Regional Climate Dynamics

Human-induced changes in Wind, Temperature and Relative Humidity during Santa Ana events

We investigated the frequency and character of human-induced changes in Southern California’s Santa Ana wind events.  This was done by comparing a 12-km-resolution WRF downscaling of two time periods of the NCAR CCSM global climate change scenario run, one corresponding to the mid 21st century, and the other to the late 20th century.  It is necessary to use a dynamical downscaling technique to examine this question:  Santa Anas do not exist in the global climate simulation due to the fact that it does not resolve Southern California's mountain complexes.   In the WRF simulations, the number of Santa Ana days per winter season is approximately 20% fewer in the mid-21st century compared to the late-20th century (see accompanying figure).  Since the only systematic and sustained difference between these two periods is the level of anthropogenic forcing, this effect is anthropogenic in origin.  In both time periods, Santa Ana winds are partly katabatically-driven by a temperature difference between the cold wintertime air pooling in the desert against coastal mountains and the adjacent warm air over the ocean.  However, this mechanism is significantly weaker during the mid-21st century time period.  This weakening occurs because of the well-documented differential warming associated with transient climate change, with more warming in the desert interior than over the ocean.  Thus the mechanism responsible for the decrease in Santa Ana frequency originates from a well-known aspect of the climate response to increasing greenhouse gases, but cannot be understood or simulated without mesoscale atmospheric dynamics.   In addition to the change in Santa Ana frequency, we investigated changes during Santa Anas in two other meteorological variables known to be relevant to fire weather conditions -- relative humidity and temperature.  We find a decrease in the relative humidity and an increase in temperature.  Both these changes would favor fire.  A fire behavior model accounting for changes in wind, temperature, and relative humidity simultaneously is necessary to draw firm conclusions about future fire risk and growth associated with Santa Ana events.

Number of days throughout the Santa season (September through March) where an index of the Santa Ana winds exceed 10 meters per second in the late 20th and mid 21st century WRF simulations.

Download the paper (Hughes et. al. 2010) describing these results in more detail.

Mimi Hughes, Alex Hall, and Jinwon Kim make up the team that performed this research.