Autumn Climate Data

Temperatures have been dropping off sharply in the past 10 days in the interior, and the change has felt more acute because of the unseasonable warmth before that.  Fairbanks didn't see anything more than a mild freeze until three weeks ago, when the temperature first dropped into the low 20s Fahrenheit (about 3 weeks later than usual).

The last week or so has felt very different.  Starting on November 4, high temperatures have been below 20°F; the first sub-zero low temperature occurred on the 9th; and then yesterday the high was -1°F.  Winter has most definitely arrived.

Last month I missed my usual monthly climate summary owing to the focus on ex-Typhoon Halong; so let's look back at September and October.  Both months brought unusually low pressure to northern Alaska and stronger than normal westerly flow across the state.  There was a particularly strong trough over the western Bering Sea in October, and this bears part of the blame for pulling Halong up from the south.  Both months saw above-normal high pressure to the south of the Aleutians.

The southwesterly orientation of the flow in October explains the unusual warmth in Fairbanks, and the anomaly was widespread; but September was somewhat cool in western Alaska, as the air had a more northwesterly origin.  Both months were very unusually warm in the southeastern interior and along the Gulf coast, according to ERA5 data:

Recall that early September saw a very unusual heatwave in the southeastern interior, and there was another rare warm extreme in early October: Northway reached 62°F on the 8th, with a low temperature of 40°F (easily the latest on record for such a high daily mean temperature).  This was followed by unrelenting warm anomalies for several weeks:

Another theme of the two months was the unusual excess of cloud cover across large parts of Alaska.  October was especially cloudy for the interior, according to ERA5 data.

Looking at cloud cover data from Fairbanks airport, the situation has been dreadful since the middle of August.  Until this week only one day (September 17) reported mostly clear skies for more than 20% of the day, and the fraction of hourly reports with mostly clear skies is easily the lowest on record for the September-October window.  This is so anomalous that I think it's worth a separate post after looking at data from the CRN solar radiation instruments.

Not surprisingly, precipitation was very abundant across much of the state in October, and northern Alaska was substantially wetter than normal for three months in a row (August through October).  Bettles saw 10.6 inches of precipitation in the three month period, the second highest on record for that period.

The active westerly pattern produced above-normal wind speeds in October (in part due to Halong of course), but September was relatively calm for the central and eastern interior.

Finally, it's worth noting that the enhanced wind speeds across the northern North Pacific produced a dramatic drop-off in sea surface temperatures from just north of Japan to the Gulf of Alaska.  Compare the two figures below (August versus October):

The North Pacific Mode (NPM) SST index, which I revisited recently here, has dropped off precipitously:

Given that La Niña will prevail during at least the first part of this winter, the disappearance of the positive NPM probably means a greater chance of cold in Alaska; but much will depend on the high-latitude flow, which is about to become very perturbed in connection with a rare upper-atmosphere disturbance over Arctic Canada.  More on that later.

First Snow, and North Pacific Warmth in Winter

Fairbanks saw its first snowflakes of the season today, just a few days earlier than normal.  Temperatures were above freezing and no accumulation occurred in town, but the scene looked a lot like "late autumn" in the hills.  Here are a couple of webcam views from Cleary Summit on the Steese Highway shortly after 9am today:

On another topic, reader AlexG asked whether this summer's extreme North Pacific warmth is likely to have an effect on winter in Alaska.  It just so happens that I gave a talk related to this question 10 years ago at UAF, and here's a link to the slides from that occasion:

https://s2s.worldclimateservice.com/wcs/uaf_sep2015_north_pacific_sst_blob.pdf

The winters of 2013-14 and 2014-15 both saw a tremendous amount of warmth in the northeastern Pacific, and that sea surface temperature anomaly became known colloquially as the North Pacific "blob".  In a 2015 paper, Dennis Hartmann of the University of Washington discussed a mode of natural variability - the North Pacific Mode - that resembled the "blob" pattern, and my 2015 talk looked into its connection to Alaska winter climate.

The bottom line from this work of a decade ago is that a positive North Pacific Mode (warm ocean) is linked to unusual winter warmth in most of Alaska except the Southeast, and it also favors wet rather than dry conditions in most areas.  The link with winter precipitation is strong in interior Alaska, because a positive NPM is associated with stronger westerly flow and less chinook flow from the south.

The other major mode of North Pacific temperature variability is of course the Pacific Decadal Oscillation (PDO), which is also closely linked to precipitation across interior Alaska and to temperatures statewide.  A negative PDO phase, which is what we have at the moment, tends to bring wetter winters to the interior (like the +NPM) but colder temperatures statewide (generally opposite to the +NPM).

Here are the canonical sea surface temperature (SST) patterns associated with positive NPM and PDO phases:

Note that the PDO and NPM are independent of each other, so a positive NPM can readily coincide with a negative PDO, as this year.  Also, a negative PDO doesn't necessarily imply cooler than normal SSTs in the northeastern Pacific; the mode expresses the spatial contrasts/gradients in SST rather than the absolute values.  This year's negative PDO phase has been very much driven by extreme warmth near and east of Japan, rather than cool water along the coast of North America.

If we look at previous winters with the +NPM/-PDO combination in the North Pacific, we see - as expected - a strong indication of above-normal precipitation across most of Alaska to the north of the southern coastal regions (where it tends to be dry), and there's also a fairly robust cold signal that apparently reflects the dominance of the PDO influence.  The fundamental circulation anomaly involves unusual high pressure (or equivalently, less low pressure) over the Aleutians, producing a stronger component of westerly flow across central Alaska.

Of course we don't yet know whether the North Pacific SST patterns will remain similar until winter and throughout winter, and indeed there's a good chance they won't.  It's therefore worth examining past years when the +NPM/-PDO combination occurred in summer, rather than assuming it will persist unto winter.  Here's the result:

Interestingly, the precipitation signal is very similar, but the cold signal is much weaker in the summer "analog".  This seems to be because several of the summer matching years ended up with El Niño and/or a near-neutral PDO phase during the subsequent winter, thereby changing the remote influences on the circulation patterns near Alaska.  In contrast, most of the winter +NPM/-PDO matches involved La Niña during winter, reinforcing the cold signal.

And for this winter?  Best estimates are that La Niña will probably prevail during autumn and early winter before fading over the course of the winter; El Niño is very unlikely.  But that doesn't mean it will be cold.  Arguably the most similar recent year in terms of the tropical and North Pacific setup is 2022-23, and that was a warm winter for Alaska (except for Southeast), while the precipitation anomalies matched the +NPM/-PDO pattern very well.

https://ak-wx.blogspot.com/2023/04/another-warm-and-wet-winter.html

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!

Winter So Far

January has been another relatively snowy month in Fairbanks, with a total accumulation of 17" - the highest January total since 2005.  The combined December-January total was almost exactly 50", which is the 5th highest on record for the two-month period.  Temperatures have averaged slightly below the 1981-2010 normal and close to the long-term (1930-present) normal, but with alternating cold and warm spells.  Snowy winters are typically colder than dry winters in Fairbanks.

As noted in earlier posts (e.g. here), the cool and snowy conditions are surprising in view of this winter's North Pacific sea surface temperature patterns.  The positive PDO phase and negative NPM phase, which continue to be evident in the latest SST data, would normally promote warm and dry conditions in the interior; we discussed this back in November.  Here's another map showing the average 500mb height anomaly (departure from normal) in November through January for 6 winters with a significantly positive PDO and significantly negative NPM:

Compare this pattern to what has actually transpired in the last 3 months:

The patterns are fairly close to the inverse of each other, and this of course explains the weather in Fairbanks: the flow pattern has produced enhanced westerly flow across western Alaska, thereby transporting moisture to the interior, and the tendency for higher heights in the Bering Sea has allowed cold air to drop south across Alaska at times.

The reason for the discrepancy appears to be that the weak La Niña in the tropical Pacific has had a much more dominant influence on the circulation pattern than expected (at least by me).  The map below shows the height pattern for the 10 strongest La Niña winters since 1950-51; this is a better match to what has happened this winter, although admittedly the La Niña flow is a colder pattern for Alaska than we've seen this winter.

The most intriguing part of the situation, I think, is how the North Pacific SST patterns have remained much more reminiscent of El Niño than of La Niña, despite the evidently profound influence of the La Niña episode on the atmospheric circulation.  A positive PDO and negative NPM are generally observed in association with El Niño, like last winter - but not this time.  But regardless of the cause, this winter's outcome will serve as a cautionary tale about the danger of relying too heavily on the North Pacific SST patterns in isolation for making a forecast.

Update Feb 2: This nice graphic from NWS Fairbanks shows the complete contrast between last winter and this winter in terms of early versus mid-winter snowfall accumulation; the totals before and after December 1 are almost perfectly reversed.

December Snowfall

Here's a quick update on yesterday's post: snowfall for December is now up to 32.9" in Fairbanks, which is the 4th highest on record (1930-present).  Nearly half of that fell in the last three days.  The airport recorded wind gusts as high as 52 mph early this morning, and sustained winds of 33 mph were measured; this sustained wind speed is the highest since February 2011.  It's also the highest for early-mid winter (pre-February) since 1970.

This morning's sounding showed an extraordinarily strong westerly flow aloft, with 850mb winds of 75 knots (86 mph).  This ties the all-time record for strongest 850mb wind speed in Fairbanks (1948-present); this has been an extreme and historic winter storm.

Returning to the topic of the relationship between snowfall and North Pacific sea surface temperature patterns, the following chart illustrates the unusual nature of this month's heavy snowfall.  The horizontal axis shows the difference between the PDO and NPM index values (PDO minus NPM), and December's snowfall is shown on the vertical axis.  When the PDO is positive and the NPM is negative, as they are now, it is unusual to have significantly above-normal snowfall.  Note that I calculated the linear trend line after transforming the snowfall data into an approximately normal distribution, because ordinary least-squares regression is not suitable for a skewed distribution like we have here (it weights the outliers too heavily).