Thursday, January 30, 2020

Modest Sea Ice Growth

Another wave of Arctic air has affected northern and western Alaska in the past few days, leading to extremely low wind chills in some places (-70°F and below in a few northern spots), and cementing this month into the solidly below-normal category for temperature in many locations statewide.

The pattern has remained remarkably persistent, and the latest cold blast has affected southwestern Alaska with a similar set-up to what happened a month ago; in particular, a strong trough over the southwest allowed a powerful low pressure system to move into south-central Alaska yesterday.  Here are the 500mb and surface analysis maps from Environment Canada for 3am yesterday (click to enlarge):

Here are the surface observations from 6am yesterday morning; note the cold and wind in the southwest.  The wind chill in Bethel got close to -50°F for the second time this winter, and yesterday was the 6th day this winter with an average wind chill below -40°.

The daily temperature anomaly chart shows the fresh surge of cold in Bethel:

As cold as this month may have seemed for many, however, it pales in comparison to some of the colder spells of the past, even as recently as 2012.  Bethel was more than 10°F colder in January 2012, with 17 days of average wind chill below -40°.  And as persistent as the chill has been in Fairbanks, January 2012 was more than 5°F colder.

Additional perspective comes from the Bering Sea ice extent data, which is showing a very modest recovery in ice for the basin as a whole (see below).  Remarkably, Bering Sea ice extent is below where it was at this date last year; but of course last year the ice coverage was decimated in February and never recovered.

Here's a side-by-side comparison of ice maps from last year (left) and this year (right).  With cold focused on southwestern Alaska this winter, the eastern Bering Sea is in good shape for ice, but there are still significant shortfalls farther west; there is mostly open water as far north as 64°N in the Gulf of Anadyr.

Friday, January 24, 2020

Why Cold Now?

To the great surprise of many observers and prognosticators (and I fall into both camps), Alaska is having something of a cold winter, at least since mid-December.  There's still plenty of winter left, and it may yet turn around, but so far it's a very distinct and striking change from recent winters, which were of course much warmer than normal.

The chart below shows the daily temperature anomaly from Fairbanks since September 1.  Nothing "different" happened until mid-December, but since then the cold has proven to have considerable staying power.  For example, today is the 25th consecutive day with a daily high temperature below +5°F, and with more cold air moving in, another week seems likely to be added.  We have to look all the way back to 1975 to find a stretch of 30 days or more below +5°F; the record is 48 days ending January 27, 1943.

An obvious question that has been much on my mind in recent weeks is, "Why?"  Why is this winter turning out so differently from recent years?  To illustrate the change in another way, consider the 500mb height anomaly since December 1 (first map below) and the striking contrast with the average Dec-Feb anomaly in the previous six winters (second map below).  What could drive such a significant pattern change?  Is there an "external" influence elsewhere in the global climate system that could explain the difference?

One aspect to notice is that this winter's circulation pattern bears more than a slight resemblance to a typical La Niña winter pattern.  The map below show the Dec-Feb anomaly in winters with strong La Niña episodes, based on CPC's ONI index.  There's less ridging over the Bering Sea this winter than in the La Niña pattern, and less of western North America has been affected by a trough, but there's obviously some similarity with the La Niña signal.

What's odd, however, is that we do not have a La Niña SST pattern in the equatorial Pacific this winter; in fact it has been considerably warmer than normal in the central tropical Pacific, and the pattern has been strongly aligned with a typical "central Pacific El Niño", or "El Niño Modoki", event.  Until recent weeks there has also been an intensely positive Indian Ocean Dipole, which is usually found in association with El Niño; so we wouldn't expect to see a La Niña influence in the mid- to high latitudes.

Are there any other factors that could be driving the pattern in a cold direction for Alaska?  Frankly, it's difficult to find any, and even with the benefit of hindsight it is difficult to see any compelling reasons why the pattern would flip this winter.  As part of my regular work responsibilities I look at a lot of seasonal forecast guidance every month, and nearly all statistical and "pattern matching" tools suggested that another warm winter was on the cards for Alaska; for instance, the maps below show objective statistical forecasts based on October-November SST patterns in the tropics and Northern Hemisphere.  (These statistical forecasts are only marginally skillful, but they're better than random chance.)

The experts at CPC also thought a warm winter was more likely than not - see below.

Perhaps the only real hint that something might turn out different this time came from some of the dynamical models in the NMME guidance.  In particular, a few of the models showed a cold signal from southern Alaska to northwestern Canada; but others showed pronounced warmth in the same area, and it was easy to dismiss the cold solutions.  In hindsight, if the winter continues the way it's going, the best forecast may have been produced by the new GEM-NEMO model from Environment Canada: not only did it show pronounced cold in much of Alaska, but it captured the unusual warmth that has prevailed in the lower 48 and Europe.

In light of this, it seems that the GEM-NEMO model "caught on" to a driving influence that turned out to be significant, leading to a good forecast.  There's a chance that further investigation might reveal what that influence was and lend some insight for the future; or it might just be that the model was "lucky" this time.  But in any case, hopefully I'll be able to do a more productive post-mortem of the winter pattern when it's all over.

Saturday, January 18, 2020

ERA5 Analysis of Cold

During the interior cold snap of the last week of 2019, when the temperature dropped well below -50°F in many places, I was eager to see what the new state-of-the-art ERA5 reanalysis would say about the minimum temperatures.  I've used the ERA5 data on previous occasions (e.g. see here), and will continue to do so, because it's an exciting new product from Europe's weather and forecasting science powerhouse, ECMWF.  Besides this, ERA5 is becoming widely used as essentially ground truth data across the globe, so it's of great interest to see how it performs in extreme or unusual weather situations.

First, here are some maps of the ERA5 daily minimum temperatures during the cold spell.  Note that I've used a 24-hour period from 3pm to 3pm AKST (midnight UTC) for simplicity in processing the data.  Click to enlarge.

When comparing to the December 27 satellite-estimated values that I showed in a previous post, it's clear that the area affected by -50°F or lower is considerably smaller in the ERA5 data, especially to the south of the Yukon River.

The absolute minimum on the ERA5 31-km grid is -58.7°F, and this is found right over Allakaket on December 27; this temperature matches very well at Allakaket itself, but as discussed before, the satellite suggests it was a bit colder on the Kanuti Flats.  Moreover, the ERA5 analysis is nowhere near as cold in the vicinity of the Eureka coop (MLYA2) report of -65°F, although of course we can't expect a 31-km resolution model to capture localized valley cold pools - we would need a far finer grid to hope for success in that situation.

It's interesting to see that the model does have a localized area of cold near the Nowitna CRN site (NWTA2), and so I thought it would be worthwhile to do a more detailed comparison of ERA5 vs CRN temperatures.  Of course the quality of the CRN measurements is extremely high.

First, the big picture: all daily mean temperatures since March 2017, when the Nowitna (Ruby 44 ESE) CRN site first started working reliably (after being installed in 2014).  See below - the comparison is fairly good at temperatures above freezing, but there are evidently some problems in the cold season; in particular, ERA5 tends to have a high bias in cold conditions, and it's large in some cases.

Here's a chart of daily mean temperatures this winter since October 1.

The main problem is immediately obvious: ERA5 stays much too warm when the temperature drops off quickly at the CRN site.  Each of these cold snaps no doubt corresponds to a period of clear and calm weather that allows a strong temperature inversion to develop, and evidently the model's boundary layer physics do not handle this situation well.  It's a bit disappointing, considering the pedigree of the ERA5/ECMWF models, but we must admit that it's a tall order to reliably represent such shallow layers of intense temperature inversion in any model.

Here are charts from the past two winters.  The same problem is evident, although not to the extent of this winter.  Interesting the mean bias from October-March was exactly the same in both 2017-2018 and 2018-2019 (ERA5 3.1°F too warm), but the bias is running at +4.9°F so far this winter.

In summary, ERA5 clearly has some problems with extreme cold in Alaska in winter.  In the recent interior cold snap, the extent of coldest conditions was too small, although part of this is certainly related to the grid-box averaging that is intrinsic to the model; the model simply doesn't represent valley-floor locations when there is higher terrain elsewhere in the grid cell.  Looking at the Nowitna CRN, there's a clear tendency for a warm bias that shows up during cold spells, and this seems to be a symptom of inadequate surface layer physics as well as insufficient horizontal resolution.

All of this goes to show that we still can't overestimate the value of real ground-truth observations such as those provided by the wonderful CRN network.  And as an aside, I for one am very grateful for the efforts of the NWS personnel who finally got the Nowitna site working properly - it's a great location and we're lucky to have such a treasure trove of data.

Monday, January 13, 2020

Cold Exits Southeast

The unusual cold spell is largely over for much of Alaska now, as the core of the chilly air mass has finally migrated to southeast Alaska and western Canada; British Columbia and Alberta are now in the deep freeze.  In the past few days the harshest cold in Alaska has been in the southeastern interior, where Northway has reached -50°F for five days in a row; this is the most since January 2009.

The nearby famous cold spot of Chicken was colder, of course, and reached -62°F on Friday.  This is the coldest since 2013, but actually it's right on par for the typical coldest temperature of the winter; I was slightly surprised it wasn't a bit colder, given the -50°F or lower in many other spots.

Last week I mentioned the persistence of cold in Bethel, and their cold snap broke at last on Saturday, when Pacific air finally bumped the temperature back up towards the freezing mark (i.e. well above normal).  In the end Bethel endured 18 straight days without rising above 0°F, which is the most since 1989.  It's an impressive cold spell for a place that has borne the brunt of excessive winter warmth in recent years.

Here's a comparison of daily temperature anomalies over recent months in Bethel and Northway.  Since October 1, both sites are still running above the 1981-2010 normal (+0.6°F and +3.7°F respectively).  Click to enlarge:

One other note in this quick update: the PDO index has plummeted in recent days in response to the outbreak of cold air over the northeastern Pacific (and in fact a broader pattern shift extending all the way down towards Hawaii).  The PDO (Pacific Decadal Oscillation) is highly correlated with winter temperatures in much of Alaska, and the daily index hasn't been this low since late 2013.  Time will tell whether the regime shift has any staying power.

Update Jan 14: here are some long-range PDO index forecasts from the NMME and ECMWF seasonal models.  It's a fairly robust signal for a generally negative PDO regime by early summer, and this is closely related to the models' expectation for La Niña-like anomalies to develop in the equatorial Pacific.  Of course this doesn't mean the North Pacific will be cool as a whole - the opposite is more likely to be true.

Tuesday, January 7, 2020

Cold Reinforced

Unusual cold has settled in over Alaska in recent days, and most of the state is now seeing significantly below-normal temperatures; and given the date on the calendar, this is pretty cold.  Temperatures near or below -40°F/C have developed over a wide region, and the colder spots are around -50°F from the interior North Slope to the Yukon Flats to the western interior.

The statewide nature of the chill is illustrated by UAF's temperature index, which combines standardized temperature anomalies from 25 stations; the latest update (see below) shows that the latest cold wave has brought a larger statewide anomaly than the cold snap just after Christmas.

The coldest conditions relative to normal have been in the southwest of mainland Alaska, and in this area there was really no break between the first and second cold waves.  For instance, as of today Bethel has been continuously below 0°F for 15 straight days.  The record is 26 days below zero, and this is not a historically cold spell for Bethel (contrast with January 1989), but it's the coldest since the winter of 2011-2012.  Today may be the coldest day so far, with a high temperature through 6pm of only -20°F.

Also of note was last Thursday's remarkable low temperature of -41°F in King Salmon on the north end of the Alaska Peninsula.  This actually beats the 2012 cold spell, as King Salmon hasn't dropped below -40° since 2006.

Perhaps equally impressive in the context of recent warm winters, Anchorage has seen four straight days with a low temperature of -9°F or below.  The six winters from 2013 to last winter only mustered five such days in total.

The change to cold conditions in Alaska is obviously a dramatic turn-around from the pervasive warmth of recent months and years, and it will be interesting to see how long it lasts.  At the moment Alaska is one of the coldest places in the Northern Hemisphere relative to normal, and this is a stark contrast with the 5-year tendency for Alaska to be a hemispheric "hot spot".

Of course with cold locked up over Alaska and the high Arctic, most other parts of the middle and high latitudes are unusually warm; and it's worth pointing out that hemispheric average temperatures are no colder this winter.  Alaska is just seeing a lucky (or unlucky, depending on your viewpoint) break from the recent trend.  To demonstrate this point, the chart below shows the fraction of the Northern Hemisphere's area in which the 850mb temperature is below -5°C; it has been running close to record lows in recent weeks.

Read more about this metric for measuring the hemispheric winter cold pool here:

Wednesday, January 1, 2020

Arctic Trough

The weather has been wild in western, southwestern, and south-central Alaska in the past few days, as an upper-level trough has deepened over the eastern Bering Sea and western Alaska.  This feature, and the associated cold air, bears some relation to the Arctic vortex that delivered last week's interior cold snap; it appears that a lobe of the Arctic air mass settled down over western Alaska, and the clash with North Pacific warmth farther south set up a stormy situation yesterday.

Here's the 500mb chart from 3pm yesterday, showing a very sharp trough over western Alaska.  The eastward kink at the southern end is very indicative of a strong surface-level cyclone.  Click to enlarge:

The surface analysis from 3am today (see below) shows the path of the 972-973mb surface pressure center, which tracked right up Cook Inlet yesterday evening.

The impacts on the south-central population were substantial: about 10" of snow in Anchorage, 21" in Girdwood, and a closed Seward Highway.  Very sadly, the storm also led to the loss of a crabbing vessel near Kodiak Island:

From a meteorological perspective, one of the most striking aspects of yesterday's events was the intense temperature contrast across the system.  While Bethel was shivering in windy sub-zero conditions, Anchorage saw a record high of 46°F before the cold air arrived in the evening.  The difference in daily high temperatures between Anchorage and Bethel was a remarkable 56°F, and this is nearly the greatest on record (59°F on Dec 23, 2004).

The map below illustrates the situation at noon: 46°F in Anchorage, 39°F and raining at McKinley Park, but with very cold air advancing from the west.  Notice the 21°F and windy at Akhiok on Kodiak Island; the waters west of the island would have been very rough, and icing may have been a significant issue.

Here's the situation 8 hours later: down to 0°F at McKinley Park and 18°F in Homer.

The frontal passage was especially striking at McKinley Park, where the temperature dropped from 39°F to 10°F in just 15 minutes as the wind direction changed from southerly to northerly.

Anchorage also saw a sudden temperature drop in early evening: 15°F in less than an hour.

One bright side of this cold blast over western Alaska and the Bering Sea is that sea ice growth will be vigorous; but that may be "cold comfort" for residents of the west.  Bethel has seen a daily average wind chill below -30°F for all but one of the last seven days, and up in the northwest, Kivalina is this evening enduring its second 60+mph ground blizzard in 48 hours.  Sounds like real winter.