Another Kotzebue Blizzard

It's only been 3 weeks since I wrote about back-to-back blizzards in Kotzebue that led to a local disaster declaration (news link courtesy of reader Gary), but another major storm affected the area last weekend.  Remarkably, hurricane-force wind gusts were reported by the airport ASOS, although the observations were missing for several hours, so perhaps the instruments malfunctioned.

By my count, this makes five real blizzards in Kotzebue this winter: November 7, December 10-11, February 21-23, March 4, and March 25.  Each of these had sustained winds above 40mph.  There was also a storm on January 27-28 that briefly met the blizzard criteria (35mph wind and 1/4 mile visibility).

In light of the recurrent storminess in Kotzebue, reader Andy's recent comment about Nome being "inundated with snow in many recent winters" is very interesting and worth a closer look.  Let's have a look at November-March precipitation for the two sites: the charts below show results both from the standard climate observations and from the nearest grid cell in the ERA5 reanalysis.

 

The large wet bias in the ERA5 data is not unexpected in model data, and it's not generally regarded as a problem - the user just has to implement a bias correction.  Much more importantly, the year-to-year correlation between ERA5 and the ground truth data is fairly good back to about the mid-1970s, giving us some confidence in both sources for the last several decades.  (The ERA5 does not use precipitation observations in creating its global gridded estimates.)

Excluding pre-1980 data, there's little trend in Nome winter precipitation, although the two very wet winters of 2017-18 and 2018-19 certainly count as being "inundated".  At Kotzebue, however, there does seem to be a significant trend towards wetter winters in both the ERA5 and ground-truth data, and the absence of a "dry" winter since 2016-17 is notable.

As for the earlier decades, the situation is more uncertain.  The Nome precipitation data doesn't seem to have enough year-to-year variability in the 50s and 60s.  The correlation with ERA5 is poor in the early decades for both sites, although that by itself doesn't invalidate the climate observations, as the ERA5 uncertainties are much greater for that time.  The pre-1970 numbers from Kotzebue are so low that they seem unreasonable at first glance, but if indeed the winter climate was more truly Arctic back then, then perhaps much lower precipitation is plausible.  Utqiaġvik averages less than 2 inches total precipitation in November through March even after becoming much wetter in recent years.

So much for precipitation; but how about high winds (the "blizzard" aspect of the problem)?

We don't have consistent wind measurements across the decades, so I again turned to ERA5 and extracted the hourly wind speed back to 1950 for both sites.  I determined the peak wind speed for each winter (November-March), calculated the median of those numbers, and then found the number of hours each winter that exceeded that "typical winter maximum".  The ERA5 winds are a bit lower than reported by the ASOS instrument of today, and in reality this "typical winter maximum" corresponds to sustained winds of about 40mph in Nome and 50mph in Kotzebue.  Half of all winters see winds of at least this strength at some point.

 

To bolster confidence that ERA5 winds are realistic, I checked the highest values in the history.  The highest ERA5 hourly wind speed for Nome was at 9pm on December 28, 2021, and indeed that day saw a major wind storm that peaked at 8:30pm.  In Kotzebue, the highest ERA5 wind speed in recent decades was at 11pm on February 18, 2006, and the observed wind speed peaked at the very same hour.

It's interesting that both sites have seen an increased frequency of winters with relatively high winds in the past 20 years; there have been few winters without high wind, especially at Kotzebue.

As for the maximum winds each winter, there's a hint of an upward trend in recent year for Nome, with few recent winters on the low side of the distribution, and the change seems a bit more pronounced at Kotzebue - see below.

Finally, looking at the number of days that met the blizzard criteria in the past 25 years (approximately the ASOS era), there's again perhaps just a hint of an increase in Nome, and Kotzebue has had 9 or more blizzard days in 5 of the last 6 winters.

All in all, it does seem that a trend is emerging at Kotzebue, although as usual the year-to-year variability makes it difficult to be sure, and formal significance testing would likely not give a resounding message.  The trends at Nome seem more tentative and uncertain.  Remind me to take another look at this a few years from now...

Colder Statewide, Snowy December in Bettles

A deep winter chill has descended upon much of Alaska this weekend, with temperatures dropping well below zero in the usual cold spots.  So far Fairbanks airport has dropped to -34°F, which is colder than anything that occurred last winter.  Of course it is a rare winter in Fairbanks that doesn't drop to at least -35°F at some point.

A few other cold spots today are:

Chicken  -49°F
Fortymile River HADS   -43°F 
North Pole  -39°F
Salcha RAWS  -38°F

The Fortymile River HADS site (on the Taylor Highway) did not rise above -40°F today, and with the cold expected to deepen somewhat in the next few days, I'd be very surprised if Chicken doesn't crack -50°F.  Only two winters in Chicken's climate record (1996-present) have failed to reached -50°F, and -60°F or colder occurs in about half of all winters.

Alaska's snow pack is below normal in many places, but one area with plenty of snow is the upper Koyukuk region; observers from Bettles up to Wiseman are reporting 30" or more of snow on the ground (click to enlarge maps below).

Remarkably, Bettles had its snowiest calendar month on record in December, with 62" of snow.  The typical annual snowfall in Bettles is close to 90" - fairly high for an interior valley location - but nevertheless 60" in a month is a lot.

A scatter plot of December snowfall and average temperature in Bettles shows a slight positive correlation, as the coldest months tend to be drier than normal; this is partly because the coldest Arctic air masses hold little moisture in winter, and partly because clear skies (associated with high pressure and dry weather) produce sustained temperature inversion and cold conditions at this time of year.

As we've noted before, the situation is quite different in Fairbanks owing to the effect of dry chinook winds from the south.  The proximity of the Alaska Range means that the warmest months, which are dominated by southerly flow, also tend to be dry, and so the snow-temperature plot shows a peak in snowfall at near-normal temperatures.

Alaska Range Snowfall

I don't often comment on the weather and climate of southern Alaska, but this week a very interesting paper was published looking at long-term snowfall accumulations high on Mt Hunter in the Alaska Range.  Here's a summary from the Weather Company

https://weather.com/news/climate/news/2017-12-19-warming-ocean-water-double-snowfall-alaska-study

and here's the study itself

https://www.nature.com/articles/s41598-017-18022-5

The paper begins by reminding us of the dramatic increase in winter precipitation that has occurred in recent decades at coastal sites in southern Alaska, for example (and most strikingly) a more than 50% increase in rain and snow at Kodiak.  The chart below shows the December-February precipitation totals since 1950 at Kodiak; note that the increase essentially occurred as a step change in 1976 when the PDO suddenly flipped from negative to positive.  The median December-February precipitation changed from 13.9" (1950-1975) to 23.6" (1976-2016), an increase of 69%, but there has been no significant trend since 1976.

The precipitation estimates from Mt Hunter, as published in the new study, show a very dramatic increase over the past 1000+ years, and they also indicate that snowfall has continued to increase in the past few decades.  Interestingly the 1976 Pacific regime shift is not particularly clear in the Mt Hunter data (see below); a more pronounced increase occurred after 1985, and the highest estimated totals occurred in the last decade of the data (which ends with the winter of 2010-2011).

The absence of a 1976 step change at Mt Hunter raises a question as to how closely the precipitation amounts are connected to the North Pacific circulation pattern.  The authors of the study argue that the long-term strengthening of the Aleutian Low is the main driver for the long-term increase in Mt Hunter snowfall, so this is an important point.

The chart below shows two measures of North Pacific atmospheric variability that are referenced in the paper: the Pacific-North American (PNA) index and a North Pacific (NP) pressure index; both of these are closely related to the strength of the Aleutian Low.  The usual definition of the NP index means that it's more negative when the Aleutian Low is stronger, but I've defined it with an inverse sign to line up with the PNA.  The 1976 regime change is evident for both indices, but as with the Kodiak precipitation data there has been no significant increase since then - in fact the PNA index has zero trend since 1976 and the NP index has a negative trend.

A scatter plot of the PNA index versus Mt Hunter precipitation (see below) reveals a fairly robust connection with - as it turns out - the same correlation for 1950-1975 and 1976-2010.  The correlations are slightly lower for the NP index (+0.39 in both periods).

It's clear from these results that the Mt Hunter data does reflect an influence of the Aleutian Low, but it's equally clear that we can't explain the most recent (post-1985) increase in snowfall by invoking continued strengthening of the Aleutian Low.  The published study alludes to this point and suggests that the explanation for the most recent changes may be a sustained increasing trend in ocean temperatures far away in the western tropical Pacific Ocean; these tropical changes may have caused shifts in the atmospheric flow near Alaska that are not reflected by the PNA/NP indices and/or the sea-level coastal precipitation measurements.  The authors describe this hypothesis as "a heightened sensitivity to tropical SST teleconnections at higher elevation in Alaska".

While I don't have any particular comments on the tropical-connection hypothesis, I thought it would be interesting to look "closer to home" for potential explanations for the most recent increase in Mt Hunter snowfall.  To begin, I calculated the portion of the Mt Hunter precipitation that cannot be explained by a linear regression with the PNA index - see below.  The most obvious aspect of the chart is the preponderance of high precipitation totals, and the absence of small totals, since 1988-89.

It's not immediately obvious what major climate regime shift could have caused a change like this in 1988; as we noted before, the PDO shifted in 1976, and the Atlantic Ocean shifted into a much warmer state (positive AMO phase) in 1995; but I'm not aware of any notable shifts in or around 1988.

To dig in a little deeper, I calculated the correlation between September-April sea-level pressure and the Mt Hunter precipitation residual.  The results show an interesting signal related to pressure over the Chukchi Sea - see below.  When low pressure prevails over the Chukchi Sea, then Mt Hunter snowfall tends to be higher than the PNA would suggest, whereas high pressure to the north and northwest of Alaska tends to occur with lower Mt Hunter snowfall amounts.

Looking at an area-average of sea-level pressure over a box from 65-80°N and 160-180°W, we see an apparent shift to lower values beginning in the late 1980s (see below), so this lines up rather well with the unexplained recent increase in Mt Hunter snowfall.  A scatter plot of the annual pressure and precipitation residual values (second chart below) reveals a 1976-2010 correlation of -0.48, which is nearly as good as the underlying PNA correlation that we started with.

Of course correlation does not prove causation, but there does seem to be some evidence here that lower pressure to the north and northwest of Alaska may have contributed to the continued rise in Mt Hunter snowfall in recent decades.  How would this work from a physical standpoint?  Lower pressure to the north would seem to favor episodes of strong westerly flow across all of Alaska, and while southerly flow is most favorable for heavy snow in the Alaska Range, it's conceivable that westerly wind events could also transport ample moisture into the high terrain.  Further investigation is clearly required to see if there is really a plausible physical connection to storminess in the Chukchi Sea.

In summary, the remarkable hydrological changes that have occurred recently on Mt Hunter and presumably elsewhere in the Alaska Range appear to have some connection to Arctic weather patterns as well as to the North Pacific circulation.  This raises the interesting possibility that the recent reduction in Arctic sea ice may also be implicated, perhaps by altering the winter flow patterns around Alaska in a systematic way.