The low-level circulation of the North American Monsoon as revealed by QuikSCAT

Five years (1999-2003) of near-surface QuikSCAT ocean winds over the Gulf of California and northeast Pacific Ocean are used to characterize the changes in the low-level circulation associated with the North American Monsoon. Our analysis shows that the onset of the summer season is accompanied by a seasonal reversal of the flow along the Gulf of California, with the establishment of a time-mean southerly wind throughout the gulf. This reversal, not evident in the global reanalysis products, occurs in late spring and precedes the onset of the monsoonal rains. In the core of the monsoon, the time-mean flow is found to be modulated by transient events, namely gulf surges, detected in the near-surface wind field as periods of enhanced southerly flow which typically originate at the southern end of the gulf and propagate northward. The histogram of the summertime along-shore winds identifies these surges as a distinct population of events, readily distinguishable from the background flow.