Southwestern United States Summer Monsoon Source—Gulf of Mexico or Pacific Ocean?

Regional simulation of the low‐level monsoon winds over the Gulf of California and southwestern United States

“…To do this, we subdivide the integration periods according to a model-based semiquantitative index (model data are used for this index because it is the only consistent data set for the two periods); the model-defined surge periods can then be compared with available observations in order to see whether the model index is justified. Some quantitative indices for identifying the passage of surge fronts have been suggested by previous authors, including 24-hour decreases in temperature, increases in surface and sea level pressure, and reversals of winds from northerly to southerly [Hales, 1974;Brenner, 1974 Brenner [1974] an additional criteria employed here involves identifying the passage of the surge front over the northern Gulf using 24-hour differences (i.e., the difference between the present value and its value 24 hours before) in meridional wind speeds over Puerto Penasco, Mexico; following Hales [1974], relative humidity could also have been used, but this selection does not affect any of the results presented here. To qualify as a surge period then, there first has to be extended (longer than 24 hours) southerly to southeasterly winds over the Empalme grid point.…”

“…The North American monsoon system is associated with large seasonal changes in wind and moisture patterns over In contrast, surge events, as defined by previous authors, are characterized by prolonged periods (3-5 days) of strong southeasterly flow throughout the Gulf of California, usually accompanied by increased fluxes of low-level moisture into the southwestern United States and northwestern Mexico [Hales, 1974;Brenner, 1974;Badan-Dangon et al, 1991]. These surge events have been associated with cyclone activity in the southern portion of the Gulf [Hales, 1974;Stensrud et al, 1997].…”

“…The North American monsoon system is associated with large seasonal changes in wind and moisture patterns over In contrast, surge events, as defined by previous authors, are characterized by prolonged periods (3-5 days) of strong southeasterly flow throughout the Gulf of California, usually accompanied by increased fluxes of low-level moisture into the southwestern United States and northwestern Mexico [Hales, 1974;Brenner, 1974;Badan-Dangon et al, 1991]. These surge events have been associated with cyclone activity in the southern portion of the Gulf [Hales, 1974;Stensrud et al, 1997]. They have also been associated with synoptic changes in the subtropical ridge (linking the Bermuda and East Pacific highs) and the passage of easterly troughs over northwestern Mexico [Hales, 1974;Brenner, 1974;Carleton, 1986;Carleton et al, 1990;Dunn and Hotel, 1994b;Maddox et al, 1995;Schmitz and Mullen, 1996;Stensrud et al, 1997;Adams and Corntie, 1997].…”

“…These surge events have been associated with cyclone activity in the southern portion of the Gulf [Hales, 1974;Stensrud et al, 1997]. They have also been associated with synoptic changes in the subtropical ridge (linking the Bermuda and East Pacific highs) and the passage of easterly troughs over northwestern Mexico [Hales, 1974;Brenner, 1974;Carleton, 1986;Carleton et al, 1990;Dunn and Hotel, 1994b;Maddox et al, 1995;Schmitz and Mullen, 1996;Stensrud et al, 1997;Adams and Corntie, 1997]. Preceding the onset of most surge events, there usually exists a surge front that is characterized by an abrupt and dramatic low-level transition of the flow from northerly to strong southerly.…”

“…Preceding the onset of most surge events, there usually exists a surge front that is characterized by an abrupt and dramatic low-level transition of the flow from northerly to strong southerly. This transition is accompanied by decreases in temperature and increases in surface and sea-level pressure and occurs quite rapidly [Hales, 1974;Brenner, 1974;Badan-Dagon et al, 1989;Douglas, 1995;Stensrud et al, 1997]. The propagation of the surge front has been associated with a density current in the lower atmosphere that moves up the Gulf over the span of ---24 hours [Hales, 1974;Brenner, 1974;Stensrud et al, 1997].…”