West Coast Storm

In the past couple of days a low pressure system has tracked up the middle of the Bering Sea, bringing blizzard conditions to the west coast of Alaska.  Kotzebue in the northwest had quite an extended episode, with sustained winds of around 30 knots and low visibility for a full 24 hours.  Here's the surface pressure analysis from 3pm AKST yesterday, showing the low near the Seward Peninsula; the storm was then winding down in Kotzebue.

How does this event compare for lengthy and severity to blizzards of the past?  It's not particularly unusual; a quick search of the hourly data shows at least 7 other events with 24+ hours of sustained 30+ knot winds and sub-1 mile visibility.  In terms of extremes, consider the storm of late February 1951: sustained winds reached hurricane force with a temperature near 0°F, and 30+ knot winds lasted for 46 hours.

Here's a chart of the frequency of windy, low-visibility conditions since 1981.  February is the peak month, but it's not too uncommon even in April.

For a look at the Kotzebue weather scene, check out the following Twitter profile:

https://twitter.com/tammaq13

Update March 31:

Here's a page from Kotzebue's monthly climatological summary in February 1951.  Note the remarkable period of persistently high wind speeds between the 20th and the 25th.

 

 

 

On a side note, the report also includes ground temperature data down to 22 feet.  It would be interesting to "dig up" all this early subsurface temperature data and compare it to modern observations.

Permafrost Study

A couple of weeks ago I mentioned a paper that was published in January, looking at changes in air and ground temperature in recent years at monitoring sites within Alaska's national parks.  Here's the link:

https://www.tandfonline.com/doi/full/10.1080/15230430.2020.1859435

The focus of the study is on the dramatic warm-up that occurred in 2013, with the monitoring data showing ground temperature increases of up to 4°C at 50cm depth.  The climate warming was concentrated in winter and was much more pronounced in western than eastern Alaska.  Here's a chart of annual mean air temperature at Nome to illustrate the change.

Among the interesting results in the study is that the measurements show much more subsurface warming in the Arctic parks than the interior parks, because the monitoring sites up north are in tundra locations with thin, wind-scoured snowpack; in this setting the ground temperature can change as much as the air temperature.  In contrast, the sites in the Denali, Wrangell-St. Elias, and Yukon-Charley areas have deeper snowpack in mostly alpine and taiga environments, and the insulating effect of the snow reduces the amplitude of the warming below ground.

The implication of the warming for permafrost is, of course, very significant, and the paper demonstrates this quantitatively by comparing the ground temperature rise to the fraction of park area with estimated (modeled) permafrost temperatures above certain thresholds.  For instance, 40% of the Bering Land Bridge National Preserve was estimated to have permafrost temperature above -3°C in 2000-2009, and so the warming of 3°C or more is likely to have brought much of this area into a thaw.  And although ground warming has been much less in the more southerly parks, these have significant areas with marginal permafrost that is already close to a thaw.

The park with the most robust permafrost situation - at least based on modeled permafrost temperatures - is the Noatak National Preserve, with only 6% of land area above -3°C prior to the warm-up.

I applaud the authors (including Pam Sousanes and Ken Hill of Howard Pass fame) for their work.  The value of this NPS monitoring network and careful analysis of the data can hardly be overstated in light of the rapid change of recent years.

Big Diurnal Range

This is the time of year for very large temperature swings between day and night across interior Alaska.  Skies are often clear and the air tends to be very dry, allowing for rapid cooling at night but strong solar insolation during the day.  Deep snow pack of course facilitates overnight cooling, with very little of the daytime solar input being stored at the ground surface.

The automated observing site on the Salcha River often produces some of the most spectacular diurnal temperature ranges, and the last 10 days have been quite extreme in this regard.  All but two of the last 10 days have seen a day-night temperature swing of more than 50°F, and March 14 saw an amazing 65°F range.

The hourly temperatures on March 14 ranged from -36°F at 7am to +29°F at 3pm, and then back to -31°F by midnight.

 

The largest diurnal range ever measured by the Salcha RAWS was 69°F, on both March 30, 2006, and March 16, 2002.  The site averages close to a 40°F diurnal range in mid-March.  For comparison, the normal range at Fairbanks airport is not quite 30°F in mid-March, although Fairbanks had a day-night swing of 48°F just yesterday.

Out of curiosity I looked at long-term normals from global gridded temperature data (derived from actual station observations), and according to this data the largest "normal" diurnal range found anywhere in the world is in central Oregon in late August: with an average low of 35°F and an average high of 79°F, the normal daily range is 44°F.  I wouldn't be surprised if this is exceeded in some desert areas with sparse observing networks, but nevertheless it shows that Salcha's typical ~40°F range is near the upper end of what's observed anywhere on the globe.

Chilly Late Winter

After a relatively warm start to winter (see this post from early January), the last six weeks have been on the colder side of normal in much of Alaska.  February was the coldest since 1999 in Fairbanks and Anchorage, and up north it was colder still; for example, it was the coldest February since 1990 in Bettles.  Here's Rick Thoman's excellent summary graphic:

March has also been somewhat colder than normal, and a cold spell late last week brought considerable discomfort to the Iditarod teams.  Fairbanks reached -35°F for two nights in a row, and a number of spots dropped below -40°F, including -44°F at the Salcha RAWS.  Here's a map from Friday morning, with temperatures in red:

When all is said and done, the extended winter period will end up near normal for temperature in many locations.  Here's the daily chart for Fairbanks: near normal to begin the winter, persistently warm in December and most of January, and generally colder than normal since then.

As I noted in the January post, the surprise in all this is that the robust La Niña episode didn't prevent a long period of anomalous warmth in the heart of winter.  This is related to the fact that the Arctic Oscillation was strongly negative from mid-December to mid-February; the negative AO phase produces cold over Eurasia and the lower 48 states of the US, but northern North America tends to be warm (see below).  However, the AO is typically positive, not negative, during La Niña winters; so a lot of ENSO-based long-range forecasts went awry this winter.

For completeness, here's Rick's temperature graphic for January.

Permafrost Update

Back in 2018 and 2019 I looked at permafrost conditions at a couple of central Alaska monitoring sites maintained by UAF's Permafrost Laboratory, and it's high time for an update now that two more years of data are available (thanks to Colby Wright at the Lab).  Here are the previous posts: here and here.

First, here's an updated time series at 52cm depth at the Smith Lake 1 site near UAF; click to enlarge.

We noted in 2019 that this data reveals a remarkable transition: at this depth below ground, the site went from permanently frozen to barely frozen even during winter; the minimum temperatures in 2018 and 2019 were -0.24°C and -0.27°C respectively.  However, colder conditions last winter (2019-20) did briefly take the temperature back below -1°C.  The data ends in May 2020, so we don't know how things are looking this winter.

To illustrate the sustained warmth that produced the change, here's a chart showing monthly air temperature anomalies since 2012 in Fairbanks.

Immediately following the coldest April on record in 2013, there was a dramatic warm-up in May (read about it on the blog archives, e.g. here), and temperatures stayed more-or-less above normal for the next six and a half years (with the exception of March 2017, which also shows up in the ground temperature trace).  But 2020 then started out with a very cold January and was a cooler year overall.

An updated chart of annual minimum temperatures in the soil column highlights just how close the Smith Lake 1 site has come to having a permanently thawed layer (I believe this is called a talik) below the seasonally frozen layer.  However, the 2020 data will show a slight recovery, as noted above.

Annual mean temperatures rose above freezing for the first time in 2018 and 2019 at depths from about 30-75cm, and remarkably the average temperature rose almost to 0°C down at 3m depth.

 

As for annual maximum temperatures, these show that the active layer depth increased to over 1m, but with thaw very close to occurring at 3m depth, it seems this permafrost is nearly gone.

Similar trends are seen at the Bonanza Creek 1 site, about half way between Fairbanks and Nenana.  In this case it's interesting and a bit odd to see the two-year cycle of increasing temperatures: there were significant jumps in 2014-15, 2016-17, and again in 2018-19.  Like the Smith Lake site, the ground barely froze below 0.5m in 2018 and 2019; but unlike Smith Lake there's no data below 1.5m to see what's happening lower down.

A time series chart for 67cm depth shows the stair-step warming, and at this site there was no recovery in early 2020; the last 3 winters in the series all failed to drop below -0.15°C at this depth, after being frozen nearly all the time in earlier years.

 

For further reading on recent permafrost trends in Alaska, check out this paper, published in January; I'll comment on it in a future post.

https://www.tandfonline.com/doi/full/10.1080/15230430.2020.1859435

Trends in Temperature Extremes

A few days ago I did a bit of historical analysis for my day job, looking at trends in temperature extremes across Northern Hemisphere mid-latitude land areas over the last 40 years.  This was stimulated by the dramatic recent freeze in Texas - clearly a very extreme event - as well as other notable swings in temperature around the Northern Hemisphere this winter.

The specific question I was seeking to answer is whether there is any evidence of increasing frequency in extremes of temperature, both warm and cold.  Obviously we expect the data to show a rise in warm extremes relative to a fixed baseline, and intuitively we would expect a decrease in cold extremes too, although there's a considerable amount of discussion around the idea that winter cold extremes could increase in certain regions.  (This is related to the "warm Arctic, cold continents" pattern that Judah Cohen, in particular, describes as a consequence of rapid Arctic warming, i.e. "Arctic amplification".)  So I looked at daily gridded ERA5 data since 1981 to see what I could find.

I won't rehash the details of the mid-latitude analysis - check out the link here if you're interested - but I thought it would be worthwhile to redo the calculations for an Alaska-centric domain.

First, here's an example of what I'm using for a temperature baseline; the figure below (click to enlarge) shows the annual cycle of ERA5 temperature near Fairbanks along with the seasonally-varying range of +/- 2 standard deviations.  The charts I'll show below illustrate historical trends in the frequency of temperatures outside this range; in a Gaussian distribution this would occur slightly less than 5% of the time (2.3% on either side), but of course the temperature distribution isn't quite Gaussian.

To track the historical frequency of these extremes, I measured the percentage of land area in which +/- 2 standard deviations was exceeded each day from 1981-2020, and then I took the annual average of that percentage for winter (November-March) and summer (June-August).  I also repeated the calculations after removing the linear trend in temperature, while allowing for the fact that the trend varies through the year.

If we start with winter extremes in reference to a fixed baseline, we see the expected rise in warm extremes and decline in cold extremes - see below.  This is for a land area box that encompasses most of Alaska except the Southeast.  2019 stands out in particular for having a lot of winter warmth, and winter cold extremes have been very scarce since 2013.  Note that these are calendar year averages, not grouped by winter, so for example 2019 includes Jan-Mar 2019 and Nov-Dec 2019.

Readers will notice that the values are considerably higher for cold extremes than for warm, and that's just because the winter temperature distribution is skewed towards the cold side; so it's more common to reach -2 SD than +2 SD.  See here for an old post showing maps of temperature skewness by season across Alaska.

Similar trends are observed for summer.  The summer of 2004 really stands out for exceptional heat, and 2013 and 2019 were also very warm.  Cold extremes have again been few and far between in recent years.

Now let's examine the data after removing the 1981-2020 linear trend; the overall trends for summer and winter are illustrated here, for the same land area box:

The winter warmth in recent years has considerably exceeded the trend (indeed the trend was actually down for the first 30 years of this period), so it's no surprise to see that cold extremes have dramatically dropped off in recent winters even after detrending:

What's more surprising is that recent years have not seen excessive warm extremes after detrending - see below.  If the temperature variability were unchanged, then we would expect to see an abundance of warm extremes from 2014-2019, because winter temperatures were generally above trend in those years.

My tentative conclusion from this is that the recent warm winters did not produce warm extremes of the magnitude that would have been expected if the temperature variance were unchanged.  Yes, it was extremely warm in the mean; but the upper end of the temperature distribution did not rise as much as we might have expected.  But perhaps this is reasonable: all else being equal, a cold climate has more variable temperatures than a warm climate, so variance probably should diminish as the baseline warms.

Finally, for completeness, below are the detrended extremes for summer.  In this case it's interesting to see the lack of cold extremes in the last 5 years, even though summer average temperatures have more or less tracked with the trend.  Again, decreasing variance may be at work, even during the relatively less volatile summer season; and this may well be related to the high humidity that seems to have been a feature of recent summers.

Late Winter Chill

It's getting rather late in the winter for harsh cold in much of Alaska (excepting perhaps the North Slope where it's more typical), but that didn't stop the thermometer from dropping to -42°F at Fairbanks airport on Monday morning.  This is the coldest of the winter so far, and the first -40° of the season in the Golden Heart City.  It's also the first time since 2007 that such cold has occurred so late in the winter.

In keeping with my last post, temperatures aloft were significantly below normal on Monday morning above Fairbanks: about -20°C at 850mb and -38°C at 500mb.  Cold air was circulating around a very intense mid-atmosphere low pressure system just to the west of Canada's Arctic Archipelago - see below.  This is a remarkable turnaround from the record high pressure that occurred in the same area just two weeks ago.

 

Here's the Fairbanks sounding from 3am AKST on Monday.  Even though winds were out of the west (and even slightly south of west in the lower troposphere), the airmass had an Arctic rather than southerly origin.

Checking in on that notorious cold spot, Chicken, they also saw -40° in the last few days.  No surprise there, but what is surprising is that Chicken has produced more -40° readings this month than in any other February: 14 of them so far (although with an 8am obs time, it's not actually 14 different nights).  Warmer air has arrived now, and the next few days will remain warmer, but there's still a chance it may be the coldest February on record for Chicken (but with data only back to 1997).  However, for the winter as a whole, the temperature is running near normal, with the -40° count approaching a typical two dozen.

Is Alaska Cold Home-Made?

My post from a couple of weeks ago about cold on the North Slope elicited a comment from a reader about the origin of the very low temperatures.  Specifically, the comment asked whether it was accurate to suggest that the cold was imported from farther north in the Arctic; my post stated that high pressure was acting to "funnel very cold air into northern Alaska".  Surely, the reader asked, Alaska's cold develops in situ when there's no competing warm influx to prevent it.  After all, there's not much severe cold over the Arctic Ocean these days, so how can the North Slope cold have its origin farther north?

The answer to this question is that both ideas have validity.  Valley-level surface cold across interior and northern Alaska certainly is generated locally when conditions are right (clear skies, calm winds), but it's also true that surface conditions are powerfully influenced by the thermal quality of the large-scale air mass.  When it's warm aloft, temperatures at the surface are usually warm and are never severely cold; but when it's cold aloft, then the surface is usually very cold and is rarely warmer than normal.

The overall correlation can be illustrated with a scatter plot of December-February 850mb temperatures and daily minimum temperatures at Fairbanks:

Slightly more than half of the variance in daily minimum temperatures is explained by temperatures aloft (850mb is around 1200-1400m above sea level).  Taking -40° as a benchmark for severe winter cold, this only happens when 850mb temperatures are below normal, and more often 850mb temperatures are significantly below normal.

The correlation with temperatures higher up in the atmosphere is smaller, of course, but it's still significant.

Similarly, the recent cold spell at Umiat (as low as -56°F 10 days ago) was associated with colder than normal conditions aloft.  The chart below shows the rather close correspondence of surface and 850mb temperatures in the past several weeks - but note that I've used upper-air data from the nearest sounding site at Utqiaġvik, about 170 miles to the northwest.

Here are the scatter plots of Umiat daily minimum temperatures and Utqiaġvik upper-air temperatures:

I also looked at the correlations in terms of daily departure from normal, rather than absolute temperatures - see below.  As it turns out, it makes very little difference in this analysis.

To summarize, historical data show that about half of the variance of winter daily temperature is explained by the temperature of the air aloft, which in turn is largely dictated by the source region of the air mass.  So while it's fair to say that Alaska's cold can be greatly amplified in situ - for example under a strong inversion with clear skies and calm winds - the coldest episodes are never just home-made; you need both cold air aloft AND favorable surface conditions to take the thermometer down to its lowest levels.

Arctic High Pressure

Harsh cold is persisting at many interior locations and also across the Brooks Range and North Slope, with numerous -40°F to -50°F readings this morning, and some even colder.  Notable low temperatures in the past couple of days include:

-49°F at Galena in the western interior

-56°F at the Birch Creek RAWS just above the Yukon Flats

-46°F at Anaktuvuk Pass, 2100' elevation in the Brooks Range

-58°F on the upper Sagavanirktok River

The latter is apparently the coldest observed in Alaska so far this winter; Chicken reached -57°F a couple of days ago.

Severe wind chill continues to afflict Arctic coastal sites, and the breeze even picked up at Umiat today: around 6am, a mean hourly wind speed of 11 mph combined with a temperature of -52°F.  This turns out to be the highest wind speed on record at Umiat with a temperature below -50°F (with hourly data back to 2007).

Looking at the surface analysis from 3pm yesterday, we see the same Arctic high pressure system that I highlighted in my last two posts - except now the high has expanded over an enormous area from the Chukchi Sea to northwestern Canada and across to Greenland.  It's really a sight to behold: click to enlarge.

The all-time record highest MSLP over the Arctic Ocean is 1069mb (per ERA5 data since 1950), so this is not far off.  It's a rare anomaly, and it's responsible not just for the cold in Alaska, but for outbreaks of severe cold far to the south in the lower 48 and also in Europe.

Frigid in the North

With high pressure over the Arctic Ocean continuing to funnel very cold air into northern Alaska, surface conditions have become downright frigid across the interior and eastern North Slope, with widespread -40s and some -50s today.  Umiat reached -55°F this morning according to the RAWS instrument, and saw a high of only -47°F this afternoon.

But at least there's no significant wind in Umiat.  From Deadhorse all the way across to the Canadian coast, wind chill values are -70°F or lower, with stiff offshore breezes creating seriously nasty conditions.

The lowest temperatures occurred in the sheltered valleys of the interior North Slope to the south and southwest of Umiat; satellite measurements suggest some spots may have dropped below -65°F.  Here's a satellite-observed temperature map courtesy of Twitter user wrighthydromet (click to enlarge).

 

 

Our favorite wind chill site, Howard Pass, also had a very extreme episode of wind chill yesterday, with sustained winds over 50mph in conjunction with temperatures below -40°F.  This is the first time in 7 years that the Howard Pass thermometer has dropped below -40°F, although missing data is an issue.

Back in December I looked at the strong inverse relationship between wind and temperature at Howard Pass.  Remarkably, -40°F or lower has only ever occurred with a wind speed of 37mph or higher, and the median wind is 48mph at such low temperatures.  The chart below zooms in on the low-temperature portion of the hourly distribution of temperature and wind.

It's interesting to observe that while yesterday's cold blast was a typical wind chill episode at Howard Pass, the temperature didn't recover much last night as the winds quickly died down to almost nothing.  Consequently, today's temperatures were easily the coldest on record for low wind speeds; this morning it was a full 10°F colder than previously observed at Howard Pass with a wind speed below 5mph.  This illustrates the intensity of the cold air mass over northern Alaska.

Here's a surface analysis chart for 3am yesterday; notice the very strong pressure gradient over northwestern Alaska, an obvious prerequisite for these extreme wind chill episodes.

Cold

Daily gains in daylight and solar strength are heralding the transition to late winter across Alaska, but temperatures plunged to their lowest levels of the season so far across parts of the interior and north today.  Umiat and Arctic Village reached -50°F on opposite sites of the Brooks Range, and a remarkable -55°F was reported by a co-operative observer far to the south, near Tanacross in the upper Tanana River valley.  Here's a look at minimum temperatures through 4pm (click to enlarge):

Wind chill has been nasty too; Fairbanks was reporting a steady breeze even as the temperature dropped into the -20s last night (bottoming out at -28°F).

Here's part of the cause of the cold outbreak: an intense high pressure system over the Arctic Ocean to the north of Alaska.

The estimated central MSLP of 1059 mb at 3pm yesterday is towards the upper end of the climatological history; for example, ERA5 data says that MSLP has reached 1060 mb at 80°N 180°W only once in the last 50 years (January 2013).  The anomaly is related to an unusual weather pattern across the mid-high latitudes of the Northern Hemisphere: the Arctic Oscillation is strongly negative, which means the usual circumpolar westerly flow is weak and disrupted in the upper atmosphere, allowing cold air to spill south from the Arctic to certain parts of the lower latitudes.

Here's a simple animation of a webcam view at Arctic Village today, showing thin layers of ice fog moving around, as is typical on very cold days.

Cooling in March

A couple of weeks ago I compared the new 1991-2020 temperature normals to the previous climate period of 1981-2010, and one of the interesting features that stood out was the widespread change to slightly cooler conditions in March.  With the exception of Utqiaġvik, which has of course seen rampant warming, all of the climate sites I looked at showed a cooling trend in March; and March is the only month with a statewide decrease in the 30-year normal temperature.

Here's a map of the March temperature difference according to ERA5:

The cooling is quite widespread in the southern half of mainland Alaska, but the magnitude of the temperature difference is small: rarely more than 1°F.  Contrast this with the much larger warming that occurred in the neighboring months of February and April, particularly across western Alaska.

 

If we look at 3 climate observing sites with noticeable March cooling, we see one key reason for the anomaly: the 1980s were exceptionally warm in March, and that decade has dropped out of the new climate normal period. 

The 1990s were very warm too - notice the complete absence of cold March's in both the 80s and 90s - but then some cold re-emerged after the turn of the century, and Bethel in particular had a spate of cold March's.  Interestingly just a few years ago Bethel's 30-year running average for March was distinctly lower than it is today, so if for example the climate normal period had changed 5 years ago, the new normal would have been more than 2°F colder than the one 10 years before.

Clearly, annual and decadal variability play a large role in the details of the changes from one climate normal period to the next, and this becomes most evident where annual and decadal variance is large compared to the magnitude of the long-term warming trend.

Some readers will have recognized already that the PDO phase is a prime candidate to explain the decadal variability we're considering here - which is to be expected, given that it's the Pacific Decadal Oscillation.  Here's a chart of the March PDO index; notice the remarkably persistent positive phase of the 1980s and the return of some significantly negative values around 2008-2013.

But an obvious question then is - if the PDO is responsible for the March cooling, then why did February and April warm so much?  The PDO tends not to change drastically from month to month, and indeed it was strongly positive in the 1980s in February as well.  However, Bethel's February temperatures (for example) have warmed dramatically in the past two decades compared to the 1980s.

The explanation for this month-to-month contrast is (I believe) that we're dealing with natural variability that has pushed the monthly decadal trends in different directions.  In other words, it just so happens that February and April have recently been warmer than we would have expected based on the long-term trend and the PDO phase; and on the flip side, March has been colder than we would have expected.  If we somehow had another "realization" of recent decades, it could equally well have been the other way around, with relatively subdued warming in February and April and much larger warming in March.

To support this idea that "luck of the draw" with the weather patterns explains the different trends, the maps below show the 2011-2020 average departure from normal for MSLP and 500mb height in March.  This is a relatively cold pattern for southern Alaska, with a ridge axis over the Bering Sea that produces a northerly component to the flow to the east of the ridge.

In contrast, February's of the past decade have tended to see a ridge over the Gulf of Alaska and low pressure over the Arctic Ocean, and this combination brings a lot of warm air up from the southwest.

The decadal pattern for April (see below) is quite different, but low pressure near the Aleutians is another typical signal for warmth in southern Alaska (this is an El Niño-like pattern).  Clearly then the monthly weather patterns of the past decade are aligned with the observed differences in trends: cool in March, very warm in February and April.

 

The pattern analysis works at the other end too, in the 1980s: it just so happens that March's from 1981-1990 had the warmest pattern of the 3 months - see below.  Consequently, March in the 1980s tended to be warmer than expected based on the background temperature baseline and the (very positive) PDO phase.  And so with unusual warmth in 1980s and a cool pattern in the past decade, it's no surprise that the 30-year normal has turned a bit cooler for this month.