The tropical monsoon climate is characterized by a seasonal reversal of winds accompanied by corresponding changes in precipitation. Its key features include:
- Monthly mean temperatures above 18°C, ranging from 30-45°C in summer and 15-30°C in winter.
- Annual rainfall typically ranges from 200-250 cm, with some areas receiving around 350 cm.
- Heavy rainfall occurs mainly in summer, while winter remains dry.
Distribution: This climate is found in the Indian subcontinent, Southeast Asia (Burma, Thailand, Laos, Cambodia, parts of Vietnam, and south China), northeastern South America, and northern Australia. In India, the summer monsoon (June to September) brings rainfall across the country, while the retreating monsoon causes rainfall along the southeastern coast during winter.
Process of Onset of Indian Monsoon:
The Indian monsoon’s origin and onset are driven by several factors:
- Differential heating and cooling of land and water create low pressure over the Indian landmass, while surrounding seas experience relatively high pressure.
- The Inter-Tropical Convergence Zone (ITCZ) shifts northward in summer over the Ganga plain, normally positioned around 5°N of the equator.
- A high-pressure area east of Madagascar influences monsoon dynamics.
- Intense heating of the Tibetan plateau in summer generates strong vertical air currents.
- The westerly jet stream shifts north of the Himalayas, and the tropical easterly jet stream moves over the Indian peninsula during summer.
The seasonal wind reversal results from the ITCZ’s shifting. In July, the ITCZ moves north, forming the Monsoon Trough, encouraging a thermal low over north and northwest India. Consequently, trade winds from the southern hemisphere cross the equator between 40°E and 60°E longitudes, turning southwest to northeast due to the Coriolis force, becoming the Southwest Monsoon. In winter, the ITCZ shifts southward, reversing winds to blow from northeast to south and southwest, forming the Northeast Monsoon.
Moisture is primarily drawn from the Arabian Sea and Bay of Bengal. The landmass shape and mountains (Western Ghats in the south and Himalayas in the north) guide wind direction. The monsoon is central to India’s agricultural cycle, as over 60% of the population depends on agriculture, which relies heavily on monsoon rains for livelihood.