The North American monsoon system (NAMS) as represented in the second-generation Hadley Center coupled ocean-atmosphere model is examined using high-time-resolution (six-hourly) records of model output. A current-climate simulation gives a realistic depiction of the NAMS, with date of onset and precipitation trends agreeing with observations. The model reproduces the observed negative relationship between precipitation over the southwestern U.S. and that over the central U.S. Changes in tropospheric circulation and moisture after onset are consistent with observed changes although mid-level moistening is less than is observed. An enhanced greenhouse-gas simulation gives higher precipitation and stronger tropospheric dynamical and moisture changes after onset. ¿ 2000 American Geophysical Union