The Northeast Monsoon 2024 started with a bang and arrived on the 15th of October. This is the earliest onset, since the year 2005 when it set in on the 12th of October. In the last 5 years, the earliest onset was on the 16th of October 2019. Last year, the monsoon has had started on a very mild note on the 21st of October. The onset of monsoon is not attempted before the 10th of October, even under favourable conditions. More often, the onset has been observed between the 11th and 27th of October. The most delayed onset ever was on 11th November in the year 1915. And in the last 40 years, the latest onset was on 02nd November in 1988, 1992 and 2000.
The most essential conditions for declaring the onset of monsoon are: Persistent surface easterly winds over the Tamil Nadu coast and preferably the depth of easterlies up to 5,000 feet or more. In addition, there should be fairly widespread rainfall over Coastal Tamil Nadu, South Coastal Andhra Pradesh and adjoining areas. Notwithstanding, the normal date for the setting of easterly winds over the southeast peninsula is 14th October and the normal date of onset of the northeast monsoon is 20th October.
The northeast monsoon is the lifeline for the South Peninsula. Northeast monsoon covers five meteorological sub-divisions namely: Tamil Nadu, Kerala, Coastal Andhra Pradesh, Rayalaseema and South Interior Karnataka. Out of these, this is the chief rainy season for the state of Tamil Nadu. This state lies in the rain shadow area during southwest monsoon. The state receives nearly 65% of its annual rainfall during this season, between October and December.
Northeast monsoon shows a high degree of variability with a 27% coefficient of variation. The season is influenced by global climate parameters, such as ENSO, IOD and MJO. El Nino, positive IOD and MJO in phases 2-4, are generally associated with good northeast monsoon. As a norm, La Nina and negative IOD do not augur well for the northeast monsoon. But, the season has defied these norms, more than once.
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