The India Meteorological Department (IMD) has officially declared the arrival of the northeast monsoon, which is primarily responsible for most of Tamil Nadu’s annual rainfall. Generally expected to set in around October 20th, this year’s onset was delayed due to the prolonged southwest monsoon season.
Understanding the Monsoon
The term ‘monsoon’ typically refers to the atmospheric phenomena experienced in tropical regions, between 20° N and 20° S latitudes. Interestingly, the climate of India is categorized as ‘monsoon’ type, a classification mainly found in the south and southeast parts of Asia.
Rainfall Patterns in India
India witnesses two main monsoon seasons. The Southwest monsoon, spanning from June to September, accounts for about 75% of the country’s annual rainfall. On the other hand, the Northeast monsoon occurs from October to December.
About NorthEast Monsoon: The Winter Monsoon
The NorthEast Monsoon (NEM), also dubbed as the winter monsoon, is a smaller-scale monsoon primarily confined to the Southern peninsula.
Factors Contributing to the Northeast Monsoon
A vital factor that ushers in the Northeast monsoon is the change in wind direction following the complete withdrawal of the Southwest monsoon by mid-October. During this time, the wind pattern rapidly transitions from south-westerly to north-easterly.
The Role of Cyclonic Activities
The period after the Southwest monsoon season – from October to December – marks the peak time for cyclonic activity in the North Indian Ocean region, which encompasses the Arabian Sea and the Bay of Bengal. The development of low pressure systems, depressions, or cyclones and their accompanying winds greatly influence this monsoon and the resultant rainfall.
Global Climate Parameters and Their Impact
The NEM rainfall is significantly affected by global climate parameters such as ENSO (El Nino/La Nina & Southern Oscillation Index – SOI), Indian Ocean Dipole (IOD), and Madden-Julian Oscillation (MJO). Experiences have shown that El Nino, positive IOD, and MJO in phase 2-4 with amplitude greater than one are typically associated with good NEM rainfall. Likewise, La Nina & positive SOI during the second half of the season also foster favourable conditions for NEM activity.
Regions Associated with the NEM
Key areas associated with the Northeast monsoon include Tamil Nadu, Puducherry, Karaikal, Yanam, coastal Andhra Pradesh, Kerala, north interior Karnataka, Mahe, and Lakshadweep. In particular, Tamil Nadu receives about 48% of its annual rainfall during these months, making this monsoon a crucial factor for agricultural activities and reservoir management within the state.