The Nature of Indian Monsoon

The Nature of Indian Monsoon

Monsoon is a familiar climatic phenomenon, yet it remains poorly understood. Despite centuries of observation, scientists are still puzzled by it. Many theories have tried to explain its exact nature and causation, but no single theory has fully succeeded. A real breakthrough came only when the monsoon began to be studied at the global level rather than the regional level. Systematic studies of rainfall causes in the South Asian region have helped clarify two important aspects: the onset of the monsoon and the break in the monsoon.

Onset of the Monsoon

Towards the end of the nineteenth century, it was believed that differential heating of land and sea during summer sets the stage for monsoon winds to drift towards the subcontinent. During April and May, when the sun shines vertically over the Tropic of Cancer, the large landmass north of the Indian Ocean gets intensely heated. This creates an intense low pressure in the northwestern part of the subcontinent. Since the Indian Ocean to the south heats up slowly, the pressure there remains high. The low pressure cell attracts the southeast trades across the Equator. These conditions help shift the position of the ITCZ northward.

The southwest monsoon can thus be seen as a continuation of the southeast trades, deflected towards the Indian subcontinent after crossing the Equator. These winds cross the Equator between 40°E and 60°E longitudes.

Inter Tropical Convergence Zone (ITCZ)

The ITCZ is a low pressure zone located at the Equator where trade winds converge, making it a zone where air tends to ascend. In July, the ITCZ is located around 20°N–25°N latitudes, over the Gangetic plain, and is sometimes called the monsoon trough. This monsoon trough encourages the development of a thermal low over north and northwest India.

Due to the shift of the ITCZ, the trade winds of the southern hemisphere cross the Equator between 40°E and 60°E longitudes and start blowing from southwest to northeast because of the Coriolis force. This becomes the southwest monsoon. In winter, the ITCZ moves southward, causing a reversal of winds from northeast to south and southwest. These are called northeast monsoons.

The shift in the position of the ITCZ is also related to the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas. The easterly jet stream sets in along 15°N latitude only after the western jet stream has withdrawn from the region. This easterly jet stream is held responsible for the burst of the monsoon in India.

Entry of Monsoon into India

The southwest monsoon sets in over the Kerala coast by 1st June and moves swiftly to reach Mumbai and Kolkata between 10th and 13th June. By mid-July, the southwest monsoon engulfs the entire subcontinent.

Break in the Monsoon …