Why south Tamil Nadu received record-breaking rain during the northeast monsoon

During the Northeast Monsoon (NEM) of 2023, Tirunelveli, a coastal district in the southern part of Tamil Nadu witnessed record-breaking rainfall of 123.9 cm. This observation was 146% higher than the seasonal average of 50.3 cm, according to data from the Regional Meteorological Centre (RMC).

On December 18, 2023, a weather station in Kayalpattinam in Thoothukudi district, recorded 94.6 cm, the highest rainfall ever witnessed in 24 hours during NEM in Tamil Nadu. This new observation is equivalent to 90% of usual rainfall received by the entire district in a year.

Cherrapunji in Meghalaya – known as one of the wettest regions of the country – tops the list of highest rainfall received in a single day in India. On June 16, 1995, Cherrapunji recorded 156.3 cm. Aminidivi in Lakshadweep is a close second, with 116.8 cm recorded on May 6, 2004.

“Although there have been times where more than 100 cm were recorded in 24 hours, most of it happened either during the southwest monsoon, in the hilly areas or in those places close to the Western Ghats. Whereas when it comes to NEM, Kayalpattinam is the only dry plain of the east coast to have received such intense rainfall which was unprecedented earlier,” weather blogger Pradeep John told Mongabay-India, based on data published by him on his page Tamil Nadu Weatherman on X (formerly Twitter).

Such extreme rainfall is usually expected during severe cyclones. However, it was not a cyclonic storm that caused the record-breaking rainfall at Kayalpattinam, but rather a mere upper air circulation, suggests another expert. “Such extreme rainfall was not expected from upper air circulation,” remarked S. Balachandran, deputy-director general of meteorology at RMC.

Weather forecasting models can accurately predict up to and above 20 cm rainfall in a day. “We can predict that the rainfall will be above 20 cm, but it is a limitation of science, that we cannot forecast up to what level. We can anticipate improved models in 20 years,” Balachandran added. RMC predicts rainfall in three categories: heavy (6 to 11 cm); very heavy (12 to 19 cm) and extreme (20 cm and above).

“The closest comparison to the extreme rainfall event at Kayalpattinam in recent years is from May 6, 2004, when Aminidivi at Lakshadweep recorded 116.8 cm and July 27, 2005, when Vihar lake in Mumbai received 104.9 cm. While the Lakshadweep event happened when a cyclone passed near the islands, Mumbai rains happened during the southwest monsoon which is generally a much larger phenomenon compared to NEM in India,” said K. Srikanth, a weather blogger who manages Chennai Rains (COMK), another page on X.

A closer look at Tamil Nadu’s rainfall history

In Tamil Nadu, the average rainfall levels during the southwest and northeast monsoons are 328.4 mm and 443.3 mm respectively. However, a close analysis of the RMC data shows that both the monsoon periods have been erratic in Tamil Nadu since 2015, resulting in extreme weather events such as floods and drought.

Southern Tamil Nadu districts witness 54-60 days of rainfall in a year. While 15 days of rains come from the southwest monsoon, NEM brings 50% of the state’s annual rainfall. “NEM brought normal to excessive rainfall between 2019 and 2023 and yielded below normal rains from 2016 to 2018. We had floods in 2015 and drought in 2016 and 2018,” said John, pointing to the erratic nature of NEM.

“While the uncharacteristically low rainfalls in 2016 to 2018 may be attributed to climate change, NEM has been erratic since 2000 and has a greater agreement with the changing climate parameters. TN’s capital city Chennai received 41.2 cm of rainfall on December 1, 2015 and 45 cm on December 4 the same year. This was the first such experience in south India,” said A. Ramachandran, a professor at the Centre for Climate Change and Disaster Management, Anna University, Chennai.

The role of climate change in rainfall

Climatologists say that it was the bare minimum system that caused such intense rainfall in southern Tamil Nadu. A bare minimum system is an event that is classified as a weak disturbance and extreme rainfall is not usually predicted at that time. “What gave rainfall over Tamil Nadu was upper air circulation. Rainfall poured in isolated locations such as Kayalpattinam. The role of climate change cannot be denied in this activity,” said Kartiki Negi, communications associate at Climate Trends.

Negi also explained how a majority of the heat generated due to global warming is absorbed by our oceans, leading to a steep rise in sea surface temperatures. “The increased temperatures cause the ocean to release more moisture into the atmosphere. The moisture-laden air moves slowly, thrashing like a turbulent bucket of water over some places. Similar conditions were thus seen over isolated regions of Tamil Nadu such as Kayalpattinam,” she added.

Extreme weather events continue to amplify multi-fold in the Bay of Bengal, turning it into a hotspot for observing global warming impacts. Climate researchers also say that the pattern of air circulation during NEM has been changing in the past ten years, owing to heavy emission of carbon dioxide. Global air circulation patterns aren’t as they once were, with unpredictable patterns being influenced by changes in greenhouse gas emissions.

“As a result, the warming potential in the atmosphere has been increasing alarmingly. Atmospheric behaviour is altered gradually due to global air circulation, which in turn has a heavy impact on oceanic air circulation. This phenomenon has resulted in changing monsoon patterns leading to climate extremities,” said Ramachandran.

Climate change and the oceans

Extreme weather events, such as intense rainfall that results in floods or droughts, were predicted by an assessment report from the Intergovernmental Panel on Climate Change (IPCC) in 2021, especially in Asia.

Several studies also reiterate the occurrence of drastic warming of oceans. The northern Indian ocean has been identified as one of the 17 climate change hotspots among the world oceans in a study published by Environmental Development in 2016. These areas are observed to be warming faster than the remaining 90% of the oceans. “These regions are expected to provide the potential for early warning and evidence of the response by natural resources to climate change. In theory, these regions at the frontline of climate change, should also be leading in assessing impacts and evaluating adaptation options,” the study mentioned.

Global warming affects the solubility of gases, including oxygen and the exchange of gases between the ocean surface and the atmosphere. The study warned that it could lead to the natural mixing between nutrient-rich deep waters and the relatively nutrient-poor surface waters of the Bay of Bengal being hindered. More stratification could make this basin far less productive than its western neighbour, the Arabian Sea, it further elaborated.

Sea surface temperatures (SST) have increased by 0.2-0.3°C along the Indian coast of the Bay of Bengal during a 45-year period from 1960 to 2005. The study predicts an increase of 2–3.5 °C by the end of the century.

Read more: [Explainer] Why are intense storms and erratic rainfall events becoming more frequent?

Is EL Niño also responsible?  

El Niño is a natural phenomenon that creates a warming effect in the Pacific Ocean periodically. The easterly wind which lifts the clouds hits the Amazon basin with heavy rainfall. Geographically located opposite to the Pacific, it brings dry weather, resulting in droughts in the Indian subcontinent.

“The El Niño pattern this year is different than the usual one and thus its impact on the southwest and northeast monsoons are not as expected,” says Raghu Murtugudde, professor of atmospheric and oceanic science and earth system science at University of Maryland and visiting faculty at IIT Bombay.

Ironically, global warming and El Niño work in tandem. “El Niño 2023 brewed under the influence of global warming. While the occurrence and frequency of this phenomenon is not unusual, warming conditions led to stronger El Niño,” said Negi. Usually, El Niño peaks during the winters but this season saw its indices peaking well from the beginning of its inception. “El Niño is also known to amplify global temperatures temporarily, which it has done this year too with 2023 most likely to be the hottest year on record. Prior to this, 2016 was the warmest year and that too was an El Niño year,” she added.

According to Murtugudde, every weather event now happens in a warmer world. Therefore, they all have some influence of global warming on them. “Warm oceans provide more moisture and warm atmosphere holds more moisture resulting in intense rainfall,” he added.

“El Niño isn’t a new occurrence; perhaps its impacts are heightened due to climate change,” remarked Kurian Joseph, a professor at the Centre for Environmental Studies at Anna University, Chennai.

Moving ahead, while dealing with cyclones or intense rainfall, it is crucial to identify the areas most prone to these impacts. “Considering the possibility of drought as another extreme event, it is important to save water, as groundwater storage cannot be improved with sudden downpours,” Joseph added.

Read more: [Interview] Ocean processes influence monsoon, but it’s not always climate change, says M.N. Rajeevan

Banner image: The aftermath of intense rainfall on December 17 and 18 in South Tamil Nadu. Photo by Thinak.