Impact of the North American monsoon on wildfire activity in the southwest United States

The North American monsoon (NAM) is an annual climate system phenomenon that develops over the Sierra Madre Occidental in western Mexico and spreads northwards into the southwest United States from June through September bringing large quantities of rainfall and lightning, which can vary greatly on intra- and inter-annual timescales. The timing of the NAM onset can lengthen or shorten the wildfire season in the southwest United States. Here we determine NAM onset thresholds and subsequent dates for the Southwest Area (SWA; Arizona, New Mexico, west Texas, and Oklahoma panhandle) and each SWA Predictive Services Area (PSA) April through September from 1995-2013. Various wildfire activity thresholds were defined to capture days or events associated with increased wildfire activity that are considered busy by wildland fire management in the context of an impact on firefighting resources. These defined thresholds allow for a unique examination of the relationship between the NAM and wildfire. Self-organizing maps (SOMs), utilizing 500-hPa geopotential heights and precipitable water, were implemented to identify atmospheric patterns contributing to the NAM onset and busy days for the SWA and each PSA. Map types from the SOMs analysis showed the transition to, during, and from the NAM. Northwards and eastwards displacements of the subtropical ridge over the SWA were associated with NAM onset. Restructuring of the subtropical ridge in time (i.e., amplification or breakdown) as inferred from map types over the SWA was directly associated with increased wildfire activity. By identifying atmospheric patterns pertinent to busy days of wildfire activity and a wildfire-based perspective of the NAM in the SWA, fire weather forecasters can proactively identify potential periods that may be particularly impactful on wildfire suppression resources.