Basics: Indian subcontinent receives most of its rainfall during Boreal summer – i.e. June to September, referred as the Indian summer monsoon. The word monsoon is derived from the Arabic "mausim", meaning "seasonal reversal of winds". Wind reversals happen on a daily time scale in coastal regions around the world due to the contrasting heating and cooling capability of land and the sea or ocean. Land heats and cools faster relative to the oceans, as oceans have higher thermal capacity (i.e. ability to store and release heat). During daytime land gets hotter faster and air rises and cooler air from the sea fills its place, thus cooler evenings on the beach. At night, land cools while the sea is still warm from the daytime heating. This is shown in the schematic figure on the right. If such a reversal of wind happens over a season and over a large scale, it results in monsoon.
As you can recognize from the schematic, for reversal of wind a significant amount of landmass and ocean should be present in warm region. Around the world, the only region with the right amount of land and ocean is the tropics (i.e. conducive for warming) and consequently leading to large scale wind reversal is the Indian subcontinent and the Northern Indian Ocean region. Together they have the right amount of land and water which leads to 180 degree shift in the winds between summer (June – Sep) and winter (Oct – Dec), as seen in the schematic above. Thus, in the pantheon of global climate, Indian Monsoon occupies a unique place in terms of its robustness, reliability and large scale signature.
Seasonal Evolution: During Boreal spring (Mar to May), also known as Indian Summer the Indian subcontinent and parts of Himalayas, especially Tibet, starts to heat up. Living in India, we all know of this heat. This heating results in raising of air (warm air is lighter and thus raises) over this large swath of land mass, which produces a strong low pressure (also known as thermal low, as it is a result of heating). Relatively the Indian Ocean is cooler and thus creating a land-ocean thermal contrast or gradient. Towards the end of May this low pressure strengthens and so does the thermal gradient, which now acts like a vacuum suction, as a result winds are sucked on to the land from the northern Indian Ocean (see the summer wind figure in the schematic above). Along with the winds moisture is also moved on to the land, thus, heralding the start of the monsoon season. As to be expected the monsoon arrives in the southwestern coast of India first before progressively advancing north by late June-early July. The typical monsoon arrival date is shown in the figure to the right.
Regional Circulation: The land - ocean thermal gradient producing the monsoon winds arriving over Indian subcontinent are configured regional as shown in the Figure below to the right. Notice that the winds (shown as arrows) along the equator and to the south that blow from east to west (also known as trade winds) deflect to the right after hitting the Somali high lands along the coast of Africa. The deflected winds head straight to the Indian subcontinent. The colours represent the rainfall. Notice that heavy rainfall occurs along the narrow coast of southwest India, eastern coast of Orissa, west Bengal and in the Himalayan foothills. Clearly, topography plays an important role in the spatial distribution of rainfall over the subcontinent.
Next edition: We will investigate the role of various land topographical features in enabling the monsoon rainfall. Especially the Somali highlands, Western Ghats, Tibetan plateau.
Prof. Balaji Rajagopalan is the chair of the Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, USA. He has a wide range of research interests from Indian monsoons to water resources and has supervised several PhD students and co-authored many articles in these areas in reputed journals. He is also a fellow of the prestigious American Geophysical union.