for the northeast rainfall is supplied by the Ganga basin. This raises questions about the impact of extensive irrigation over central India on the North East floods.

The heatwave season arrived early this year and the duration, intensity and area covered by the heatwaves broke all records. It is a scary image for the future in terms of how global warming can combine with natural variability such as La Niña to produce destructive and persistent heatwaves.

El Niño and La Niña also impact the monsoon. A La Niña that emerged in 2020 continued into 2021 and is forecasted to continue to the end of this year; a rare three-year La Niña. La Niña tends to favour a normal monsoon but alas now “normal” has no meaning for the monsoon.

Moisture-laden winds

Late pre-monsoon cyclones now occur more often thanks to the weaker monsoon circulation. During the last few years, the cyclones dragged the monsoon trough onto Andaman and Nicobar and the Bay of Bengal to bring on-time monsoon onsets. This year saw no strong cyclones but the onset was slightly early although rainfall has been deficit over much of the country.

And yet, the northward tilt of the southwesterlies has driven the moisture into the North East to create massive floods. Moisture-laden winds are forced to rise and expand when they run into mountains. This causes an expansion and a cooling leading to condensing of the moisture and heavy rains. Significant moisture is also pumped in from the Bay of Bengal to exacerbate the deadly deluges.

What has caused the northward tilt of the low-level jet over the Arabian Sea? A prime candidate is the accelerated warming over West Asia. Since 1990, West Asia to the Mediterranean region has warmed nearly twice as much as India and the Arabian Sea. This produces a pressure gradient between the Arabian Sea and West Asia that is likely to exert a northward pull on the low-level jet.

Spotty rainfall distribution

All changes over the Indian Ocean and the monsoon region tend to project onto the so-called intraseasonal timescale which is also the active/break timescale. The rainfall distribution thus far appears quite spotty and yet the deficient rainfall over Kerala thus far and the increased rainfall over the northern Western Ghats are consistent with the rainfall trends over recent decades driven by the northward orientation of the southwesterlies.

The India Meteorological Department continues to offer early warnings that have reduced the loss of life due to floods. The granularity of these forecasts needs to get finer in producing local information which is in the offing.

Disaster management based on reliable early warnings are the best option for strategic disaster prevention, mitigation and recovery. Motherhood statements about disaster-proofing the country will be ineffective without such warning systems.

Raghu Murtugudde is a Professor of Atmospheric and Oceanic Science and Earth System Science at the University of Maryland and visiting faculty at the Indian Institute of Technology, Bombay.