The role of tropical-extratropical interactions on Gulf of California Surges and the intra-seasonal variability of the North American Monsoon

Gulf of California (GoC) moisture surges are transient disturbances that propagate northwards along the Gulf of California during the July-September North American Monsoon (NAM) season. They are typically associated with positive intense northward winds along the GoC and positive precipitation anomalies in the NAM region, with widespread thunderstorms in the arid Southwest US. Their frequency and intensity therefore have an important impact on the rainfall summer budget of this area. Although traditionally regarded as pure mesoscale processes, there is evidence that GoC surges are related to large-scale dynamical controls, such as tropical easterly waves (TEWs) transiting over the eastern Pacific ocean and upper-level westerly disturbances passing by over Northern America.

Using reanalysis products, we perform an EOF analysis of the GoC alongshore wind to investigate the intra-seasonal variability of the GoC surges and its relationship with precipitation anomalies. The first and second principal components, which explain more that 80% of the summertime variability of GoC winds, provide very effective indexes of strong and weak surges, with the former being associated with coherent wind anomalies throughout the GoC and the latter being primarily confined over its northern half. Linear regressions analyses are used to identify synoptic patterns that lead to the development of GoC surges and allow to differentiate between strong and weak surges. Inter-comparisons with other surge indexes based on dew-point temperature in Yuma and precipitation in Southwest US are performed in order to assess the suitability of these different indexes for analysis of coarser resolution GCMs in the CMIP5 archive. Eddy fluxes of moisture, temperature and momentum associated with such disturbances are quantified and related to GoC events.

These analyses will then be extended to simulations in the CMIP5 archive, to explore how well state-of-the art climate models represent transient activity associated with GoC surges and their impact on the simulated temporal and spatial distribution of the NAM rainfall