Thursday, March 16, 2023

Lingering Cold

Temperatures have been volatile in Alaska over the past month or so, but overall the past 5-6 weeks have produced more cold than warm weather, and this week is certainly on the cold side for much of the state.

Last night dropped into the -20s and -30s Fahrenheit in many of the colder locations: click to enlarge the map below.


This kind of cold is a bit unusual for the middle of March, although nowhere near record territory.  For instance, today's record low at Bettles is -49°F, and just two years ago Bettles saw a remarkable -39°F on April 9.  Read more here on the great April 2021 cold snap.

It's interesting to note that the northern interior has had an unusual number of days with much colder conditions than in, say, Fairbanks this winter.  For example, Bettles typically sees half a dozen days with a low temperature 25°F colder than Fairbanks, but there have been 13 such days this winter.  This is the fourth straight winter with 10 or more such days.

Here's this morning's mid-atmosphere (500mb) map, courtesy of Environment Canada.  A deep trough extending south from the Arctic means a cold air mass, a weak pressure gradient means light winds, and with the main jet stream activity far to the south, clear skies provided the final ingredient for sharp overnight cooling.

Views from the FAA webcams this afternoon are simply glorious; here's Bettles and the Chandalar Shelf DOT site:

Saturday, March 11, 2023

February and DJF Climate Anomalies

Just a quick post this weekend as I'm traveling - a look back at February and climatological winter (December-February).

February was the coldest month of the winter for the state as a whole, which happens about 20% of the time (compared to 47% for January and 33% for December).  The month was colder than the recent normal, but not by a large margin, and it would have been near-normal in earlier decades.


The winter as a whole was significantly warmer than normal because of the very warm January:

February precipitation was widely above normal for western and interior Alaska, but it was a relatively dry month for the northern Gulf coast.  This fits the bill for a typical La Niña winter pattern, and the DJF precipitation map shows broadly the same thing, despite the warmth being uncharacteristic of a traditional La Niña.

Finally, according to the ERA5 model, February wind was notably lacking for much of interior, southern, and western Alaska, despite the first of two big blizzards occurring in the latter part of the month in Kotzebue.

Monday, March 6, 2023

Kotzebue Blizzards

Kotzebue has been seeing some rough weather lately, with the second big blizzard of the past two weeks winding down today.  Twitter user Tundrabilly has been documenting the conditions.


Here's a chart showing the frequency of blizzard conditions as a percent of all hourly ASOS observations in the last 25 years.  February is peak blizzard season, and the climatological frequency doesn't drop off a whole lot in March.  However, the percentage only peaks at about 1%, so a true blizzard is unusual weather even at this time of year.


The much lower frequency in January might be at least partly a statistical artifact, but it appears in both the first and second half of this 25-year period, so I'm inclined to think it is a real feature of the climate.  Presumably the jet stream drops far enough south in January that strong storms are less common in Kotzebue; but this is a topic for further investigation.

If we exclude the visibility requirement of the blizzard definition, then the December and November frequencies jump up above the February and March frequencies, respectively.  It seems that low visibility is more readily attainable in late winter, perhaps because there's much more snow lying on the ground throughout the region (available to be blown around), or perhaps because the air tends to be drier in late winter (making snow more powdery).

Saturday, March 4, 2023

February Regime

As noted in my last post, February was quite snowy again this year in Fairbanks, and it's worth following up on this.  With 21.1" of snow, last month makes 5 of the last 7 Februarys with over 20", and all 7 have seen above-normal snow.

In the preceding 87 years (1930-2016) there were only 7 Februarys with 20" or more, so the probability of randomly getting 5 of 7 years above that mark is extremely small indeed (less than 0.00001, assuming no year-to-year correlation).  Obviously this indicates that "something has changed"; the global circulation and the February weather patterns around Alaska have behaved differently in the last 7 years than in earlier decades.

Here's the average sea-level pressure anomaly for February in the last 7 years. 

On average, SLP has been more than 10mb higher than normal over the mid-latitude North Pacific to the south of Alaska.  That's a very large departure from normal, and it signifies a stronger than westerly component to the flow over southern Alaska.  The map below shows the departure from normal of the wind vectors (speed and direction).

The more westerly flow regime is favorable for importing Pacific and Bering Sea moisture to interior Alaska, and the pattern also tends to set up a (cloud/snow-producing) frontal zone across southern Alaska.  Here's the lower-atmosphere temperature anomaly: it has tended to be colder than normal in western Canada and northern Alaska, but warmer than normal across the northern Pacific.

The pattern is substantially similar to a typical La Niña winter pattern, except that the North Pacific ridge often extends more strongly to the northwest across the Bering Sea during La Niña.  In contrast, recent Februarys have seen low pressure in the western Bering Sea and a ridge axis farther to the east over Alaska itself (see the SLP map above).  This means that there has been far less cold for Alaska than you would expect during La Niña.

Of course, we have indeed seen a preponderance of La Niña in the past 7 winters; 4 of the 7 years had bona fide La Niña conditions in February.  Winters 2018-19 and 2019-2020 were more El Niño-like (although not strongly so), and it may be no coincidence that the two recent Februarys with under 20" of snow in Fairbanks were those ones.

However, the recent state of affairs is not as simple as a recurring La Niña regime causing excess snow in Fairbanks; some of the strongest La Niña's of the past have been dry, not snowy, winters in Fairbanks.  Compare the sea surface temperature maps below; the first shows the typical SST pattern for La Niña in February, and the second shows the pattern from 2017-2022.  (The ERA5 data for February this year isn't available yet, but I've included an ERSSTv5 map for last month as well.)

What strikes me here is that recent Februarys have seen much more warmth in both the Bering Sea and the western Atlantic Ocean than is typical for La Niña.  It seems clear that the western Bering Sea warmth is associated with the recurrent low pressure there - which again is not typical of La Niña - and both the flow trajectory and the excess of moisture (from excess evaporation) have favored abundant snowfall in Fairbanks.

Evidence of excess atmospheric moisture upstream of Fairbanks can be seen in the anomaly of precipitable water for the last 7 Februarys: it has been above normal over the Bering Sea.

As for the abundance of warmth in the western Atlantic, this has helped reinforce (and has been reinforced by) the U.S. East Coast ridge that is part and parcel of the continental-scale flow regime.  Here's the global 500mb height anomaly for February in recent years.

The connection between the North Pacific ridge and the East Coast ridge is an example of a long-distance "teleconnection", and the pattern has been pronounced in recent years.  It's not coincidental, then, that the southeastern U.S. has seen unusual February warmth for - you guessed it - the past 7 years, and this February was one of the warmest on record.

To illustrate, here's a scatterplot of Fairbanks February snowfall versus February temperature at my location in northeast Georgia.  There has always been a relationship, with a snowy Fairbanks often corresponding to a warm Georgia, but the joint anomaly has been very striking in recent years.

When will the pattern change?  It's impossible to say, of course, but next winter has a good chance of seeing El Niño, and so the North American pattern should be significantly different.

Sunday, February 26, 2023

Snow and Snow Ratio

Fairbanks has seen a fair bit of snow this month, with measurable accumulation on 16 days and a total of 20.6" so far.  The record number of days with accumulating snow in February was set just last year, at 18 days.  The long-term median February snowfall is just 7 inches.  However, we're nowhere near the February 1966 record of 43".

It's interesting to note that this is now the 7th consecutive February with above-median snowfall, including 5 of 7 with over 20".  Remarkably, only 7 of the preceding 87 years (since 1930) had this much snow in February.  It used to be rare - fewer than 1 in 10 years - but is apparently now common.

Last Thursday's snowfall of just under 6" contained nearly half an inch of liquid equivalent, so it was fairly dense stuff by interior Alaska standards.  This got me thinking about snow ratios and the long-term trend thereof.  Is there any evidence that the snow ratio (snow depth divided by liquid-equivalent precipitation) has changed over time?

Yes, absolutely: the last decade or so has seen generally lower snow ratios than in the previous six decades, and there have been no winters with a historically high ratio (e.g. over 20) since 2009-10.  Last winter had the lowest ratio on record for Fairbanks, only 10.5, and a large reason for that was of course the epic rain storm of Christmas 2021.

It's interesting to see that mostly low ratios also prevailed in the 1930s and 1940s, although I suppose there's a chance that different measuring practices could explain at least some of this.  However, we know there were some notable, indeed extreme, winter rain events in the 1930s, so a regime of low ratios is perhaps not unexpected.

The recent decrease in ratios is more noticeable for November-December than for January-March, although the absence of high ratios clearly stands out for both:

The low ratio in Nov-Dec 2010 reflects the exceptional rain event of that time - as documented here in the early days of this blog:

As for the low ratio in January-March 2016, that reflects the extraordinary rain at the end of March, which produced as much precipitation as the rest of the (very dry) 3-month period combined.

A decrease in snow ratio is what we would expect if the atmosphere is warming overall.  Snow tends to be more dense at higher temperatures, and of course increased warmth favors occasional winter rain, which crushes the total snow ratio.

The ERA5 global reanalysis (model) also shows signs of a change since 2010-11, with most years having a lower ratio of total snow water to total precipitation - see below.  This is not the same thing as snow ratio, as it's only affected by the rain vs snow partition, and not by snow density - so the ERA5 is just showing an increased frequency of significant winter rain.  It would be highly worthwhile to look at the spatial distribution of this change in the model data.

Wednesday, February 22, 2023

Weather Contrast

The weather has been bad today in western Alaska, with wind, snow, and rain moving up from the south.  Kotzebue developed a white-out this afternoon with 40-50 mph winds.


But far to the east on the other side of the ridge axis, it has been a stunning day (after a chilly start) in eastern Alaska and the Yukon, with lots of glorious sunshine.  Here's the webcam view from Dawson:

And in video format:

Here's the 3am 500mb analysis, courtesy of Environment Canada: click to enlarge.  Note that ridge axis right over Alaska.

Monday, February 20, 2023

Follow-Up on La Niña

It's worth following up on a couple of points from last week's post.

First, to extend the comments on what happened in January, the Arctic as a whole did not see the widespread and unusual warmth that affected Alaska, although conditions were highly variable across the basin:

The European side was even warmer than the Alaskan sector, but much of inland Arctic Siberia was colder than normal.  I say "inland" because the most northerly Russian sites around 90°E were much warmer relative to normal, illustrating the moderating influence of the ocean despite its ice cover.

The relationship between ENSO and winter temperatures is not as simple for the Arctic as a whole as it is for Alaska, because there are strong regional differences across the Arctic.  The Arctic-wide temperature trend since 1980 is similar for both El Niño and La Niña, judging from the 32 sites I've been using for realtime monitoring:


Notice that of the three extremely warm winters of 2015-16 through 2017-18, one was an El Niño winter, one a La Niña, and the other one was neutral.  

The neutral-ENSO trend is slightly less than the El Niño and La Niña trends, which is similar to the result I showed last time for Alaska.  But let's discuss that result a bit more.  It turns out that the neutral-ENSO temperature trend is very sensitive to the ENSO classification method, i.e. which index we use to determine whether a winter is El Niño, La Niña, or neutral.  Last week I used the Multivariate ENSO Index, which is a comprehensive measure of ENSO's expression in several different variables.  This was the result for Fairbanks:

In contrast, here's the analysis using the Southern Oscillation Index to define the occurrence of El Niño and La Niña.  The SOI is a simple but venerable index based solely on pressure differences between Darwin (Australia) and Tahiti.

What a difference this makes to the neutral-ENSO trend!  It now shows no significant discrepancy from the El Niño and La Niña trends, and all three are similar.  The trends are also quite similar if we use data back to 1930 (the SOI is available back into the 19th century):

How could the ENSO classification method make such a large difference?  Part of the answer is that these linear trend estimates are inherently uncertain (as I noted last time) because of the small sample size in each category, and the large year-to-year variance.  And in particular, the classification of neutral-ENSO years is subject to considerable uncertainty, because the phase estimate can differ in either direction.  In contrast, the stronger El Niño and La Niña episodes are unambiguously classified with more confidence.

As an example, it seems that three very warm winters in the first half of the MEI series were classified as neutral ENSO winters according to the MEI, and all three had a significantly positive PDO phase: 1969-70, 1976-77, and 1980-81.  The warmth in these earlier years contributes to the lack of warming trend in the ENSO neutral category:

But according to the SOI, all three winters were marginally El Niño winters, and so they are excluded from the neutral-ENSO trend calculation.

Similar differences are found for the statewide Alaska temperatures, and remarkably the situation is now reversed with the neutral-ENSO trend, which is now the steepest of the three:

Another interesting point here is how much more shallow the long-term trend is when including the data prior to 1950.  Starting the trend in 1950, as I did for the MEI classification, produces very steep trends because of the cold (negative PDO) era of the 1950s and 1960s.

In conclusion, there's obviously very large uncertainty around the neutral-ENSO trend, but the SOI classification supports the consistency in Alaska's warming trend for both La Niña and El Niño winters.