j.g. pausas' blog

We often tend to think that the main driver of fires is drought, but in many cases wind is extremely important; and the perfect combination is a strong wind after a long drought period. Foehn-type (adiabatic) winds are especially important for fires as they are fast-moving hot and dry winds that quickly dry out the vegetation, and thus they spread the fire very easily. In the recent months, we have witnessed several very destructive wind-driven fires, affecting many infrastructures and lives. Wind-driven fires are common in Southern California where the Santa Ana winds blow after the summer. They typically occur in October, although this year they came later, in December. For instance, the Thomas fire started at the early December, and has now become the largest wildfire in California history (> 110,000 ha, Fig. 1) with more than 1000 houses destroyed, more than 100,000 residents evacuated, and several fatalities. This fire has been largely driven by Santa Ana winds.

Although less frequently, there are also wind-driven fires in Northern California, the wind is called Diablo wind. This year Diablo driven fires has been particularly important and destructive; during October more than a dozen wildfires north of San Francisco had killed more than 40 people, burned approximately 65,000 ha and destroyed more than 7,000 structures (see also, washingtonpost.com, treehugger.com).

Both Santa Ana and Diablo winds are Foehn winds going down from the mountains (inland) to the coast (Santa Ana and Diablo winds at a glance). Wind-driven fires are natural in California and have been generating large fires since long ago, but the increasing population living in the wildland-urban interface is making these fire more destructive than ever. In addition, climate change is extending the fire season into the late fall and winter, increasing the probability of large fires.

Wind-driven fires have also occurred this year in north-western Iberia (Spain and Portugal), caused by the hurricane Ophelia. Typically, tropical hurricanes do not get to Europe, but this year the Ophelia touched western Europe (probably due to the warming of the ocean) and spread massive fires in Portugal and Spain that were fuelled with large poor-managed forest plantations; the ashes from these fires reached England and Ireland (Fig. 2 below).

In conclusion, global change is likely modifying wind patterns, and thus to understand new fire regimes we need to predict wind regime; however, predicting future wind regimes is more difficult than predicting temperature changes.
 

Figure 1. The beginning of the Thomas fire (started in Ventura, Southern California) was clearly driven by Santa Ana winds (Image: NASA / MODIS, December 5, 2017). This fire has grown and become the largest wildfire in California history (> 110,000 ha).

Figure 2. Massive October wildfires in NW Iberia were fueled by the hurricane Ophelia; smoke and ashes from these fires reached England and Ireland  (Image: NASA Terra / MODIS, October 16, 2017); see also Severe Weather Europe. You can see an animation from NASA Earth here.

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