Under the Torrent: Causes Behind Tamil Nadu's Heavy Rainfalls

In December 2023, Tamil Nadu, nestled in southern India, experienced an unprecedented onslaught of rainfall, unlocking a sequence of unfortunate events.

This surge in precipitation stemmed from the active Northeast monsoon (NEM) stretching over the southern peninsula, a seasonal phenomenon known to last about 70 days, impacting coastal Tamil Nadu and Andhra Pradesh. 

December commenced with Chennai and neighbouring regions witnessing notable downpours due to the emergence of Cyclone Michaung. After an interval of nearly 15 days, Tamil Nadu encountered an overwhelming downpour, primarily on December 17 and 18. 

This exceptional weather endured throughout the week, inundating several southern districts with extreme rainfall, resulting in the loss of 31 lives and causing widespread flooding. 

Severity of the Event

Between October and December, Tamil Nadu received 453.7 mm of rainfall, slightly surpassing the expected normal of 430.7 mm.

Some districts experienced rainfall nearly equivalent to or surpassing the anticipated annual levels for the region. 

According to the Indian Meteorological Department (IMD), the three districts below experienced a significantly higher amount of rainfall.

The persistent havoc led to the cancellation of about 15 trains and disruptions in flight operations, along with infrastructure damage.

Unveiling the Catalysts

The IMD pointed to a vigorous NEM over Tamil Nadu and its southern regions, with the El Niño phenomenon playing a pivotal role in its intensification. 

Niño, part of the El Niño-Southern Oscillation cycle, involves sporadic warming of sea surface temperatures in the central and eastern tropical Pacific Ocean. 

A cyclonic circulation formed on December 16 in the southwest Bay of Bengal. Gradually moving westward, this system heavily influenced NEM winds, persisting across southern Tamil Nadu on December 18 and 19. 

Vineet Kumar Singh, a researcher at the Typhoon Research Centre of Jeju National University, emphasized that this circulation led to substantial upper atmosphere divergence, resulting in a strong rising of moisture-laden winds. These conditions built deep convective clouds with high rain rates (in the range of 60-70mm/hr), precipitating the extreme rainfall due to the convergence of moisture-rich easterly winds.

Reflecting on historical parallels

Drawing parallels from 2015 NEM, the occurrence of a similar event linked the intense floods in South India to powerful El Niño conditions and dominant easterly wave activity which supported the intensification of synoptic systems developing over Bay of Bengal.

Experts believe this current event, akin to the intense El Niño episodes in 1982, 1997, and 2015, could stem from an extraordinary El Niño occurrence. 

With a view toward climate change, expectations arise for heightened robust El Niño events in the future, combining increased temperatures and El Niño occurrences, amplifying the probability of extreme weather conditions. 

Challenges in Prediction

The IMD's red alert projected rainfall exceeding 210 mm, however Kayalpattinam in Thoothukudi district recorded a staggering 950 mm in a single day, eclipsing the district's annual average of 700 mm. 

Experts highlighted this as a relatively shallow system, surpassing expected intensity due to higher moisture from easterly winds.

The weather prediction models encounter challenges in predicting the scale of extreme rainfall events, which posed difficulties in accurately predicting outliers, in this case 950 mm rainfall in a day, underscoring the need for thorough satellite analysis and real-time data.

Forecast Ahead

IMD's latest forecast does not predict significant rainfall or warnings in Tamil Nadu. However, the outlook suggests the likelihood of light to moderate rain in certain parts of Tamil Nadu, Puducherry, and the Karaikal region until Friday due to an easterly wind surge.

According to IMD, Under the influence of a fresh easterly wave and the above Low-Pressure Area; light to moderate rainfall at some places is very likely over south Tamil Nadu, south Kerala and Lakshadweep from 31st December 2023 to 04th January 2024 with isolated heavy rainfall over Lakshadweep on 02nd January 2024.

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