Tamil Nadu has witnessed heavy rainfall in December

What’s in today’s article?

• Why in news?
• What is northeast monsoon?
• News Summary: Tamil Nadu has witnessed heavy rainfall in December
• Tamil Nadu generally see rain in December
• How much did it rain over southern Tamil Nadu this week?
• Reasons behind the record rainfall over southern TN
• IMD’s forecast

Why in news?

• Tamil Nadu has battled heavy rainfall throughout December.
  • In the beginning of the month, parts of Chennai and its neighbourhood experienced massive flooding because of Cyclone Michaung.
  • Earlier this week, Kalyanapattinum in Thoothukudi district saw excessive amounts of rain, as high as 950mm in 24 hours, leaving a trail of destruction.

Northeast monsoon

• About
  • The northeast monsoon is another major and permanent feature of the Indian subcontinent’s climate system.
  • Its name is derived from the direction of the monsoon winds, which blow from the northeast to the southwest.
  • During their movement, these winds pick up moisture from the Bay of Bengal.
  • This moisture is then dumped over the southern states of Tamil Nadu, Andhra Pradesh, Kerala, and even south Telangana and Karnataka.
  • It is also referred to as the winter monsoon, retreating monsoon, or reverse monsoon.
• Period
  • It remains active during the three-month period between October and December.
• What causes and influences the northeast monsoon?
  • One of the primary causes of the northeast monsoon is the southward movement of Inter Tropical Convergence Zone (ITCZ).
    • ITCZ is a dynamic region near the Equator where the trade winds of the northern and southern hemispheres come together.
  • This southward movement of the ITCZ, coupled with the heating up of the Indian Ocean, reverses the direction in which the lower-atmosphere moisture-laden winds blow (from southwest to northeast), thereby triggering the NEM.

News Summary: Tamil Nadu has witnessed heavy rainfall in December

• 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).

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.
• Hence, rainfall during these months are normal over Tamil Nadu.

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).

Reasons behind the record rainfall over southern TN

• Northeast monsoon
  • The northeast monsoon remained vigorous over Tamil Nadu since the start of this week, brining in steady rainbands, especially over the south Tamil Nadu and neighbouring Kerala.
• Development of a cyclonic circulation
  • 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.
• Heavy cloud convection
  • The above-mentioned 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) in these areas.
    • Cloud convection is the process by which warm air rises and cool air sinks, transferring heat and mixing within a fluid.

IMD’s forecast

• 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.

Q1) What is India Meteorological Department (IMD)?

The India Meteorological Department (IMD) is a government office and weather forecast agency. The Government of India established the IMD on January 15, 1875. The Ministry of Earth Sciences is the IMD's parent organization.

Q2) What is Inter Tropical Convergence Zone (ITCZ)?

The Intertropical Convergence Zone (ITCZ) is a low-pressure band that circles the Earth near the equator. It is where the trade winds of the Northern and Southern Hemispheres meet, causing frequent thunderstorms and heavy rain.

Source: Why Tamil Nadu has witnessed heavy rainfall in December | The Weather Channel