Tamil Nadu rains due to depression, onset of Northeast and withdrawal of Southwest monsoon

Tamil Nadu rains due to depression, onset of Northeast and withdrawal of Southwest monsoon

Sub: Geo

Sec: Climatology 

Context:

• Tamil Nadu, particularly Chennai, has been experiencing heavy rainfall since October 14, with predictions of continued downpours in the coming days. The primary causes are a low-pressure area off the Tamil Nadu coast, the onset of the Northeast monsoon, and the sudden withdrawal of the Southwest monsoon on October 15.

Meteorological Factors for heavy rainfall:

• Low-Pressure Area:
  • As of October 15, a well-marked low-pressure area was observed in the central Bay of Bengal, which is expected to intensify into a depression and move towards northern Tamil Nadu, Puducherry, and southern Andhra Pradesh in the next 24-48 hours.
• Cyclonic Circulation:
  • A cyclonic circulation off the coast of Andhra Pradesh in the west-central Bay of Bengal is also contributing to the rainfall.
• Northeast Monsoon Onset:
  • The IMD declared the onset of the Northeast monsoon, which is the main rainfall season for Tamil Nadu and southern states like Kerala, Andhra Pradesh, and Karnataka. The conditions for its onset include easterly and northeasterly winds and ongoing rainfall in southern Peninsular India.
• Southwest Monsoon Withdrawal:
  • The complete withdrawal of the Southwest monsoon, initially expected to occur over several days, was completed on October 15, coinciding with the typical withdrawal date.

About Northeast Monsoon:

• The Northeast monsoon marks the rainy season for the southern states of India, primarily affecting Tamil Nadu, Kerala, Andhra Pradesh, Karnataka, and the Union Territory of Puducherry.
• It typically begins in mid-October and lasts until December.
• Unlike the Southwest monsoon, which delivers rain across most of India, the Northeast monsoon is critical for the southeastern part of the country.

Key Features of the Northeast Monsoon:

• Onset Timing:
  • The Northeast monsoon usually begins in the second half of October, around the time the Southwest monsoon withdraws from the Indian subcontinent.
  • The onset is marked by the establishment of easterly and northeasterly winds over southern India and the Bay of Bengal.
• Main Rainfall Region:
  • Tamil Nadu receives around 48% to 60% of its annual rainfall during this period, making it the most crucial rainfall season for the state. Andhra Pradesh, Karnataka, and Kerala also benefit from these rains.
  • The monsoon is driven by weather systems such as low-pressure areas, depressions, and cyclonic circulations that form in the Bay of Bengal.
• Weather Systems:
  • The formation of low-pressure systems in the Bay of Bengal is critical for triggering heavy rainfall along the eastern and southeastern coast.
  • These systems often move westward, bringing rainfall to Tamil Nadu and Andhra Pradesh, and sometimes affecting Kerala and southern Karnataka.
• Influence of Global Phenomena:
  • El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) can influence the intensity and variability of the Northeast monsoon.
  • Positive IOD phases typically bring more rainfall, while El Niño conditions may lead to reduced rainfall.
• Impact on Agriculture:
  • The Northeast monsoon is vital for winter cropping in the southern states. Crops like rice, groundnuts, pulses, and millets depend heavily on this monsoon.
  • The timing and amount of rainfall can significantly impact crop yields and water availability in these regions.
• Urban Flooding:
  • Cities like Chennai and coastal areas of Tamil Nadu and Andhra Pradesh are often prone to urban flooding during this season due to heavy rains, compounded by poor drainage systems.

Importance of the Northeast Monsoon:

• The Northeast monsoon accounts for nearly 30% of the total annual rainfall for the Peninsular region.
• Tamil Nadu relies heavily on this monsoon for replenishing water reservoirs, which is critical for drinking water supply, agriculture, and hydropower.

Source: DTE