Monsoons

The term monsoon derives from the Arabic word for season, mausim, and refers to the seasonal reversal in atmospheric circulation and the associated precipitation in tropical and subtropical regions. Such reversal of wind direction occurs with the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) in conjunction with the annual north-south progression of the sun. The movement of the ITCZ induces a temperature difference between the land surface and the ocean. The resultant difference in surface pressure between the ocean and land areas fuels the monsoon winds. This entry presents a brief review of how the monsoonal regions throughout the globe are demarcated and examines in detail the characteristics of the South Asian monsoon.

Monsoonal regions are presently identified by seasonal changes in cross-equatorial flows, changes in wet- and dry-spell transitions, and seasonal changes in divergent circulation in the upper troposphere associated with the large-scale atmospheric overturning taking place in tropical regions. Cross-equatorial flows occur when the mean wind flows across the equator along pressure gradients established due to land-ocean temperature contrasts. Monsoonal climates are characterized by distinct wet and dry spells with wet spells occurring during the monsoon season and dry spells at other times. The large-scale overturning of the atmosphere refers to the three-dimensional circulation associated with the monsoons. When the continental land masses heat up with the ITCZ overhead, large-scale convection or rising motion takes place. Such rising motion in the lower troposphere around 10° latitude is offset by subsidence around 20°. This results in divergent circulation in the upper troposphere. Simultaneously, meridional and zonal circulation cells are set into action with winds blowing in from the cooler surrounding oceans in response to the pressure gradient (see Figure 1).

Current methods emphasize that the monsoon is a global phenomenon where the different regional components interact with each other and the overall general circulation of the atmosphere.

The regional monsoonal components include the tropical and subtropical regions of Asia, Australia, Africa, South America, and southwestern North America. These regions can be visually identified through the seasonal reversal in mean summer (July) and winter (January) winds at the 850-hPa (hectopascal) geopotential (potential energy of unit mass relative to sea level) height level (Figure 2). In South America, seasonal reversal of the winds is not as distinct as in other regions. The change appears in the direction of the cross-equatorial flow in the Amazon region. In North America, there is no cross-equatorial flow. However, there is a distinct seasonal reversal in mean winds.