Wednesday, June 13, 2018

Chilling in Summer

Northern and interior Alaska has seen some very chilly weather for the time of year in the past few days, as a strong upper-level trough and an unseasonably cold air mass plunged south out of the Arctic at the beginning of the week.  Despite the fact that the summer solstice is now less than 10 days away, and daylight is continuous, sub-freezing temperatures have occurred in many of the usual cold spots in the interior.

In the Fairbanks area, three consecutive days have seen temperatures falling into the 30s, with upper 20s at the colder spots like North Pole and the Goldstream valley.  The Smith Lake site on UAF's North Campus recorded 26°F yesterday morning; but the chart below (note the Celsius scale) shows that only a few days since late May have NOT dipped below freezing at this spot - even when daily high temperatures were well into the 70s.


The airport has seen 37°F, 36°F, and 37°F in the early mornings of the past three days, which is a remarkably cold series of daily minimum temperatures for this time of year.  In fact, this is the closest to the solstice that Fairbanks has ever observed 3 straight days with low temperatures of 37°F or lower at the official climate site (1930-present).

It's also interesting to note that with a high temperature of only 53°F, Monday's daily mean temperature was a mere 45°F.  It's been almost 70 years (1949) since Fairbanks saw such a chilly day this late in June (or in July).

The mid-level atmospheric pattern that created the midsummer chill is evident in the sequence of maps below.  The charts show the 500mb analysis at 24-hour intervals from 4am on Saturday through 4am today, and for ease of reference the red dot shows Fairbanks' location.  Notice the very tight pressure gradient and associated strong northerly flow that rushed down from the high Arctic into Alaska at the beginning of the week - this was a remarkable cold blast for the time of year.

Saturday:

Sunday:

Monday:

Tuesday:

Wednesday:

Finally, here's a nice view from Monday of the fresh snow that fell at Toolik Lake (2400' elevation) on the north side of the Brooks Range.  The lake is still mostly frozen despite the fact that the air temperature reached 60°F earlier this month.


Friday, June 8, 2018

Fire Season Begins

Lightning has been widespread over Alaska in the past several days, and wildfires have sprung up as an inevitable consequence.  According to the latest information on akfireinfo.com, fires have burned about 25,000 acres statewide so far this season, which is about normal for the time of year.  Fire activity typically ramps up quickly in June, with burn acreage often exceeding 200,000 acres by the end of the month.

Year-to-year variability of fire acreage in Alaska is a very interesting topic and a fascinating and challenging prediction problem.  I'd like to do an in-depth study of it one day, but today I'll just make a couple of points.  First, consider the map below, showing a 23-year correlation between sea surface temperatures in May and the subsequent fire acreage rank.  I've used the rank of the fire acreage (with higher rank for higher acreage) rather than actual acreage numbers because the distribution is strongly non-Gaussian.

The color scheme on the map rather exaggerates the statistical significance of the correlations, as the highest values are only 0.4-0.5, but nevertheless it's interesting to see that fire activity is favored by warmer ocean conditions in both the northern North Pacific and the central tropical Pacific.  The horseshoe-shaped pattern looks quite reminiscent of the PDO pattern, but actually it's a bit different; the typical PDO horseshoe hugs the coast of North America more closely and has a strong inverse correlation with SSTs extending east of Japan to south of Alaska.  Alaska fire acreage is actually nearly uncorrelated with the PDO index in May.



There is a better correlation (+0.44) between Alaska fire acreage and the North Pacific Mode (NPM) index.  The NPM pattern is focused between 40 and 50°N across the North Pacific, and according to the first map above, this is an area that shows some connection with Alaska fire activity.


So what do current conditions look like?  The map below shows the May analysis; the NPM was slightly positive, as it has been for the last 4 months, but it's not a dramatic anomaly (excepting the Bering Sea warmth).  This suggests that ocean temperature patterns are only slightly favorable for enhanced fire activity this year.  As an aside, there seems to be no sign of the strongly positive NPM phase that the long-range models were predicting earlier in the year (and are still predicting).



We can also search for fire-acreage-related precursor patterns in the atmosphere.  According to the map below, there is a statistically significant - but not highly robust - correlation between 500mb heights over Alaska in May and subsequent fire acreage.  This makes sense; if the weather pattern sets up with a ridge over Alaska during May, then dry and sunny conditions will reduce fuel moisture, and the next month or two are also more likely than not to be warm and dry.



How about May 2018?  Rather than having a ridge over the state, there was a trough over the southwest, and most of the interior was wetter than normal.  So this points to reduced fire activity, albeit with low confidence.



And now perhaps the most interesting result that I've stumbled upon in this brief analysis.  The map below shows the average SST anomaly in winters following the 6 most active fire seasons since 1995.  Most readers will recognize the pattern immediately: the warm band along the equator in the central and eastern Pacific is a classic El Niño pattern.  This suggests that very active fire seasons in Alaska have a strong tendency to be followed by significant El Niño episodes.

The chart below confirms the rather remarkable statistical connection; the 4 strongest El Niño's since 1995 were preceded by Alaska fire acreage in the top quartile (6 of 23) since 1995.  Naively this suggests we can use Alaska fire acreage as a predictor for El Niño - but why would this be?  My take is that the atmospheric and oceanic patterns that evolve into major El Niño events are already unfolding in the summer months prior to the classical winter peak of El Niño, and those patterns happen to be very favorable for Alaska wildfire.


The last point to make is that the latest data from the long-range computer models have recently shifted quite decisively in favor of El Niño for the coming winter (2018-19); so it will be most interesting indeed to see how the rest of the fire season evolves in Alaska.

Friday, June 1, 2018

North Slope Warmth Subsides

After a winter of record-breaking warmth in Alaska's northernmost city, temperatures have returned to near-normal levels recently, and even a bit below in the past week or so, as easterly flow has kept a chilly Arctic air mass in place.



The slight preference for cool conditions lately has prevented Utqiaġvik (Barrow) from seeing a sustained thaw so far this season; there has not yet been a day with mean temperature above freezing this year.  It has been a few years since the first such day occurred as late as June, but we would have to go another 10 days or so for the absence of warmth to become really unusual.  The chart below shows the long-term trend towards earlier first thaw day, and earlier first 50°F; both dates have advanced by more than a day per decade over the long-term history at Utqiaġvik.



The relative magnitude and persistence of the recent warming trend at different times of year is illustrated by the chart below.  For each month in the past 15 years, I've plotted up the monthly mean temperature as a departure from the 1971-2000 normal, and the red markers highlight the anomalies since 2013.  As we all know, warming has been most amplified in autumn as a direct result of sea ice loss.  The absolute magnitude of change has been smaller in summer, as Arctic summer temperatures are rather strongly constrained by the presence of at least some sea ice; but the summer warming is nevertheless very significant as the variance is much smaller in the warm season.



The year labels at the top of the columns indicate months in which the monthly-mean temperature records were broken in the past winter.  Remarkably, from November-March, 4 of 5 months saw monthly mean temperatures higher than any observed before.

Just to drive home the magnitude of what happened in the winter that recently ended, the chart below shows the November-March mean temperature for each winter since 1930-31; and the high-quality CRN data are fully consistent with the airport temperatures over the past 16 years.  A linear trend obviously doesn't capture what is happening here.


Friday, May 25, 2018

Sunshine at CRN Sites

After reviewing the status of Alaska's CRN data last week, curiosity led me to take a look at the solar radiation measurements from these sites.  For example, it's interesting to find out what these instruments have to say about how solar radiation varies across the state - at least for the last few years.

The chart below shows a summary of April-October total solar energy for the 11 sites that have been in operation since at least 2013 and have essentially complete data for these months.  The horizontal black lines show the 5-year mean for each site, and the blue bars indicate the minimum and maximum values within this brief 5-year period.


The sites near Fairbanks and Tok are the only interior sites represented here, and as we would expect they are at the top for solar energy.  Interestingly Metlakatla, on Annette Island in Alaska's far southeast, is in 3rd place and close behind Fairbanks.  It's not surprising to find that rainy Sitka is easily in last place; but note that the difference in solar energy between the cloudiest site and the sunniest site is less than 50%.

The range between minimum and maximum seasonal totals is remarkably high at the Red Dog site, but this is mostly because of a very low total from 2013; this looks suspicious and might be incorrect.  (If we exclude 2013, Red Dog actually comes in above Fairbanks for mean solar energy, and the Red Dog instrument also reported the highest seasonal total of any site in any year.)  More robust, perhaps, is the very low variance of solar radiation at the sites near Barrow (Utqiaġvik) and Tok; it's quite extraordinary, actually, how consistent the solar energy has been at these locations in the past 5 years.  But in the case of Barrow, data from earlier years showed higher values - see below.

The decrease of solar energy at the Barrow CRN site, evident in the chart below, is highly statistically significant even over such a short period.  Presumably this is related to the increase in open water area in the Arctic Ocean and therefore higher evaporation and increased cloudiness in recent years.


Friday, May 18, 2018

Improvement in CRN Data

I've mentioned the US Climate Reference Network program on previous occasions - it's a national effort to install high-quality climate monitoring instruments throughout the nation in locations that are unlikely to be affected significantly by urbanization in the coming decades.  So far 21 such sites have been installed in Alaska, with the most recent being near Toolik Lake and Cordova last summer.  Several more are planned:



Read more about the program here:

https://www.ncdc.noaa.gov/crn/

https://www.ncdc.noaa.gov/news/new-climate-observing-stations-alaska

In previous years it has been very disappointing to see that several of the sites had serious and persistent problems with missing temperature data in the winter months.   This is apparently caused by the fuel systems being unable to produce sufficient electrical power to operate the instruments and other electronics during cold weather.  It's a little surprising that the system engineering wasn't up to the challenge of Alaska's climate, but one must concede that it's no small task to run a complex array of instruments without external power throughout the deep cold and dark of the high-latitude winter.

But happily there is now some good news: some modifications were made in last summer's maintenance visits, and the past winter saw a significant reduction in the amount of missing data from a number of sites.  The chart below shows (in green) the statewide percentage of all November-March days for which daily high and low temperature data are available in GHCN, and the black columns show the number of sites.  The network was in a rather sorry state in winter 2015-2016, with more than 20% of days missing from the 18 sites around the state, but in the past winter only 7% of days were missing statewide.  If the improving trend continues, we'll soon be in good shape.


Here's the percent complete over the lifetime of each site.  Notice that the really bad locations are some of the coldest and most remote locations, whereas the instruments in southeast Alaska are performing just fine.

The sites that improved dramatically this winter were Deadhorse, Nowitna (Ruby 44 ESE), and the sites near Selawik and King Salmon: whatever tweaks were made at these locations worked nicely.  On the other hand, there are still significant problems at Ivotuk and Denali 27 N.  Let's hope that one more year does the trick to get the network running as intended.

Saturday, May 12, 2018

Chena Basin Snowpack

Continuing with the theme of snowpack, here's an update on the conclusion of the snow season in the Chena Basin above Fairbanks.  Back in March I noted that the amount of snow on the ground was at near-record levels based on data from 5 SNOTEL sites in the hills, and it turns out that the snowpack did indeed reach an unprecedented level by late in April.  Historical data goes back to 1981.

As of a week ago, 3 of the 5 sites were reporting a greater snowpack (in terms of water equivalent) than in any other year on the same date.  Of course the major warm-up in the past few days has melted most of the snow at the lower elevation sites - Teuchet Creek (1640') melted out yesterday - but Mt Ryan (2800') still has 8" of water equivalent on the ground.  Even more remarkably, Munson Ridge (3100') is reporting 15.2" still on the ground (a record for the date) and a 47" snow depth.  The beginning of hiking season will be somewhat delayed in the White Mountains this year.

Here's a chart of snow water equivalent at Munson Ridge for this year and some other notable years.  The greatest snowpack of record occurred in 1991, but no other year retained so much snow at this late date.  It will be interesting to see how long it lasts; in 1982 there was still snow on the ground on June 13.



And here's an update to the multi-station chart that I showed before; the late winter snowpack has been much greater than in any recent year.  (The rapid rise in percent of normal at the end of April reflects the rapidly diminishing median value, as the median snowpack drops to zero in early May for 3 of the 5 sites.)


Tuesday, May 8, 2018

Snowpack Season

The winter snow cover melted out on Friday in Fairbanks, according to measurements by the National Weather Service at the airport.  This is somewhat later than normal, which is not surprising in view of the healthy snowpack at the end of winter and the rather cool temperatures of late.

The chart below shows the 88-year history of dates when the continuous winter snowpack began and ended in Fairbanks.  The variance of dates is greater for snowpack onset, but that's nearly all because of two major outliers in 1934 (the great December chinook) and 1992 (early arrival of winter).


The dashed lines show long-term linear trend lines for the two series of dates, but instead of least squares regression I've used quantile regression, with each trend line showing the best estimate of how the median has changed over time.  The reason to go with quantile regression is to avoid an undue influence from outliers, which seem likely to be a problem here, especially for the snowpack onset dates.

It's interesting to see that the regression estimates indicate that snowpack onset has become earlier by nearly a week, and meltout has become slightly (about 3 days) later, over the 88 years.  But it's important to note the degree of uncertainty: the 90% confidence interval includes a zero trend for both series, so neither trend is statistically significant; we can't confidently rule out the possibility that the trends are just a reflection of random chance.

One thing we can say with confidence, however, is that snowpack meltout has not become significantly earlier over time, and this is intriguing.  We know that April - when most of the melting occurs - has become significantly warmer, so how is the snow not disappearing more quickly?  There might be a variety of explanations involving changes to sunshine or precipitation during melt season, but the simplest explanation may be the best: it seems there is just more snow on the ground these days.  The chart below shows the snow depth in Fairbanks on April 1st each year, and there's an upward trend that is significant at p~0.05.


More investigation will be required to determine whether and how this trend is related to precipitation changes during winter; my impression from previous work is that Fairbanks winter precipitation has NOT increased over the long haul, so it seems this is a rather interesting question.