A Climatological Perspective of Transient Synoptic Features during NAME 2004

Abstract

This note provides a first look at a recently developed long-term climatology of transient synoptic features in northern Mexico. Key features investigated include inverted troughs, cutoff lows, cold fronts, and open troughs (westerly short waves). This 35-yr analysis of transient systems crossing northern Mexico (1967?2001) was developed to help place the summer climatology of the 2004 North American Monsoon Experiment (NAME) into a broader perspective. Inverted troughs are found to be the most commonly occurring transient synoptic feature during the monsoon with a mean frequency of occurrence of 55 days per summer season (June?September). Inverted troughs are found to contribute from 20% to 25% of the average summer rainfall observed in northern Mexico. Rainfall doubles during inverted trough days compared to days without transient systems being present. In 2004 the monsoon season was greatly shortened due to a poorly developed subtropical high. Compared to long-term means, inverted troughs contributed less rainfall to the region in 2004 and this was, in part, associated with the shortened monsoon season. In contrast, frontal penetration into the region was almost double the 35-yr mean. These climatologies are designed to provide NAME researchers with benchmarks to assess model performance relative to how these models handle these systems and their associated rainfall. The work presented is a small portion of a much larger study that aims to determine the impact of all of these rain-bearing transient systems on the monsoon in northern Mexico.