Atmospheric river, not just low-pressure system, caused 2023 flooding

CHENNAI: A rare atmospheric river and embedded high-intensity storm bands were the key drivers behind the record-breaking rainfall that inundated southern Tamil Nadu in December 2023, according to a new study, with weather experts saying improved forecasting tools could help predict such extreme events in the future.

The study, conducted by Sivachitralakshmi S and Chitra P of the SRM Institute of Science and Technology and published in the Journal of Water & Climate Change, identified a thermodynamically enhanced atmospheric river and narrow convective bands, termed ‘AR rapids’, as the primary drivers of the extreme rainfall event that battered Thoothukudi and neighbouring districts.

The findings challenge the conventional understanding that the disaster was largely caused by a low-pressure system and favourable ocean conditions. It argues that the rainfall was amplified by a concentrated corridor of atmospheric moisture that remained nearly stationary over the region for almost two days.

On December 17 and 18, 2023, Kayalpattinam recorded an unprecedented 946 mm of rainfall in just 24 hours, surpassing its average annual rainfall. Tiruchendur received 690 mm, while Srivaikuntam recorded 620 mm, resulting in severe flooding that disrupted transport networks and left widespread damage.

Using high resolution Weather Research and Forecasting (WRF) model simulations, satellite-based rainfall observations and ERA5 climate reanalysis datasets, the researchers reconstructed the atmospheric conditions that led to the disaster. They found that a narrow corridor carrying large quantities of water vapour developed over the equatorial Indian Ocean and stretched towards Tamil Nadu. At its peak, moisture transport exceeded 850 kg m⁻¹ s⁻¹ (kilograms per metre-second) and was aided by a strong low-level jet over the Bay of Bengal.

As the moisture plume approached the coast, frictional effects and atmospheric convergence over the Gulf of Mannar slowed the system, causing it to remain nearly stationary over southern Tamil Nadu for close to 48 hours. The process was further intensified by warmer than normal sea surface temperatures associated with the El Niño phenomenon.

Warming climate makes Chennai vulnerable to atmospheric rivers

Researchers also identified narrow convective bands between 5 km and 15 km wide moving at speeds of nearly 108 kmph. These features, referred to as ‘AR rapids’, generated the most intense rainfall bursts during the event.

According to the study, the AR rapids contributed 513 mm, or 54.22%, of the total rainfall recorded at Kayalpattinam despite being active for only about one third of the storm’s duration. During the peak hour, when rainfall intensity reached 90 mm per hour, the rapid bands alone accounted for 75 mm per hour, representing more than 83% of the total rainfall intensity.

The study is believed to be among the first to document such atmospheric river rapid bands in a tropical monsoon environment without the influence of strong cold fronts. It also warns that many operational weather forecasting models may not possess sufficient resolution to detect these highly localised but destructive features, potentially leading to underestimation of flood risks.

Speaking to DT Next, independent weather analyst Srikanth said the concept of an atmospheric river can be understood as a river of moisture flowing through the atmosphere, similar to how rivers transport water across land.

“Just as we refer to a channel of flowing water on the ground as a river, an atmospheric river is essentially a continuous stream of highly moisture laden clouds moving through the atmosphere. When these moisture corridors become concentrated over a region, they can trigger extremely intense rainfall,” he explained.

Srikanth pointed out that atmospheric river conditions had also played a significant role during the devastating Chennai floods of 2015 and the extreme rainfall recorded in Sirkazhi three years ago, where nearly 40 cm of rain fell within two hours.

Referring to the 2023 Thoothukudi floods, Srikanth noted that meteorologists had successfully forecast the presence of a low pressure circulation and the likelihood of heavy rainfall. However, the unprecedented magnitude of the event was not anticipated. Now, with advances in weather modelling and atmospheric observations, it is possible to identify atmospheric river events in advance.

He also warned that coastal districts such as Chennai remain particularly vulnerable because moisture-rich cloud bands originating over the sea typically make landfall first along coastal regions.

While atmospheric rivers are relatively common in parts of North America, they remain rare over Tamil Nadu. However, researchers caution that a warming climate could increase the frequency and intensity of such events, underscoring the need for high resolution forecasting systems capable of detecting these hidden atmospheric threats before they cause disasters on the ground