Northeasterly Post Summer Monsoon of Indian South Peninsula

The period October to December is referred to as Northeast Monsoon season over southern peninsular India. Earlier this period was also referred to as "Post-Monsoon Season" or "Retreating southwest Monsoon Season". But the structural feature of Transition Monsoon Season (Oct-Dec) is distinctly different from a quasi - geostrophic type structure of the mid-summer monsoon circulation , and is similar to the equatorial wave mode ( although not exactly symmetric about the equator ) , as evident in the attached 850hPa map . Convections are quite active over the equatorial BBG and SCS due to merging of the equatorial westerlies from the Indian Ocean with the easterlies from the Pacific .

Northeast Monsoon season is the major period of rainfall activity over south peninsula, particularly in the eastern half comprising of the meteorological subdivisions of Coastal Andhra Pradesh, Rayalaseema and Tamilnadu-Pondicherry. For Tamilnadu this is the main rainy season accounting for about 48% of the annual rainfall. Coastal districts of the State get nearly 60% of the annual rainfall and the interior districts get about 40-50% of the annual rainfall.

Though the principal rainy season for Interior Karnataka, Kerala and Lakshadweep is the Southwest Monsoon season, rainfall continues till December in these sub-divisions, the period October-December (Northeast Monsoon ) contributing about 20 % of the annual total.

The increase in rainfall activity over Andhra-Tamilnadu coasts which takes place sometime around middle of October is generally considered as the "setting in of Northeast Monsoon". Normal date of onset of the northeast monsoon is around 20 October with a deviation of about a week on either side.

The rainfall over south peninsula towards the end of southwest monsoon season is mainly in the interior districts and it generally occurs in the afternoon , evening or early part of the night.