Warm season moisture fluxes over the Gulf of California

Juan Carlos Leal

CICESE

Ensenada, Baja California

Mexico

Michael Douglas

National Severe Storms Laboratory

Norman, Oklahoma

Numerous studies have indicated that enhanced moisture transport up the Gulf of California during the summer months is linked to outbreaks of convective storms over the southwestern deserts of North America. Unfortunately, because of inadequate spatial resolution, routine NCEP analyses do not adequately depict the observed southerly flow over the Gulf, and thus these analyses cannot be used to accurately quantify the variability of this moisture flux. Therefore, the relative importance of variations in this flux, and its relationship to rainfall variations over the southwestern deserts has remained unclear. This talk describes the results of research that have investigated the variability of moisture transport parallel to the axis of the Gulf of California, from diurnal to interannual time scales. Some of the key results of this study are:

1) The up-Gulf flux of moisture is largest during the July- September period. During the winter it is also mostly positive (up gulf), but with mean monthly values less than 25% those during the warm season.

2) During the summer, the monthly mean up-Gulf moisture flux is largest at the surface, goes to zero around 875mb, and is negative above. This reflects the mean southerly wind at low-levels that weaken rapidly with height.

3. There is a large diurnal variation in the moisture flux, associated with the strong sea-breeze circulations at Empalme. Estimates of meridional moisture flux based on observations at only one synoptic time would be strongly biased.

4. There are major synoptic time-scale variations in the up-Gulf moisture transport. These appear to be associated with "gulf surges" - major fluctuations in the winds in the lower troposphere over the Gulf.

The structure of the variations in moisture flux were investigated from the Empalme radiosonde data. Events were stratified into different types of "surges", from 1 to 4 day duration. Composites of these moisture surges were produced for temperature, moisture, wind, and moisture flux. The structure of these composite surges will be presented.