Physical Background

Abstract

A monsoon is a circulation system with a set of well-defined features. During summer, winds in the low-level troposphere flow from the colder oceanic regions of the winter hemisphere toward heated continents. Conversely, winds flow from the summer to the winter hemisphere in the upper troposphere. Moreover, the so-called monsoon trough determines the occurrence of rainfall during summer (e.g., Webster et al. 1998, 2002; Webster and Fasullo 2003). This trough of low pressure locates in the surrounding regions of the heated continents and the adjacent oceans and seas, on which precipitation mostly takes place. Most summer rainfall is associated with synoptic disturbances that propagate through the regions aforementioned. These disturbances are referred to as “active monsoon periods,” being grouped in periods of disturbed weather and heavy rainfall lasting from 10 to 30 days. The intervening periods of disruption in this strong convective activity are referred to as “monsoon breaks.” The location of the monsoon trough and maximum monsoon precipitation is generally poleward of the position of the oceanic ITCZ, within which most of the tropical precipitation occurs. This location of maximum precipitation is the so-called tropical rain belt. As stated in the introduction (see Sect. 2.2), each system is different in terms of intensity and atmospheric circulation features. Purely monsoon climates exhibit a single rainfall peak during the solstices, along which dry seasons occur for equatorial climates.