The South Indian peninsula lies adjacent to the equator. The seasonal monsoon winds cause widespread rainfall due to their southwesterly direction of travel. The southwest monsoon, which reaches the subcontinent in May, sustains the wet season during its life span of about four months. This year, since May, rainfall over most parts of the southern peninsular region has been deficient, and relatively fewer number of days with rains have been recorded in many districts. This indicates that the northeast monsoon has remained subdued this year.
India receives rainfall during two seasons, the Southwest monsoon and the Northeast monsoon. The country has three main rain belts: The Himalayan region, the Peninsular Plateau and the northeastern part of the Deccan Plateau.
Southwest monsoon: About 75% of the country’s annual rainfall is received from the Southwest monsoon between June and September.
Northeast monsoon: The Northeast monsoon occurs during October to December, and is a comparatively small-scale monsoon, which is confined to the Southern peninsula. It is called the winter monsoon.
Rainfall in parts of the country is mainly due to Northeast Monsoon. It is because of this reason that the month of October receives average rainfall over some parts of the country.
Implementation and continuation of northeast monsoon depend on the temperature conditions over the Pacific Ocean and Western Asia.
The period after the Southwest monsoon season, from October to December, is the peak time for cyclonic activity in the North Indian Ocean region covering the Arabian Sea and the Bay of Bengal.
The rainfall associated with the Northeast monsoon is important for Tamil Nadu, Puducherry, Karaikal, Yanam, coastal Andhra Pradesh, Kerala, north interior Karnataka, Mahe and Lakshadweep.
Tamil Nadu records about 48% of its annual rainfall during these months, making it the key factor for undertaking agricultural activities and reservoir management in the state.
Some South Asian countries such as Maldives, Sri Lanka and Myanmar, too, record rainfall during October to December.
La Niña conditions enhance the rainfall associated with the Southwest monsoon, but has a negative impact on rainfall associated with the Northeast monsoon.
La Niña (Spanish for ‘little girl’) refers to the large-scale cooling of the ocean surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall.
It usually has the opposite impacts on weather and climate as El Niño, which is the warm phase of the so-called El Niño Southern Oscillation (ENSO).
The current position of the ITCZ has also contributed to the poor rainfall during the ongoing monsoon season.
The ITCZ is a low-pressure belt, whose northward and southward movements along the equator determine the precipitation in the tropics.
Currently, the ITCZ is located to the north of its normal position.