Regional simulation of the low-level monsoon winds over the Gulf of California and southwestern United States

Using a fine-scale (10-20 km) nested regional modeling system, synoptic var iations in climatological summertime low-level wind fields over the Gulf of California and the southwestern United States are described. Under appropr iate synoptic conditions, strong surge events can develop. These are charac terized by low-level southerly flow over the entire Gulf of California with southerly winds extending into Arizona, California, and southern Nevada. V ertically, these southerly winds are present through the bottom 1-2 km of t he atmosphere. Southerly flow is persistent throughout the day with some lo cal diurnal cycling over the foothills of the Sierra Madre Occidental and n orthwestern Mexico. Under nonsurge conditions the simulated low-level winds have a significantly different geographic structure. Nighttime southerly f low is limited to the northeastern Gulf and small portions of southwestern Arizona. Flow over the central and southern Gulf is northerly with weak, va riable winds over the foothills of the Sierra Madre Occidental. During the day, southerly winds are present over the central and northern Gulf of Cali fornia, extending into southwestern Arizona; however, this southerly wind p attern does not support continuous flow from the mouth of the Gulf into nor thwestern Mexico. Instead, there is a westerly component associated with pr edominantly upslope flow over the foothills of the Sierra Madre Occidental and the Sonora Desert.