An anomaly in the occurrence of the month of maximum precipitation distribution in northwest Mexico

This paper explores the existence and potential causes of a stationary spatial anomaly, or pattern incoherence, in the seasonal distribution of warm season rainfall in western Mexico. The anomaly is evident in maps of the Month of Maximum Precipitation (MMP) variability over northwest Mexico (NWMx). Instead of occurring progressively as latitude increases, the MMP occurs in July in the southern part of western Mexico and along the main axis of the Sierra Madre Occidental (SMO) to the north, and in August in the central portion of the region, in Nayarit, Durango and Zacatecas. The Month of Maximum Streamflow (MMQ) occurs in a similar spatial pattern but shifts one month later. This feature is explored through inspection of the daily rainfall climatology of the region for stations with more than 20 years of data between 1940 and 2004. We hypothesize that this anomalous pattern in rainfall (and subsequent streamflow) is likely influenced by the complex thermodynamic interaction between the orography of the SMO and the regional monsoon circulation. The possibility that the canyon system known as the San Pedro Mezquital helps drive the anomalous increase of rainfall in August across latitudes limited to of Nayarit is explored. The correlation between rains that fall over watersheds on the western slope of the SMO and rainfall over the Mexican Plateau is also discussed. We also explore the relationship of the rainfall anomaly to the seasonal march of the Western Hemisphere Warm Pool (WHWP) and near shore Sea Surface Temperature (SST) variations as they may impact the land-sea thermal contrast. The results of this study based on daily rainfall and streamflow climatologies suggest that a region exists in Nayarit where atmosphere-ocean-land surface interactions lead to a relative phase-shift in the maximum precipitation and streamflow.