Why Tamil Nadu has witnessed heavy rainfall in December 2023

Does Tamil Nadu generally see rain in December?

The northeast monsoon is key for Tamil Nadu, Kerala, Puducherry, Karaikal, coastal Andhra Pradesh, Rayalaseema and Yanam in southern India. Of the total annual rainfall, Tamil Nadu receives nearly 48 per cent (443.3mm) during the October to December months, important for rabi cultivation.

So, rainfall during these months is normal over Tamil Nadu.

Since October 1, the rainfall recorded over Tamil Nadu has been 450mm. Only 14 out of a total of 38 districts have received deficient rainfall during the ongoing season (till December 20).

How much did it rain over southern Tamil Nadu this week?

Three districts of southern Tamil Nadu saw ‘exceptionally’ heavy rainfall during December 17–19.

The India Meteorological Department (IMD) stated that these districts had received about 100 per cent or more surplus rain during these three days over the previous week (December 6 – 13, 2023).

Some exceptional rainfall figures, not commonly occurring during a 24-hour time span, were reported by nearly 40 meteorological stations across Thoothukudi (Tuticorin), Thirunelvali and Kanyakumari districts.

For example, in Kalyanapattinam, the 24-hour rainfall recorded on December 18 was 950 mm, which is close to the annual rainfall of Lucknow and Ambala (960 mm each) and more than the annual rainfall of Allahabad (918 mm). In two days, this station received a total rainfall of 1160 mm, which was greater than the annual rainfall of Chandigarh (1, 070 mm) and Bengaluru (1,000 mm).

Some of the wettest places on December 18 were Nalumukku (470mm), Oothu (500), Manjolai (550mm) in Tirunelveli district, Tiruchendur (690mm), Old taluk office Srivaikuntam (630mm), Kakkachi (360mm) and Kulasekarapattinam in Thoothukudi district.

The annual rainfall of India’s southernmost district, Kanyakumari, is 760mm, whereas the district has already recorded 1050.7mm during October 1 – December 20.

What caused the record rainfall over southern TN?

The northeast monsoon remained vigorous over Tamil Nadu since the start of this week, bringing in steady rainbands, especially over the south Tamil Nadu and neighbouring Kerala.

On December 16, a cyclonic circulation developed in the southwest Bay of Bengal, located off the western Sri Lankan coast. As this system moved westwards and reached southern Tamil Nadu, it gave impetus to the northeast monsoon winds. This system persisted over the southern Tamil Nadu region on December 18 and 19. A heavy cloud convection was observed here, resulting in exceptionally heavy rainfall (more than 200mm in 24 hours) over Thoothukudi, Tirunelveli and Kanyakumari districts of Tamil Nadu and Idukki district of Kerala.

What is the IMD’s forecast now?

With the cyclonic circulation having completely moved away from the Indian land mass and currently located over southeast Arabian Sea, there is no significant rainfall forecast or warning over Tamil Nadu.

However, the IMD has said that light to moderate intensity (upto 64mm in 24 hours) rainfall will occur in some places in southern Tamil Nadu till Thursday, after which there will be reduction in the overall rainfall activity.

Anjali Marar works at the Raman Research Institute, Bengaluru.