Indian Climate II

Date: December 13, 2014

Indian Climate

Factor Affecting Indian Climate :

  1. Latitude – Tropic of Cancer passes through Central India; from Runn of Kuchchh to Mizoram. There sub-tropical climate in North of Cancer tropic and Tropical Climate in South.
  2. Altitude – Himalayas acts as a barrier for the cold winds from Central Asia, Siberia.
  3. Winds and Pressure – India lies in the North-East winds. These winds origins from the high pressure belts in sub-tropics. These North Easterlies moves towards south and get deflected towards right because of Corilis forces, and moves towards Equatorial low pressure belts. They generally blow from land surface, hence no or less rain.

During winters there is high pressure above Himalayas and cold winds from here move towards the low pressure area in the ocean. In summers, there is low pressure area over India. So the winds from high pressure area from South of Indian Ocean moving in South East direction after crossing Equator turns to right and moves towards the low pressure area in Indian sub-continent. These are known as South West monsoon winds. These winds gather moisture form Indian Ocean and bring rainfall in India.

The main component of upper air circulation is Jet stream. These Jet streams are under the influence of westerly flow and hence are known as subtropical westerly Jet streams. Other than summers these Jet stream flows throughout the South of Himalayas. These Jet Streams are located 12 km (troposphere). In summers speed of Jet streams is 110/hr. and in winters 180-190 km/hr. With the apparent movement of sun, the sub-tropical westerly Jet stream shifts towards North of Himalayas in summers. Another Jet stream known as sub-tropical Easterly Jet streams flows over 140N of Peninsular India.

Western Cyclonic Disturbances – Westerly flow of Jet streams from Mediterranean sea in Winters causes rainfall in North and North-West of the country.

Indian Monsoon

Indian Ocean surface, Tibet plateau and Jet streams plays an important role in the origin of Monsoon winds.

  1. Jet Streams – Because of Himalayas and Tibet plateau, Jet stream divides and one branch flows North of Himalayas.

               During summers the jet stream flows in Trans-Himalayas and with the beginning of June month the Southern branch of Jet streams merges with Trans Himalayan branch. Because of this a low pressure centre is developed in the North, North-West part of India which helps in the origin of monsoon.

               Inter Tropical Convergence Zone – This is broad low pressure through in the equatorial latitudes. Here North-East and South-East trade winds converge. This ITCZ lies parallel to the equator and moves in North-South direction with the apparent movement of sun.

               In the June moth the strong Jet streams on the Southern Hemisphere force the ITCZ to move towards North. In summers the position of ITCZ is over the Gangetic Plain. This is known as Monsoon through.

  1. Tibet Plateau – This plateau got heated up in summers due to which the Jet stream flowing in the South of Himalayas gets weak. It activates the East Jet stream which further strengthens the High pressure over the Indian Ocean which turns on the South West Monsoon.

               In the October month, atmospheric condition changes Western Jet stream again starts flowing in South of Himalayas, East Jet stream disappears. This activates the North East Monsoon winds.

  1. Oceanic Area
  • El Nino – This is warm ocean current which develop periodically along the coast of Peru as a temporary replacement of cold Peruvian Current. This cold regular current is known as Humbolt or La Nina. During Normal period, the water of East Pacific is cold (Peru, Equador) and the West Pacific Water is warm (Indonesia, West Australia). This condition ensures a healthy monsoon. But due to El Nino affect, this situation changes and East Pacific water is warm and West Pacific water is cold. This condition weakens the Indian Monsoon.
  • Southern Oscillation – When there is high pressure in East part of South Pacific Ocean, then there is low pressure in East of Indian Ocean. Opposite to this, when there is low pressure on South Pacific Ocean, there is high pressure on Indian Ocean. So this change of pressure is Southern Oscillation. The difference in the pressure over Darwin (Australia) and Tahiti (Pacific Ocean), tells the intensity of monsoon. When the pressure difference is negative, then the monsoon is average and late. El Nino is related to the Southern Oscillation. So this process is known as ENSO also.
  • Walker Cell – This cell extends in East-West direction. Generally in the Peru coast region the temperature of Ocean floor is less and at Indonesia, Australia coast the ocean floor temperature is high. Because of El Nino and negative Southern Oscillation, the Peru coastal part of Walker cell shifts towards East which causes weak monsoon.
  • Somali Current – This is a warm current. During Summer this current flows from South to North of Equator to 100 From there is turns towards Indian Coast. There are two gyres in this current. The North Gyre lies between 50N-90N and South gyre lies between equator to 40N. When the monsoon is active over the Indian mainland, the Southern gyre moves northward and merges with North gyre. Sometimes the Northern gyre moves away from its place and the two gyres didn’t meet. When there is good, healthy monsoon, the intensity of Southern gyre is high and in weak monsoon, the intensity is lens.

Onset of Monsoon and Withdrawal

               The Monsoon period is from June to Mids of September rainfall begins with thunder and lightening. This is called ‘Monsoon Burst’. From the Southern tip of peninsula, it divides into two branches – 1. Bay of Bengal Branch  2. Arabian Sea Branch.

               Arabian sea branch moves from Kerala coast (near 1st of June) arid reaches Mumbai till 10th of June. In the mids of June monsoon reaches Saurashtra, Kutchchh and parts of Central India.

               Bay of Bengal Branch reaches Andaman Islands till 20th May and Tripura, Mizoram till 1st of June. Within the 1st week of June spreads throughout North-East India. From there moves West towards Gangetic Plain along with Himalayan axis. It reaches Kolkata on 7th June, Patna 11th June and Varanasi 15th June. After this, the two branches merges with each other and spread over West UP, Haryana, Punjabi, East Rajasthan. Within June whole of the country is under the Monsoon. In the 1st Week of July, monsoon enters Kashmir, West Rajasthan as a weak current.

Withdrawal – Monsoon withdraws from the North-West part of country during mids of September. With the end of November, monsoon withdraws whole of country. Tamil Nadu coast receives rainfall from returning monsoon till mids of December.

Break in Monsoon Rainfall – These breaks in rainfall are related to the Monsoon trough. This trough moves North and South because of many reasons. When this monsoon trough is over the plains, then these area receives heavy rainfall. When the trough is active near Himalayas then plains didn’t get rainfall and the Himalayan rivers receives lot of rain water which in turn cause flood in plains. These flood cause massive destruction.

Distribution of Rainfall

  • Western Coast, Sehyadri, Meghalaya, Arunachal Pradesh, Sikkim, Darjiling receives rainfall above 200 cm.
  • Rest of the North East states, West Bengal, Orissa, East Bihar, Chattisgarh, Tarai Belt and Uttarakhand gets 100-200 cm rainfall.
  • South and West UP, North and West UP, East Maharashtra, East Gujarat and North Andhra receives 50-100 cm rainfall.
  • Rajasthan, West Gujarat, South Andhra Pradesh, Karnataka Plateau, Tamil Nadu, Haryana, Punjab, Kashmir receives less than 50 cm rainfall.