Summertime surges over the Gulf of California: Aspects of their climatology, mean structure, and evolution from radiosonde, NCEP reanalysis, and rainfall data

This paper describes aspects of summertime northward surges of low-level moisture over the Gulf of California, based on 9 yr (1980-88) of radiosonde observations and also from NCEP reanalyses. These events are usually referred to as "gulf surges" by forecasters in the southwestern United States. A composite structure of 38 well-marked surge passages at Empalme, Mexico, during this period is presented. Radiosonde observations were composited to obtain the synoptic-scale structure of the surge from 2 days before to 2 days after the surge passage at Empalme. The composites reveal that the surges, strongest in the lower troposphere, are associated with low-latitude cyclonic perturbations that pass south of the Gulf of California. The composite cyclonic perturbation associated with the surges can be traced back to the western Gulf of Mexico 2 days prior to surge passage at Empalme. Composites based on the NCEP reanalyses for the same dates also show a similar evolution, though with somewhat weaker amplitude. Rainfall data from Mexican stations along the eastern side of the Gulf of California show that the surges modulate the climatological daily rainfall totals by ~15%-30%. The evolution of surges is shown to be related to the propagation of tropical storms over the eastern Pacific Ocean.