The North American monsoon is accompanied by large-scale changes in circulation and precipitation over much of Mexico and the United States during summer. Here, the influence of the North American monsoon is analyzed in terms of midlevel changes to the thermodynamic energy equation, circulation, and precipitation associated with the monsoon onset in northwest Mexico, for the 1948–2004 period. In addition to the well-known strong increase in rising motion over the core region of the monsoon during the onset, there is also a decrease in upward motion over the northern Baja California Peninsula and into the southwest United States, directly in the path of monsoon development. This area of decreased vertical motion is linked to cold advection caused by the onset itself, as the Gill-Matsuno response to the monsoon precipitation thermodynamically interacts with the mean circulation. It is in this sense that we propose that the monsoon is self-limiting.
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