THE INDIAN MONSOON
Following facts are important in formation of monsoon:
- The Sun causes differential heating and cooling of land and water. This creates low pressure on the landmass of India and high pressure over the ocean surface.
- The Inter Tropical Convergence Zone (ITCZ) is normally positioned about 5°N of the equator. It shifts over the Ganga plain during summer. It is also known as the monsoon trough during the monsoon season.
- The high pressure area, east of Madagascar is approximately 20°S over the Indian Ocean. This area affects the Indian Monsoon.
- The Tibetan plateau gets intensely heated during summer. This results in strong vertical air currents and formation of high pressure over the plateau. This high pressure zone is about 9 km above the sea level.
- The westerly jet stream move to the north of the Himalayas, and the tropical easterly jet stream moves over the Indian Peninsula during summer.
- In normal circumstances, when the tropical eastern South Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. Such changes in the pressure conditions over the southern oceans also affect the monsoon.
- But in certain years, there is a reversal in the pressure conditions. In this case, the eastern Pacific Ocean has lower pressure compared to the eastern Indian Ocean.
- This periodic change in pressure conditions is known as the Southern Oscillation or SO.
- The difference in pressure over Tahiti and Darwin is computed to predict the intensity of the monsoons. Tahiti (18°S/149°W) lies in the Pacific Ocean and Darwin (12°30’S/131°E) lies in northern Australia. If the pressure differences are negative, it means a below average and late monsoon.
This is a feature which is connected with the SO. El Nino is a warm ocean current that flows past the Peruvian coast; in place of the cold Peruvian current, every 2 to 5 years. El Nino significantly affects the changes in pressure conditions. Hence, the phenomenon is called ENSO (El Nino Southern Oscillations).
THE ONSET OF MONSOON
Generally, the monsoon arrives at the southern tip of the Indian peninsular by the first week of June. Subsequently, it divides into two branches, viz. the Arabian Sea branch and the Bay of Bengal branch.
- The Arabian Sea branch reaches Mumbai about ten days later, i.e. around 10th of June. The Arabian Sea branch of the monsoon arrives over Surashtra-Kuchchh and central part of the country by mid-June.
- The Bay of Bengal rapidly advances and reaches Assam in the first week of June. The monsoon winds are then deflected by high mountains and move towards west over the Ganga plains.
- The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains.
- Delhi usually receives monsoon showers from the Bay of Bengal branch by the end of June.
- Western Uttar Pradesh, Punjab, Haryana, and eastern Rajasthan experience monsoon by the first week of July. The monsoon reaches Himachal Pradesh and the rest of the country by mid-July.
Withdrawal of Monsoon
Withdrawal or the retreat of the monsoon is a more gradual process.
- The monsoon begins to withdraw from the northwestern states of India by early September.
- The monsoon withdraws completely from the northern part of the Indian peninsular by mid-October.
- The monsoon withdraws from the rest by the country by early December.
- The islands receive the very first monsoon showers from the first week of April to the first week of May; progressively from south to north. The withdrawal of monsoon in the islands takes place from the first week of December to the first week of January.
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