The North American summer monsoon is documented, using precipitation data together with gridded data for outgoing long-wave radiation (OLR), geopotential height and wind at various levels. The upper level divergence field is diagnosed and compared with the precipitation field. A simple wet-dry precipitation index is used to date the monsoon onset at stations with daily precipitation data. The analysis shows that the monsoon rains advance northward rapidly from late June to early July. The monsoon onset is accompanied by the development of a pronounced anticyclone at the jet stream level, by sea-level pressure rises over the southwestern United States, and by decreases in climatological mean rainfall over adjacent regions of the United States, Mexico and the Caribbean. This coherent pattern of rainfall changes, that covers much of North and Central America, is shown to be dynamically consistent with the circulation changes aloft. Hence, the monsoon onset is embedded within a planetary-scale pattern of circulation changes. The demise of the monsoon and the associated upper level anticyclone, which takes place around September of the year, is more gradual than the onset, and it is accompanied by an increase in rainfall throughout much of the surrounding region. The monsoon exhibits substantial interannual variability with regard to intensity and onset date.

The North American summer monsoon is documented, using precipitation data together with gridded data for outgoing long-wave radiation (OLR), geopotential height and wind at various levels. The upper level divergence field is diagnosed and compared with the precipitation field. A simple wet-dry precipitation index is used to date the monsoon onset at stations with daily precipitation data. The analysis shows that the monsoon rains advance northward rapidly from late June to early July. The monsoon onset is accompanied by the development of a pronounced anticyclone at the jet stream level, by sea-level pressure rises over the southwestern United States, and by decreases in climatological mean rainfall over adjacent regions of the United States, Mexico and the Caribbean. This coherent pattern of rainfall changes, that covers much of North and Central America, is shown to be dynamically consistent with the circulation changes aloft. Hence, the monsoon onset is embedded within a planetary-scale pattern of circulation changes. The demise of the monsoon and the associated upper level anticyclone, which takes place around September of the year, is more gradual than the onset, and it is accompanied by an increase in rainfall throughout much of the surrounding region. The monsoon exhibits substantial interannual variability with regard to intensity and onset date. / Arts, Faculty of / Geography, Department of / Graduate