The North American monsoon system (NAMS) spans over different spatial scales that are usually treated separately in model studies due to current computational limitations. The core monsoon has mesoscale features that are characterized by distinct sea surface temperature fields in the Gulf of California, the presence of a northward Low-level Jet (LLJ), and episodic moist surges also progressing northward along the Gulf. These mesoscale features, and the diurnal cycle, are crucial for describing the moisture transports (and sources) associated with the monsoon precipitation. It is also known that the continental scale evolution is characterized by a seesaw pattern between the monsoon precipitation and the southern Great Plains.

The purpose of this study is to investigate the nature of the seesaw pattern using a stretched grid variable resolution model. The focus is on the summer of 1993, but it is shown that many of the features discussed here are common to other years as well. Particular attention is given to the monsoon’s onset and evolution, including the regional circulations over the Gulf of California. The onset of the monsoon is accompanied by increased moisture flux convergence, large scale ascending motions and upper-level divergence. The simultaneous decrease of precipitation over the southern Great Plains seems dynamically induced, with a change in the sign of moisture flux convergence (becoming divergent), a reduction in the intensity of the ascending motion (which remains close to zero) and predominant wind convergence in the upper-levels. The onset of the monsoon over Arizona suggests similar features as those of the core monsoon, although of much smaller magnitude. The study is completed by analyzing such local and remote effects of the monsoon on the water cycle of diverse basins of the United States.