INTRASEASONAL VARIABILITY OF THE SUMMER MONSOON OVER SOUTHEAST ARIZONA

Intraseasonal fluctuations associated with the Mexican monsoon system are examined for the semiarid Sonoran Desert region. Daily rain gauge accumulations, radiosonde reports, satellite imagery, and global analy ses are all analyzed. Composite wet and dry periods during July and Au gust of 1985-92 are compared, and the statistical significance of diff erences between the composite fields are assessed. Significant differe nces exist between many of the wet and dry fields over the Sonoran Des ert. As the monsoon shifts from dry to wet conditions, the subtropical ridge moves similar to 5 degrees lat northward, and the middle- and u pper-tropospheric (700-300 mb) winds back from southwesterly to southe asterly. The midtropospheric transport of water vapor from the southea st strengthens, and the precipitable water values increase by as much as similar to 1.2 cm (similar to 0.5 in.). Middle-tropospheric air par cels ascend into the region from the southeast, while low-level air pa rcels continue to stream inland from the Gulf of California and up the slopes of the Sierra Madre Occidental. The surface and midtropospheri c air parcels rise at an average rate of similar to 50-75 mb per day a nd would saturate within 2 days if undiluted. This combination of cond itions leads to a more unstable atmosphere and an increase in convecti ve activity. Spectral analysis of precipitation data for southeast Ari zona indicates that 75% of the temporal variance is contained in fluct uations longer than 7 days and that a statistically significant peak e xists in the 12-18-day band. These results suggest that large-scale, l ow-frequency dynamics might play an important role in modulating the v ariability of convective activity over the Sonoran Desert.