Interannual variablity of wildfires and summer precipitation in the Southwest

As described in the accompanying Gallery of Geology article, springtime is wildfire season in the Southwest. The table of recent fires in New Mexico on page 25 is dominated by events in May and June. Hot, dry, windy conditions in the spring promote wildfire development. Drought conditions make some years more fire prone than others, and long-term climatic changes in temperature and precipitation affect the general fire regime in the Southwest (Meyer and Frechette, this issue). The onset of the North American monsoon, usually around the beginning of July, brings higher humidity and intermittent rainfall, dampening grass and dry fuel and generally marking the end of the fire season (and the acceleration of plant growth that provides fuel for the next year's fire season). It is also possible that wildfires affect the regional climate, although this connection is harder to quantify. Large fires inject large quantities of soot and smoke high into the atmosphere. Winds then blow these particulates across the Southwest, so the impact of locally injected particulates can spread far beyond the source. Other sources of particulates, such as widespread air pollution, have been shown to block sunlight and depress precipitation. A study of the South Asian monsoon showed that black carbon soot associated with air pollution decreases the radiative heating of the surface, thereby altering atmospheric stability and largescale temperature gradients that drive monsoon circulations (Ramanathan et al. 2005). Compared to the South Asian brown cloud, southwestern wildfires in most years generate much smaller quantities of particulates that remain airborne for a shorter period of time. However the timing of the southwestern fire season in late spring is potentially just right to affect the onset of the monsoon. The onset date is highly correlated with total seasonal precipitation, such that late onset is usually a precursor of low total summer rainfall (Higgins et al. 1997). We describe here a preliminary assessment of the hypothesis that large spring wildfires could depress monsoonal precipitation, by comparing a 25-yr time series of acreage burned in southwestern wildfires in June with the subsequent precipitation observed in July and August (Van Alst 2009). Data