On the structure and efficacy of gulf surges

Six years (1999-2004) of QuikSCAT near-surface ocean winds over the northeast Pacific and the Gulf of California (GoC) are used to study the North American Monsoon. The wind data show that the onset of the summer rains is associated with a seasonal reversal of the low-level flow, with the establishment of a mean southerly wind throughout the gulf. In the core of the monsoon, the time-mean flow is found to be composed of periods of enhanced southerly winds associated with transient events, namely gulf surges. An EOF analysis of the daily summertime along-shore wind anomalies identifies gulf surges as the leading mode of synoptic-scale variability of the low-level flow along the GoC, allowing us to interpret the leading principal component time series as an objective index for gulf surge occurrence. This index is used as a reference time series for regression analysis and compositing of meteorological fields of interest, to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon, as well as the manifestation of these transient events in the large-scale circulation. It is found that, although seemingly mesoscale features confined over the GoC, gulf surges are intimately linked to patterns of large-scale variability of the eastern Pacific ITCZ and greatly contribute to the definition of the northward extent of the monsoonal rains.