Northeast Monsoon has set in over South Peninsula on 25thOctober, with a marginal delay of 5 days. Last year, it was stretched further and the onset happened on 28th October. The coastline of Tamil Nadu, Coastal Andhra Pradesh and parts of south and central Bay of Bengal(BoB) have been predominantly swept by northeasterly and easterly winds in the lower levels of atmosphere, an essential condition to be fulfilled at the time of onset.
The rainfall activity has been confined to southern parts of Tamil Nadu and the northern half along with Coastal Andhra Pradesh mostly stayed dry. Karaikal, Nagapattinam, Trichi and Kanyakumari received 25mm, 34mm, 34mm and 5mm rainfall respectively in the past 24hr. Chennai, Vellore, Dharmapuri, Kanchipuram Madurai, Thanjavur, Nellore, Kavali and Ongole remained dry. In case of resounding start, all these places also rain with varying intensity of moderate to heavy falls.
The variability of Northeast Monsoon is very high and in that, month of October is highly unpredictable. October rainfall is largely dependent on the timings of southwest monsoon withdrawal from the country and succeeded by the arrival of winter monsoon.
Northeast monsoon is immensely influenced by global climate parameters like ENSO, IOD and MJO. El Nino, +ve IOD and MJO over the Indian Ocean with moderate amplitude is considered favorable for monsoon activity. Even La Nina with +ve Southern Oscillation Index (SOI), is associated with good northeast monsoon rains during the 2nd half of season.
Northeast Monsoon is the main rainy season for Tamil Nadu. The long period average (LPA) of annual rainfall of the state is 943.7mm. The share of southwest monsoon is just 35% (342mm) and the major contribution comes from the northeast monsoon with 48% of LPA (447.4mm). Northeast Monsoon of last year ended within normal range, but leaving large deficiency for Kerala.
The seasonal performance for the 5 sub divisions was : Kerala & Mahe -26%, Tamil Nadu & Puducherry +6%, Coastal Andhra Pradesh & Yanam +24%, Rayalaseema +54% and South Interior Karnataka 0%. Also, last year NEM season got extended till 1st half of January and cessation of rains over the peninsular India took place on 19thJanuary 2021.
Courtesy cyclonic circulation over southwest BoB and another one over southeast BoB, conditions are becoming favorable for active opening phase of the season. Moderate rains with isolated heavy falls are likely for Tamil Nadu and South Coastal Andhra Pradesh till 30thOctober. Thereafter, rainfall will increase, both in terms of intensity and spread. Heavy to very heavy rainfall is expected over North Coastal Tamil Nadu (including Chennai) and South Coastal Andhra Pradesh.