Indian sub-continent has three marked rainy seasons: Southwest Monsoon, Northeast Monsoon and Winter rains. Lots of studies have been done and indicators worked out towards likely performance of Southwest Monsoon and winter rains. However, Northeast Monsoon remains a less rehearsed and researched subject. The period October to December is referred to as Northeast Monsoon season over peninsular India.
Northeast monsoon season is the major period of rainfall activity, particularly in the eastern half of peninsula comprising of the meteorological sub- divisions of Coastal Andhra Pradesh, Rayalaseema and Tamil Nadu- Puducherry. For Tamil Nadu, 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 areas of the state get about 40-50% of it.
The increase in rainfall activity along South Andhra Pradesh and Tamil Nadu coast, which takes place sometime in the first half of October, has generally been considered as the setting of Northeast Monsoon
Like the El-Nino, ENSO, MJO etc. are linked with the performance of Southwest Monsoon over the Indian region, QBO(Quasi Biennial Oscillation) has a teleconnection with the Northeast Monsoon rainfall. QBO is a periodic oscillation of equatorial zonal winds between easterlies and westerlies in the tropical stratosphere. Few other noticeable features of this phenomenon are:
- The average period for change from westerlies to easterlies and vice-versa varies between 25 to 29 months.
- Easterlies are generally stronger( 30-35m/s) than westerlies (15-20m/s)
- The amplitude of easterly phase is about twice as strong as that of westerly phase
- Both, period and amplitude, vary considerably from cycle to cycle
- Maximum amplitude of both phases typically occur near 20 hPa(20-25km)
- Waves responsible for this effect remain debatable
- Probably, Gravity Waves are the major cause/contributor
Some of the effects of QBO are:
- Responsible for rainfall variation in several regions of the globe
- Modification of NE monsoon precipitation in India
- Strength of Jet Stream
- Severity of winters in the Northern Europe and Eastern USA
Some of the features of QBO with respect to Northeast Monsoon are:
- Northeast Monsoon is basically a lower tropospheric phenomenon
- QBO is a lower stratospheric phenomenon
- Probability of excess rainfall during easterly phase is only 23%
- Probability of deficit rainfall during westerly phase is only 10%
- During westerly phase, the probability of NE Monsoon being normal is 55% and its being normal/excess is 90%
- During easterly phase, the probability of NE Monsoon being normal/excess is 36%
We may infer that probability of NE Monsoon being deficient during the westerly phase and excess during easterly phase is low. The westerly phase of QBO has concluded three months back and it has entered the easterly phase now. We may also conclude that no single factor can sway a large scale phenomenon like monsoon and therefore keep our fingers crossed for a reasonable rainfall during the Northeast Monsoon.