Influence of sea surface temperature anomalies in the Gulf of California on North American monsoon rainfall

[1] Linkages between sea surface temperatures (SSTs) in the Gulf of California and the North American monsoon rainfall were examined using observational data and regional model experiments. SSTs in the Gulf start to increase in late June and reach a maximum in August. Monsoon onset dates in the Southwest were determined from the gridded rainfall analysis. Monsoon onsets can occur before SSTs in the Gulf reach a maximum. There is no relationship between monsoon rainfall onset dates or seasonal total rainfall over the Southwest and SSTs in the Gulf. Regional model experiments were then performed to study the impact of local SSTs on monsoon rainfall using the NCEP regional spectral model (RSM). RSM experiments were performed with the observed SSTs for four summers (July-September 1997-2000). The experiments were repeated with the same initial and boundary conditions but with the climatological SSTs in the Gulf of California and its vicinity. The model is able to capture the general features of monsoon rainfall and the diurnal cycle. Warm (cold) SSTs in the Gulf are responsible for more (less) rainfall along the western slopes of the Sierra Madre Occidental. Over the Southwest, the impact of local SSTs is small. Large-scale flow has more influence on monsoon rainfall than SSTs in the Gulf of California because the SST forcing does not produce significant changes in the low level flow needed to influence rainfall over the Southwest.