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

Abstract

Abstract. Synoptic forcing of the Gulf of California summertime low-level wind field is described using a nested regional modeling system. Under appropriate synoptic conditions, strong surge events develop that are characterized by persistent, vertically extensive (1-2 km) southerly flow extending along the entire Gulf of California and into Arizona, California, and even southern Nevada. These surge periods are initiated either by tropical-cyclone activity to the south of Baja California or by the westward propagation of lower tropospheric troughs from over the Sierra Madre Occidental to the eastern Pacific. The flow over the Gulf is primarily geostrophic and is associated with the presence of these low-pressure centers to the south and west of Baja California.

Hydrometeorological Response of the Modeled North American Monsoon to Convective Parameterization

“…In all three simulations (Figs. 4a,c, [Note: this feature has also been found in the recent analyses by Berbery (2001), Anderson and Roads (2001), and Stensrud et al (1995).] The largest differences between simulations are along the coast of western Mexico.…”

“…However, there are notable differences in the modeled transient flux fields and in the way in which transient features are represented. In this region, tropical storms, easterly propagating waves, and, to a lesser degree, midlatitude (short) waves all likely contribute to the mean transient flux (Schmitz and Mullen 1996;Anderson et al 2000a), and the simulations with alternative convective schemes differ in their representation of such features. Over the Gulf of California, the direction of the transient flux component is northwest in all three schemes, indicating that transient features carry moisture away from the core monsoon region over the Sierra Madre Occidental.…”

“…Several authors have examined the atmospheric branch of the NAMS (e.g., Schmitz and Mullen 1996;Higgins et al 1997;Berbery 2001;Anderson and Roads 2001;Anderson et al 2000a) using analyzed or reanalyzed data. Using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data at T106 spectral resolution, Schmitz and Mullen (1996) showed that moisture flux into the core NAM region of western Mexico and the southwestern United States is attributable to low-level stationary components traveling up the Gulf of California (GoC), while larger-scale circulation around the summertime continental anticyclone over the southeastern United States is subsequently responsible for transporting the moisture (generated by deep convection over the Sierra Madre Occidental) from midtropospheric levels, northward into the southwestern United States.…”

Relationships between Gulf of California Moisture Surges and Tropical Cyclones in the Eastern Pacific Basin

“…The development of southerly low-level jets (LLJs) over the northern GOC region during the monsoon season has been linked to gulf surges in field campaigns (e.g., Meitin et al 1991;Reyes et al 1994;Higgins et al 2004b) and in diagnostic and modeling studies (e.g., Douglas 1995;Douglas and Li 1996;Stensrud et al 1997;Anderson et al 2000). The GOC LLJ is one of the primary mechanisms by which moisture is transported from the northern GOC into the northern Sonoran Desert (e.g., Fawcett et al 2002).…”

“…Stensrud et al (1997) showed that the detailed characteristics of surges could be reproduced by a mesoscale numerical model. Recent diagnostic and modeling studies of gulf surges have emphasized their relationships to tropical storms south of Baja California, tropical easterly waves, and midlatitude westerly waves (e.g., Fuller and Stensrud 2000;Anderson et al 2000).…”