Tuesday, July 2, 2019

Wildfire Activity

Increasingly unusual warmth in the past couple of weeks has allowed wildfire activity to pick up substantially across Alaska, and as of today the state has seen about double the normal fire acreage for the date.  The typical midpoint for statewide fire acreage is only about a week away, and so 2019's acreage is close to exceeding the median value for the entire season.  The last 3 years were relatively normal for statewide fire activity (at least based on a "modern" normal since 1995), but with another round of heat currently developing across Alaska, there's little doubt that 2019 will end up with significantly above-normal wildfire activity.  Click to enlarge the figure below.

The fire that is drawing the most attention from fire suppression experts is the Shovel Creek fire, which is very close to Fairbanks - just the other side of Murphy Dome.  The fire is not nearly as large as some other fires in the state, but it is expected to grow and there is a high risk to properties near Fairbanks.  Consequently a national-level command (Type 1 IMT) is taking over the suppression efforts tomorrow; I'm not sure how often this occurs in Alaska.

Here are a couple of maps showing how the Shovel Creek fire grew between Friday (3,424 acres) and yesterday (10,008 acres).  Ester Dome (on the west side of Fairbanks) is located in the lower right of the map domain.  Let's hope the trend does not continue.

Last year I commented on the positive correlation between North Pacific sea surface temperatures and Alaska wildfire activity, so perhaps it should be no surprise that this fire season is becoming active.  The Bering Sea and Gulf of Alaska surface temperatures have been far above normal since winter, and Rick Thoman has been documenting (via his Twitter feed) temperature anomalies of several degrees Celsius to the west of Alaska in recent weeks.  From a global perspective, the warmth in the North Pacific is one of the more striking features in the global oceans at present; here's a map of May SST departure from normal in terms of standard deviations.

To reinforce the idea of a link between Alaska fire and North Pacific temperatures, the chart below shows a 25 year history of the North Pacific Mode index in May and subsequent Alaska fire acreage.  Remarkably, all of the very active fire seasons in recent decades have occurred when the NPM index was positive.  Recall that the NPM represents a specific pattern of SST anomalies that extends west-east across the North Pacific to the south of Alaska; not surprisingly, the NPM was positive in May and has become strongly positive in recent weeks (see figures below).

The long-term history of the NPM index does not have a rising trend over time, but this is by design: the NPM definition proposed by Hartmann (2015) removes the linear trend from the SST data before doing the EOF analysis.  In contrast, if we look at area-average SSTs from 40°N to the Bering Strait, there is a very significant warming trend, and the past several years have been persistently very warm.

It stands to reason that higher temperatures would favor increased wildfire activity, all else being equal, because earlier snowmelt allows fuels to dry out earlier in the year, and increased evapotranspiration creates larger moisture deficits if rainfall does not increase.  The long-term increase in temperature has probably therefore produced a background increase in fire activity, or an increase in what is "normal" for a multi-year or decadal period.

However, the year-to-year variability in ocean and atmosphere patterns is still very large and obviously controls the level of fire activity in any given year; and the NPM appears to be closely connected to the key weather factors that influence Alaska fire activity.  A lot more research could be done on this, and it seems likely that useful seasonal fire predictions might be possible.  If anyone is interested in collaboration, leave a comment!


  1. Well, like always, I'm not an expert on anything, but I've lived in Fairbanks a while now (40 yrs) so there's that. What this summer reminds me of is the big fire yrs of '04-05 when I believe we had persistent high pressure over the interior that led to warm dry conditions and lots of wild fires. So maybe that's something that we're getting again?

    1. Yes indeed, 2004 and 2005 were the number 1 and 3 fire acreage years. Other than 2015, no other years have come particularly close in recent decades.

  2. I agree with BJ's comment about the blocking high pressure system. Coming from Southcentral, this heat wave follows Anchorage's wettest May on record. Grasses and other understory plants had optimal growing conditions this spring. No doubt the hot, drying effect of the extended high pressure is at play. I'm curious what the ocean temperatures were like in the big fire years of 04-05'. Certainly these warm waters this far north aren't helping matters.