Large-scale forcing of summertime monsoon surges over the Gulf of California and the southwestern United States

Synoptic forcing of the Gulf of California summertime low-level wind field is described using a nested regional modeling system. Under appropriate syn optic conditions, strong surge events develop that are characterized by per sistent, vertically extensive (1-2 km) southerly flow extending along the e ntire Gulf of California and into Arizona, California, and even southern Ne vada. These surge periods are initiated either by tropical-cyclone activity to the south of Baja California or by the westward propagation of lower tr opospheric troughs from over the Sierra Madre Occidental to the eastern Pac ific. The flow over the Gulf is primarily geostrophic and is associated wit h the presence of these low-pressure centers to the south and west of Baja California.