Tuesday, March 30, 2021

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:


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.

Friday, March 26, 2021

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:


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.

Tuesday, March 23, 2021

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.

Wednesday, March 17, 2021

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.

Friday, March 12, 2021

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.


Friday, March 5, 2021

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.