In winter when sub-tropical westerly jet stream establishes itself South of Himalayas, an induced anti-cyclonic circulation develops over North India, which triggers the onset of North-East monsoon. The anti-cyclonic circulation picks up enough moisture from the Bay of Bengal and pours it on the Eastern side of the Eastern Ghats. Almost entire Tamil Nadu get rainfall from this North-East monsoon.
These disturbances are the low pressure systems originate in West Asia and over Mediterranean Sea travel Eastward across Iran and Pakistan to reach India during winter after getting their moisture contents from Caspian Sea and Persian Gulf. These depressions are stirred into Indian by the Westerly jet stream and are most active during December and February. These depressions are responsible for much of the winter rainfall over the great Indian plains and snowfall over the higher riches of Himalayan belts. It also brings winter rainfall in North-East in Assam and Arunachal Pradesh. The amount of rainfall decreases from West to East. This is extremely useful in saffron cultivation and Lirabi crops in North-West India.
During October monsoon low pressure trough becomes weaker and monsoon with draws from North-West India following the course of Southward movement of Sun. The retreat of Monsoon is marked by clear skies and rise in temperature. The land is still moist, owing to high temperature and the weather condition in highly oppressive. This is known as October heat.
Sometimes in January and February the Westerly jet stream unusually stays over Southern Himalayan region and creates unexpected high pressure over the region. Due to high pressure, cold winds are thrown out to Indian plain or Northern states, sometimes night temperature go below the freezing point. Such periods of unusually cold weather are called Cold Waves.
Rainfall Distribution in India
The distribution of rainfall in India is a quite uneven and the regional variations are apparent. The average annual rainfall is about 125cm, but has great spatial variations. The highest rainfall occurs along the West coast, on the Western ghats, as well as the sub-Himalayan areas in the North-East and the hills of Meghalaya (Khasi, Jaintia and Garo).
Though, the Southern part of Meghalaya plateau gets more than 1000 cm, but its Northern frontier along with Brahmaputra valley gets less than 200 cm rainfall. The Western part of Jaisalmer (Rajasthan) is one of the driest parts of the world having only about 9 cm rainfall. Total rainfall increases generally eastwards and with height. The monsoon depressions causes wide spread rainfall in the North-Eastern part of the Indian Plateau and the Ganga Plain.
The larger part of the Gangetic plains and the central uplands receive a moderate amount of rainfall. The rainfall over parts of Punjab-Haryana, Kutch and Kathiawar region of Gujarat is below 60 cm. A narrow strip of land in the lee side of Sahyadris is lying in the rain shadow area and has below 60cm of rainfall.
Variability is an important factor of rainfall distribution that determines to a large extent the incidence of drought, is computed in terms of coefficient of variations. Region with high rainfall are having lower coefficient of variation i.e. West Coast, Sikkim etc and regions with scanty rainfall have higher coefficient of variations i.e. Rajasthan desert, Kutch etc and are prone to drought.
Rainfall in Tamil Nadu
Tamil Nadu receives most of its rainfall during the months of October, November and December. The Easterly depression and presence of ITC account for this rainfall. A seasonal low pressure is also located off Tamil Nadu coast during these months, which strengthens North-East Monsoon rainfall further, but only over a narrow coastal belt.
Easterly depressions and cyclones also originate west of 90°E meridian and between 7°N and 10°N latitude, bring heavy rainfall along the coast. So the rainfall is higher in the coast than the interior. The coastal strip between Point Calimere and Cuddalore receives the highest rainfall during these months.