05 The South American Monsoon System: Climatology and Variability

A typical Monsoon System is characterized by a reversal in the low-level wind direction between summer and winter seasons, and distinct wet (summer) and dry (winter) periods. The changes in low-level atmospheric circulation are related to changes in the thermal contrast between oceans and continents. During summer, the air over continents is warmer and more convectively unstable than air over adjacent oceanic regions. Consequently, lower pressure occurs over land and higher pressure occurs over nearby oceanic areas. This pressure pattern causes low-level moist air to converge onto the land, resulting in precipitation, especially during the late afternoon and evening hours. During winter the temperature contrasts and low-level atmospheric circulation are reversed, resulting in dry conditions over continents.https://digitalcommons.usu.edu/modern_climatology/1004/thumbnail.jp