Tamil Nadu receives highest January rainfall in last 100 years; heavy crop losses in Cauvery delta
Tamil Nadu’s normal January rainfall is 18 mm. Already, this month, the state has received 124.8 mm. The previous heaviest recorded rainfall was 141.2 mm in January 1921, and the state looks all set ready to break that record.
Pic credit: SANDRP
Tamil Nadu receives a large chunk of its annual rainfall (48 per cent) in the northeast monsoon season from October to December. By January, the rains subside. However, this year has been unusual as the state continues to receive very heavy rainfall, which is expected to go on till January 18. Extended rainfall has led to large-scale crop losses across the state.
As per the data of the India Meteorological Department (IMD), between January 1 and January 14 this month, the meteorological sub-division of Tamil Nadu and Puducherry has registered a rainfall departure of 1,057 per cent. As against a normal rainfall of 10.8 mm, this meteorological sub-division has received 125 mm rainfall. There are a total of 36 meteorological sub-divisions in the country.
All the 37 districts in Tamil Nadu have received ‘large excess’ rainfall in the initial two weeks of this month, shows the Met department’s data. For instance, Thiruvarur, Thanjavur and Ariyalur districts recorded a rainfall departure of 1,111 per cent, 1,190 per cent and 1,273 per cent, respectively. The highest rainfall departure is recorded in Tiruppur that received 98.7 mm rainfall against its normal of 4.8 mm, thus a rainfall departure of 1,957 per cent.
“Historic January rains witnessed in Tamil Nadu and Kerala. Almost all districts recorded large excess rainfall. The interplay of local dynamics and synoptic scale processes could be attributed to this freak rainfall event,” Sridhar Balasubramanian, an associate professor of mechanical engineering and an adjunct faculty member at IDP Climate Studies at the Indian Institute of Technology Bombay, told Gaon Connection.
The rainfall data analysed by Pradeep John, a weather blogger from Tamil Nadu, shows that January 2021 (with half month still left) has seen a number of weather records breaking in the southern state. As against the normal January month rainfall of 18 millimetres (mm), till January 14, 2021, the state has already received 124.8 mm rainfall. And the rains are expected to continue till January 18.
This means there are high chances of the previous highest January rainfall record of 141.2 mm in 1921 being broken this month. In Jan 1922, the state received 136 mm rainfall and in Jan 1920 it had received 135.9 mm rainfall.
January 2021 has seen several station-wise records broken. For instance, Chennai Airport (Meenambakkam) station has broken ATR (annual total rainfall) for both 24 hours and monthly rainfall for January. On January 28, 1947, the station received the highest rainfall of 91.7 mm in 24 hours. Recently on January 6, the same station recorded 126.2 mm rainfall in 24 hours, breaking the previous record.
The Chennai Airport (Meenambakkam) station’s highest January month ATR was 170 mm in 1986. This month, till January 14, the station has already received 230.9 mm rainfall.
Similarly, both 24 hours and monthly rainfall records have been broken in other stations across the state including Cuddalore, Erode, Tirupattur, Vedaranyam, Coonoor, Karaikkal, Thanjavur, Thoothukudi, etc.
In a press release, PR Pandian, president of the Coordination Committee of All Farmers Associations of Tamil Nadu, said that one million acres of the samba crop in the delta region and 2.5 million acres across Tamil Nadu were submerged.
On January 13, the state chief minister Edappadi K Palaniswami announced the government would release compensation to the affected farmers. The state’s farmers have already suffered huge crop losses due to the two back-to-back cyclones — Nivar and Burevi.
What explains the heavy rainfall in January 2021?
Balasubramanian explains the possible reasons behind the freak January rainfall this year. “Firstly, by the end of December or early January, the Inter Tropical Convergence Zone, commonly known as ITCZ [a belt of low pressure that circles the Earth generally near the equator], should move very close to the Equator, highly favouring the Southern Hemisphere. However, the sluggish movement of the ITCZ allowed it to linger near South India, allowing transient low pressure systems to form in the North India Ocean,” he said.
The second possible explanation is the ever-changing state of the Arabian Sea cannot be discounted, as it is resulting in erratic monsoon seasons, both southwest and northeast monsoons. “The southeast Arabian Sea was warmer than normal, aiding formation of low pressure systems. This allowed moisture to be pulled in from the Bay of Bengal resulting in copious amounts of rainfall. The Arabian Sea mini warm pool has a strong inter-annual variability, which could result in massive rains,” he explained.
Lastly, La Niña has reached its peak state. “The peak La Niña is allowing strong moisture-laden winds to move in from the South China Sea, complementing the local dynamics…
A train of Rossby waves was seen in January resulting in unseasonal rains over South India as well as Maharashtra,” he added.
Excess rainfall in northeast monsoon 2020
The five meteorological subdivisions of Tamil Nadu, Coastal Andhra Pradesh, Rayalaseema, Kerala and south interior Karnataka of south Peninsula together receive about 30 per cent of their annual rainfall during the northeast monsoon season between October and December. Tamil Nadu in particular receives about 48 per cent of its annual rainfall during this season.
The overall rainfall during the northeast monsoon 2020 was in ‘excess’ category with 10.3 per cent above the long period average of 33.76 cm. It is highest during the last five years (2016-2020), notes an IMD document released recently on January 7.
According to the Met department, in the year 2020, the progress of northeast monsoon rainfall over Tamil Nadu has been highly variable as it was subdued till November 11 with the season’s cumulative rainfall departure from normal during October 1 to November 11 at minus 46 per cent.
However, rainfall over Tamil Nadu and Puducherry and adjoining southeast peninsula significantly enhanced during the second fortnight of November 2020 and the first week of Dec 2020 due to formation and movement two back-by-back cyclones towards Tamil Nadu coasts — very severe cyclonic storm Nivar and the cyclonic storm Burevi.
As a result, during the period of October 1 to December 9 last year, the cumulative rainfall in terms of per cent departure from normal over Tamil Nadu improved from minus 16 per cent by December 2 to nine per cent by December 9.
Thereafter for the remaining days of December 2020, no major rainfall spell occurred over the peninsular region due to absence of any major weather system. Thus, the cumulative northeast monsoon rainfall 2020 over Tamil Nadu ended at six per cent of the long period average in the ‘normal’ category.
But, in January this year, the rains picked up again and have continued to pound the state, leading to huge crop losses and a flood like situation in several districts.
In its rainfall forecast for the week of January 7 to January 13 this year, the IMD had warned: “Under the influence of cyclonic circulation over Southeast Arabian Sea and another cyclonic circulation over south Tamil Nadu coast & neighbourhood in lower tropospheric levels, fairly widespread to widespread rainfall with isolated heavy falls and moderate thunderstorm, & lightning very likely over southern peninsular India.”
“One needs to brace for such unseasonal events, which are primarily tied to changes in the local state of the Indian Ocean, that is warming, wave action, moisture incursion, modulating ITCZ movement, etc.,” said Balasubramanian.
Meanwhile, yesterday on January 14, the World Meteorological Organization released its 2020 global temperature update. It has confirmed the last decade to be the warmest decade recorded. “@WMO confirms we just had the warmest decade & warmest 6 years on record. Abnormal warming in 2020 in spite of a La Nina, which normally cools, suggesting role of greenhouse gases,” tweeted Madhavan Rajeevan, secretary, Union ministry of earth sciences.
The IMD’s recent report recorded deaths of at least 1,680 people in India last year due to high impact weather events, such as cyclones, floods, lightning and heavy rainfall. Also, 12 out of 15 warmest years in India were between 2006 and 2020, it noted.