Wildfire Risk in California No Longer Coupled to Winter Precipitation

[OKIsItJustMe]

https://uanews.arizona.edu/story/wildfire-risk-california-no-longer-coupled-winter-precipitation

Wildfire Risk in California No Longer Coupled to Winter Precipitation

Recent large fires may be harbingers of things to come, report Valerie Trouet, a UA tree-ring researcher, and her colleagues.

Mari N. Jensen, UA College of Science

March 4, 2019

Wet winters no longer predict possible relief from severe wildfires for California, according to a new study from an international team that includes a University of Arizona scientist.

From 1600 to 1903, the position of the North Pacific jet stream over California was linked to the amount of winter precipitation and the severity of the subsequent wildfire season, the team found. Wet winters brought by the jet stream were followed by low wildfire activity, and dry winters were generally followed by higher wildfire activity.

After 1904, the connection between winter moisture brought by the jet stream from December through February and the severity of the wildfire season weakened. The weakened connection between precipitation and wildfires corresponds to the onset of a fire suppression policy on U.S. federal lands, the team reports in the March 4 issue of the Proceedings of the National Academy of Sciences.

The connection disappeared altogether after 1977.

…

http://dx.doi.org/10.1073/pnas.1815292116

[PoindexterOglethorpe]

1. I suppose they are simply extrapolating about the position of the jet stream

for 1600 to 1903, given it wasn't discovered until 1939. Although it would have been observed much earlier, given that Mt. Everest reaches into the it, and photos you see of a strong wind blowing snow off the top is the jet stream in action.

[OKIsItJustMe]

2. Re: Atmospheric Circulation Field Reconstruction

…

Atmospheric Circulation Field Reconstruction.

We used an analog assimilation method similar to that applied in other studies (18, 39⇓⇓–42) to reconstruct 3D fields of the atmospheric circulation, based on a combination of state-of-the-art climate model output [MPI-ESM-P (43), National Center for Atmospheric Research Community Climate System Model, version 4 (44), and Goddard Institute for Space Studies Model E2-R (45) last-millennium transient-forcing simulations; see SI Appendix, Earth System Model Selection and Evaluation] and three gridded paleoclimate reconstructions. The reconstruction predictors include tree ring-based reconstructions of June to Augustt soil moisture [the North American Drought Atlas (NADA)] (46), water-yeart (Octobert−1 through Septembert) precipitation (15, 20), and February to Marcht near-surface air temperature (47) over portions of western and southwestern North America. For a given calendar year t, the predictor fields are compared with the simulated seasonal mean fields of the same variables through the length of the climate simulations; the years for which the simulated seasonal fields are most similar to the gridded reconstructions are considered the “analog years,” and the atmospheric circulation (the predictand) of the corresponding winter (Decembert-1 to Februaryt) season is selected to form the model-based reconstruction. To define the most similar analogs, the three climate variables were merged into a single vector for each year after they were regridded to a common 2.5 × 2.5 longitude/latitude grid and then normalized so that each variable had the same weight in the definition of similarity to the simulation output.

The physical basis for this strategy is the strong relationship between these variables and the winter atmospheric circulation over the northeastern Pacific–western North American sector. Winter temperatures are directly influenced by this circulation via air mass advection, and the two hydrological variables also strongly depend on the moisture delivery from the Pacific onto land that occurs during the winter season. As noted, CA receives more than half its annual precipitation during winter (19), and the NADA has been shown to be strongly related to antecedent winter moisture delivery south and west of a line connecting the northwestern United States to northeastern Mexico (48), which is the regional portion utilized here. Employing the analog method, the global 3D field of the atmospheric circulation can, in principle, be reconstructed, although the actual reconstruction skill will be spatially heterogeneous and generally focused in the regions where the atmospheric circulation most strongly influences the predictors (SI Appendix, Figs. S5 and S6). We note the similarity of this reconstruction process to the Proxy Surrogate Reconstruction method outlined in ref. 49.

We extracted 200 hPa zonal (u) and meridional (v) wind components from the reconstructed circulation fields for analysis of winter NPJ conditions. We analyzed the latitudinal position and strength of the reconstructed NPJ during extreme wet/dry years in CA using a tree ring-based reconstruction of Octobert-1 to Junet precipitation for the southern Sierra Nevada mountains (23), which is nearly independent of the NPJ reconstruction. In a similar way, we stratified NPJ winter conditions according to high- and low-fire years, as reconstructed from a completely independent record of Sierra Nevada fire history (24) (SI Appendix, Fig. S2). We note that the annual precipitation at the fire record sites is generally similar to that in the precipitation reconstruction subregion, which is included within the spatial extent of the fire record near its southern end (50). We also note that the spatial coverage of the extreme precipitation stratification in the 20CR (Figs. 1 E and F and 3 A–D) is spatially as close to the independent Sierra Nevada paleoprecipitation record as possible, given the gridded nature of the reanalysis, and that the 20CR seasonal coverage also matches that of the Sierra Nevada record (Octobert-1 to Junet).

As noted, winter dates are labeled according to the year in which January occurs. Winter 2013 values from the 20CR are based on December 2012, the end of the 20CR record. Further methodological information is provided in the SI Appendix, Additional Methodological Detail.

…

[Wellstone ruled]

3. Thanks for this info.

Complex but understandable .

What should be added is,the extreme growth of non native grass species as well as non native shrubs and vine type plants. These mature in June and are pure Fire Fuel from that time forward.