El Nino-fuelled Northeast monsoon caused the deluge in south Tamil Nadu

Subject: Environment

Section: Climatology

Context

Many parts of southern Tamil Nadu got flooded due to Cyclone Michaung in early December. The cause for this is an active Northeast monsoon (NEM) fuelled by the El Nino phenomenon, which is peaking right now.

Details:

The El Nino is the warmer-than-normal phase of the El Nino Southern Oscillation Phenomenon (ENSO) in the central and eastern Pacific Ocean and generally enhances rainfall extremes during the NEM.
El Nino provides the intensity while low-pressure systems or cyclonic circulations around the region provide the required moisture convergence for rainfall events to the NEM.
- Cyclonic circulations are whirls of winds in the upper layers of the atmosphere. They usually induce low-pressure areas in the atmosphere’s lower layers, which eventually cause rainfall. Sometimes, the cyclonic circulations can cause rainfall all by themselves by bringing moisture-laden winds into the region.
- Upper-level divergence is the spreading out of air in the upper layers of the atmosphere, which causes air from below to rise. This leads to other winds taking their place. This process is known as ‘low-level convergence’.

What are North-East Monsoons (NEM)?

Also known as the “retreat of monsoon”, NEM enters India from the northeast. The wind blows from the sea to the land in this type of monsoon.
The monsoon trough weakens and shifts southward as the monsoons retreat. As a result, the pressure gradient is minimal.
The moisture from the Indian Ocean (Bay of Bengal region) is carried by the monsoon winds.
It is limited to south India, bringing rain to Tamil Nadu, Puducherry, Karaikal, Yanam, Andhra Pradesh, Kerala, Mahe, and south interior Karnataka from October to December.
Low-pressure systems, depressions, and cyclones cause the associated rainfall, also known as the winter monsoon.
This is Tamil Nadu’s main rainy season, with the state receiving 48% (447.4mm) of its annual rainfall during these three months.