Tuesday, December 6, 2016

50 Below Reached

The Chalkyitsik RAWS reported -51°F this morning, which is the lowest observed temperature of the winter so far in Alaska (except for a dubious reading from the Bettles SNOTEL, which seems to have a cold bias).  The Chalkyitsik RAWS is located about half way between Fort Yukon and the village of Chalkyitsik, and is more than 100 feet lower in elevation than the latter.  Climate observations from Chalkyitsik itself were recorded only from 1962 to 1972, and the data are quite incomplete.

Temperatures in Fort Yukon were nearly steady at around -45°F today.  Below is a simple animation of the south-pointing FAA webcam view during the 3 hours of sunshine.  It's interesting to see the apparent fluctuations in the concentration of condensate or particulates in the shallow surface layer.  I'm not quite sure if the haze is composed of smoke or ice crystals, although the airport reported ice crystals in the air for most of the day, and the reported visibility varied from 0.5 to 2.5 miles.  With a strong temperature inversion undoubtedly present in the lowest levels, the haze was trapped very close to the surface - even below the tops of the trees at times, judging from the visual appearance.  A lot of interesting physics goes on in the surface layer of a winter day north of the Arctic Circle.

Monday, December 5, 2016

Cold Spell Persists

It's been quite a while since interior Alaska has seen persistently cold conditions during winter, so the current lengthening cold spell is a notable change of scene.  Including today, Fairbanks has been colder than normal for 17 of the last 18 days.  Here's an update to my chart showing the daily statewide minimum temperature; this is based on 24-hour minimum temperature reports ending at various times of day, so the daily measurement windows are not the same everywhere, but the trend is clear: the colder spots in Alaska have produced -40s for more than a week now.

The coldest area last week was the north-central interior, with the Norutak Lake RAWS reaching -40° or colder for 8 consecutive days ending yesterday.  The coldest conditions have now shifted to the northeast, with the Chalkyitsik RAWS sitting at -48°F this evening and apparently on track to break -50°F tonight.  Last winter no station in Alaska quite reached -50°F, although the Kanuti Lake RAWS got very close.

Environment Canada's surface chart from yesterday afternoon showed strong high pressure over northeastern Alaska, and the Fairbanks sounding measured a cold column with 850mb temperatures well below -20°C.  However, strong warming has been occurring aloft in the past 24 hours as the high pressure produces subsidence; note the difference between the two soundings below (3pm yesterday and 3pm today).

The 1.5-meter temperature measurements from UAF's Poker Flat Research Range show the warm-up nicely; this morning the temperature jumped from -28°F to -7°F in only half an hour.  The Poker Flat site is located near Chatanika on the Steese Highway north of Fairbanks at an elevation of about 700 feet above sea level.

Thursday, December 1, 2016

Lack of Warming in November

A couple of months ago I made a passing reference to the rather noticeable absence of a long-term warming trend in November upper-air temperatures as measured by balloon soundings from Fairbanks.  Based on sounding data from 1950 to present, November mean 850mb temperatures actually show a slight cooling trend, in contrast to the adjacent months of October and December - see the charts below.  Note that Octobers have been unusually warm since 2002, and Decembers have been warmer since the early 1980s, but November mean temperatures show neither of these features.

I've drawn linear trend lines on these charts, but this is just to gain a simple impression of the long-term changes; obviously the changes have not occurred in a linear fashion.

Looking at surface temperatures from Fairbanks over a longer period (1930-2015), we see a similar pattern: temperatures have been warmer in the past 15 years in October, and December has become much warmer over the long haul, but November has seen very little change.

At the risk of boring readers with too many charts, I think it's worth looking also at the Alaska statewide average temperatures from NOAA's climate division data.  It's quite a similar picture here again, although as a whole the state has seen some slight warming in November; this warming has all been in the western half of the state.  I'll make a map of the regional trends when I have more time.

The obvious question, of course, is why has November not warmed in Fairbanks and the eastern interior - not even in the past decade or two.  We might be tempted to pin part of the blame on the PDO phase becoming slightly more negative over the long term (see below), but this hasn't alleviated the strong warming trend in December (and the PDO trends are very similar for the two months).

My working hypothesis for further investigation is that the atmospheric circulation patterns have shifted over the decades so that relatively cooler conditions are now favored in November for interior Alaska; for example, perhaps low pressure is now less common over the Bering Sea in early winter.  This kind of shift could allow a localized region to defy broader hemispheric trends towards warmer conditions; but even if we can show this has occurred, the deeper question of "why" may be difficult to answer without delving into global climate model simulations.  But hopefully I'll be able to report back here with some more answers before too long.

Tuesday, November 29, 2016

Low-Elevation Sunshine

Colder than normal conditions have settled over the interior, and so far the RAWS thermometers at Norutak Lake and at Prospect Creek/Jim River have measured the coldest conditions: -45°F this morning.  A report of -49°F from the Bettles SNOTEL site seems suspect as that site always runs cold.

Norutak Lake is located just north of the Arctic Circle and just to the east of the upper Kobuk River where it drains south out of the Brooks Range.

It is interesting to note that yesterday's hourly temperature observations from Norutak Lake showed a rise of 8°F in association with some very weak solar radiation during the day (see below).

The sun only rose to an elevation of 2° above the horizon yesterday at Norutak Lake, so I wouldn't have expected to see such a large diurnal temperature change.  However, a closer look at the topographic map reveals that the RAWS site is located on a south-facing slope at the north side of the lake.  The RAWS elevation is given as 800', but Norutak Lake itself is at 712', so the thermometer is almost 100' above lake level.  It seems that the south-facing exposure allowed for a bit more solar heating than would occur at lake level, and the daytime warming may even reflect the warm bias that RAWS thermometers are known to encounter on calm, sunny days.

With a surface-based temperature inversion undoubtedly in place over Norutak Lake, it's likely that temperatures near the lake surface were at least a couple of degrees lower than at the RAWS site, and perhaps substantially lower.  At Fairbanks airport it is normal to see an inversion of 4-5°F per 100 feet at the surface during winter, and 10°F per 100 feet is not uncommon.

Saturday, November 26, 2016

Deepening Cold

Temperatures below -30°F have been observed in a number of interior locations in the past week, and it seems it won't be long before -40° cold arrives in some areas.  [Update Sunday: the Norutak Lake RAWS recorded -40° this morning.]  The latest GFS MOS forecast shows -45°F for Arctic Village on Tuesday and -26°F in Fairbanks.  Here's a chart of the lowest temperatures observed each day across Alaska in the past couple of months.  The Chalkyitsik RAWS site was the location of the coldest temperature report on 17 of the 57 days shown here, although some of these were ties.  The next most frequent cold spot (7 days) has been Chicken.

In a change of scene for Fairbanks, below-normal daily mean temperatures occurred for 7 consecutive days ending Thursday, the first such period since mid-July.  It's gratifying to see that my earlier speculation of a turn to colder conditions around November 19-21 worked out well - as we all know, even a blind squirrel finds a nut occasionally.

Looking again at the MJO phase, it's interesting to note that colder conditions tend to occur about 10 days after phase 7 even when the PDO phase is positive (as it is now) - see the graphic below.  (But note that the sample size is quite small for this analysis.)  Unusual warmth is much more likely to develop soon after an active MJO wave progresses through phases 1-3, which is where the MJO has been in the past week (second image below).  We might therefore expect warmth to return to Fairbanks rather soon, although the MJO anomaly is now fading and predictability may be lessened as a result.  In any case, the tropical MJO wave is only one of many influences on sub-seasonal climate variability in Alaska, so we wouldn't want to rely too heavily on it.

Wednesday, November 23, 2016

Low Snowfall Winters

As a long time reader and life time weather enthusiast I've been invited to share some of my observations about weather in our great state of Alaska.  I'm sure not going to surprise anyone when I say that this winter has been shaping up as a dry one. In fact the Fairbanks airport is reporting a snow pack depth of only 2 inches from a total of 4.1 fallen inches .  Normally we'd see about 2 feet of measured snow by now. Indeed Fairbanks has only received 0.26" of liquid equivalent since October 1st.

I suspected that low snow years may correlate with persistent southerly steering flow. After talking with Richard James he suggested I look at 700 mb level wind direction. I reviewed wind data from 2000-2016 during the winter period of each year (Oct 1st-April 1st). I found that when winds are coming from almost due south (180 degrees) Fairbanks had low snow years.

No surprise the Alaska Range is real good at blocking out moisture from the Pacific. So for you snow lovers out there; southerly winds are bad.  I should note that there is a small correlation to snowy years when winds are more from the south west and off the Bering sea. I'll expand on this more later. Thanks for reading my first post.  -Mike

High Arctic Warmth and ENSO

The recent very unusual warmth in the Arctic has drawn considerable attention from various quarters, and justifiably so; the conditions have been quite extreme.  Here's an estimate of the temperature anomaly for the week ending November 20, based on NOAA's CFS reanalysis data.  Temperatures have been 15-20°C above even the relatively recent 2001-2010 normal over parts of the Arctic Ocean; but parts of Siberia have endured a correspondingly extreme cold anomaly.

Pressure and temperature analyses over the same period (see below) show that the recent Arctic warmth was connected to a circulation pattern involving low pressure in the Norwegian and Greenland Seas and high pressure over central Russia.  The flow associated with these features transported warm air into the Arctic from far to the south over western Europe and the North Atlantic Ocean.  Therefore an absence of Arctic sea ice is not the only reason for the warm conditions, but of course the mutual reinforcement of open water and warm air helped to amplify the anomaly.

One question that occurred to me is whether the Arctic warm anomaly may be a lingering result of last winter's strong El Niño episode.  Could a warm early winter in the Arctic be an amplified reflection of the jump in global temperatures that always accompanies El Niño?  The chart below shows that the answer is no, and in fact the reverse is true: on average, El Niño tends to be followed in the early part of the next winter by colder November-December conditions in the high Arctic (north of 80°N), whereas La Niña quite reliably brings unusual warmth in the subsequent winter.

The inverse correlation that's evident here is partly caused by twin long-term trends towards less frequent El Niño conditions and warmer Arctic conditions in recent years, as seen in the chart below.  In other words, recent years have been warmer and have also seen a preference for La Niña conditions, and this by itself explains part of the inverse correlation.

Nevertheless, a fairly robust correlation remains after removing the long-term trends - see below.  It's interesting, however, that the two strongest El Niño's (1982-83 and 1997-98) were followed by near-normal Arctic temperatures relative to the long-term trend.

Looking at a map of the correlation coefficient, we see that the region from the Barents and Kara Seas to the North Pole shows the largest inverse correlation between early winter temperatures and the ENSO conditions of the previous winter.  This region is similar to the area that has seen the most anomalous warmth recently, but this year's conditions are opposite to what the historical correlation shows!  If the historical connection were being reflected this year, then the high Arctic would be cold, not warm, at the moment.  This is puzzling, but it seems to show rather clearly that we can't blame El Niño for the current warmth.

Looking at the MSLP correlation to the previous winter's ENSO index, the map below shows that El Niño tends to be followed by high pressure over western Europe and low pressure over Siberia in November-December of the next year.  Again this is roughly the opposite of what we have seen this year.

Sunday, November 20, 2016

Blog Changes

Long-time readers of Deep Cold will know that the blog's authorship has changed over time; I didn't start the blog, but I've been posting semi-regularly since mid-2013, and I've enjoyed just about every minute of it.  There's nothing more fascinating to me than Alaska's weather and climate, and I have no intention of stopping now.  However, I am probably going to post less often in the next couple of months or so, as I try to devote some spare time to writing an article for publication.

As I step back, however, I expect that a new author will begin to contribute occasionally; I've invited Mike Garrison, a long-time reader and commenter, to add his thoughts and analyses on interior and northern Alaska weather and climate.  Mike is an Alaska resident and avid weather enthusiast, and he works with weather and climate data in the course of his professional employment.  I expect we'll see a few words from Mike on the blog soon.

In the meantime, I was asked recently about a chart of Fairbanks temperature percentiles that I showed here last year.  I decided to clean up the chart and create parallel versions for maximum and minimum temperatures; so here they are.  They serve as a useful reference for the climate in Fairbanks.  Many comments could be made, but I'll just point out one feature that stood out to me: the greater variability of daily high temperatures in summer compared to daily low temperatures.  It's clear to even the casual observer that winter temperatures are much more variable than summer temperatures, but this difference is less significant for high temperatures than for low temperatures, and I hadn't quite appreciated this distinction before.  The reason, I believe, is that the presence or absence of clouds and rain in summer makes a bigger difference for high temperatures than for low temperatures.