About American Monsoon

  • The North American Monsoon System

The North American Monsoon System (NAMS) is also known as the Southwest United States monsoon, the Mexican monsoon or the Arizona monsoon. There is pronounced increase in rainfall from an extremely dry June to a rainy July until mid-September. The major circulation feature is the Bermuda High and its westward extension. Moisture source for the NAMS is complex with important roles played by the Gulf of Mexico, Pacific Ocean and on a smaller scale by the Gulf of California. The inter-annual variability in monsoon rainfall is partly influenced by the Pacific-North America pattern and to some degree by the El Nino Southern Oscillation phenomenon. This monsoon accounts for at least 50% of the annual rainfall during the summer monsoon period (July through September). Figure A (Ropelewski et.al.; Review Topic 4: American Monsoon) shows the percent of annual rainfall (50% is shown in dark green) during the NAMS (July through September).


Figure A


  • The South American Monsoon System

The South American Monsoon System (SAMS) is one of the major monsoon systems of the Southern Hemisphere. SAMS wet season in tropical South America begins in early October over the Brazilian Highlands and spreads northward. The mature phase of SAMS prevails between December and February and demise in April. The annual cycle of precipitation is most pronounced in Southern Amazon, where some of the largest seasonal rainfall occurs. Moisture for the SAMS has its origin in the easterly trade wind circulation rooted in the Atlantic Ocean. Figure B (Vuille et.al. 2012) shows the percentage of annual precipitation falling during the mature stage of the SASM (December through February). On inter-annual scale the SAMS is significantly influenced by the variability associated with the El Nino Southern Oscillation, Pacific Decadal Oscillation, Atlantic Multi-decadal Oscillation and the Atlantic variability.


Figure B


  • Minutes of teleconference on the Formation of the Americas Monsoon Working Group, 1700UTC 12 Feb 2016 

Click here to access the PDF (Passwod Protected) : http://www.clivar.org/documents/minutes-teleconference-formation-americas-monsoon-working-group-1700utc-12-feb-2016