Surges over the Gulf of California during the Mexican monsoon

Northward surges of relatively cool, moist, maritime air from the tropical Pacific into the southwestern United States occur via the Gulf of California every summer during the Mexican monsoon season. These surges advect large amounts of moisture northward and promote increased convective activity in Arizona. Mesoscale model output from 32 successive 24-h simulations are used to examine both the large-scale and mesoscale features associated with surge events. Model data are compared to high-resolution aircraft measurements on two days, showing clearly that the model is capable of reproducing many of the detailed structures found in the observations. Results from a large-scale analysis indicate that strong surges are produced in the model when a midlatitude trough is in the proper phase relationship with a tropical easterly wave. Weak surges occur when this relationship is missing.