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 analyses are all analyzed. Composite wet and dry periods during July and August of 1985-92 are compared, and the statistical significance of differences between the composite fields are assessed. Significant differences exist between many of the wet and dry fields over the Sonoran Desert. As the monsoon shifts from dry to wet conditions, the subtropical ridge moves ∼5° lat northward, and the middle- and upper-tropospheric (700-300 mb) winds back from southwesterly to southeasterly. The midtropospheric transport of water vapor from the southeast strengthens, and the precipitable water values increase by as much as ∼1.2 cm (∼0.5 in.). Middle-tropospheric air parcels ascend into the region from the southeast, while low-level air parcels continue to stream inland from the Gulf of California and up the slopes of the Sierra Madre Occidental. The surface and midtropospheric air parcels rise at an average rate of ∼50-75 mb per day and would saturate within 2 days if undiluted. This combination of conditions leads to a more unstable atmosphere and an increase in convective activity. Spectral analysis of precipitation data for southeast Arizona indicates that 75% of the temporal variance is contained in fluctuations longer than 7 days and that a statistically significant peak exists in the 12-18-day band. These results suggest that large-scale, low-frequency dynamics might play an important role in modulating the variability of convective activity over the Sonoran Desert.