Tuesday, March 31, 2020

Cold and Snow

As is only fitting, winter is coming to an end with another spell of unusual cold in much of Alaska; here are a few notable reports from the past several days.

-44°F  Umiat RAWS
-36°F  Eagle COOP
-34°F  Chicken COOP
-34°F  Tok 70SE CRN
-32°F  Salcha RAWS
-23°F  Goldstream Creek COOP

In Fairbanks, March is ending as the fourth month in a row with average temperature below the 1981-2010 normal, and it's the coldest first quarter of the year since 2007.

March was also a very snowy month in Fairbanks, as Rick Thoman illustrated nicely in the following plot (via Rick's Twitter feed); click to enlarge.

With 27.5 inches of snow, March was the snowiest month of the winter in Fairbanks.  Since the winter of 1929-30, this has happened 8 times before, so it's unusual but not rare.  Average (median) March snowfall is less than 5 inches, but it's a very skewed distribution, with occasionally much more occurring.

April is even more skewed, of course; the median is only 1.7", and yet even April has been the snowiest month of the winter on 4 occasions - most recently in 2008 (14.7").

Here's a frequency histogram of the snowiest month of the winter in Fairbanks, Bettles, and Anchorage.  Fairbanks stands out in terms of having October as the snowiest month rather frequently; I find this particularly surprising as rain is more common than snow in the first week or so of October in Fairbanks.

Other interesting features of the chart include the notable November-December peak in Bettles, and the enhanced frequency in late winter (February through April) in Anchorage.  Surprisingly, March has more often been the snowiest month than January in Anchorage, and average March snowfall lags only very slightly behind January.  I suspect part of this is an artifact of sampling variability, but there may well be a physical reason why January tends to underperform for snow; I don't think it's as simple as "it's too cold to snow", but if readers have any ideas, I'm all ears.

Friday, March 27, 2020

Why Cold Now - Part 3

In the first two installments of this discussion (here and here), I made a few comments about Alaska's surprisingly cold winter and the difficulty of finding a good explanation for it.  Of course from one perspective the reasons are obvious: the circulation pattern produced more northerly and westerly flow (and less southerly flow) than usual over the state, leading to a much-reduced influence of mild Pacific air.

The pattern is illustrated by the 500mb height anomaly map, which shows an unusual trough centered near the southeastern interior and a strong ridge over the central North Pacific.

A very strong westerly flow is implied between the ridge and the trough, and this strong and stable jet stream pushed warm air eastward into Canada and the Lower 48 rather than taking occasional (or frequent) northward excursions into Alaska.  Here's a map of the vector wind anomaly at 250mb; the westerly flow was more than 12 m/s stronger than normal to the south of Alaska.

For reference, here's the normal upper-level wind pattern in winter:

The map below shows the resulting temperature anomaly pattern, which really highlights that the configuration was "perfect" for bringing cold to the heart of Alaska; the state experienced an "island of cold" in a sea of unusual warmth (and I'm not showing Eurasia, which was much warmer still).

Another key aspect of the flow regime was a positive phase of the Arctic Oscillation (AO), indicating that (i) low pressure was unusually low in the Arctic, (ii) the mid-latitude westerly jet stream was unusually strong, and (iii) there was a strong contrast between unusual warmth in the mid-latitudes and relatively cold conditions in the Arctic.  The positive AO anomaly became increasingly pronounced as winter progressed, and the daily AO index reached all-time record positive values in February.

The AO phase has a modest connection to Alaska temperatures in winter, as illustrated by the chart below.  Note that I'm using detrended temperatures based on data since 1950, and yes, it was the coldest winter relative to trend since 1950.  However, the trend since 1950 is very steep (over 1°F/decade), and this winter's cold was less remarkable when compared to a longer-term trend or simply a long-term average.

The chart indicates that below-trend temperatures tend to be favored when the AO phase is positive, and this provides a reasonable connection between Alaska's cold winter and the larger Hemispheric flow pattern.  However, it's interesting to see that this winter's flow pattern over Alaska was not particularly well-aligned with the typical positive AO pattern.  The map below shows the average 500mb height departure from normal in the top 10 positive AO winters of the past, and while the same strong north-south gradient is evident across the North Pacific, the AO pattern usually has a trough axis near the Chukchi and Bering Seas rather than over eastern Alaska.

The temperature pattern associated with the positive AO phase reveals a cold signal over southwestern Alaska (see below), which is certainly consistent with what happened this winter; but it seems the positive AO is not typically associated with more widespread cold across the interior and north.  As an aside, it's interesting to see a negative PDO-like temperature pattern across the North Pacific in these winters, and indeed the PDO was negative in 7 of the 10 winters (as it was this winter).

A closely related, and significant, feature of the Northern Hemisphere winter was the unusual strength of the polar vortex up in the stratosphere.  The stratospheric polar vortex forms every winter and has a fairly close connection to the lower-atmosphere AO phase, but the two don't always behave in tandem over the course of a winter.  However, this winter the polar vortex was strongly coupled, especially in late winter, and the upper-level vortex became remarkably intense by late winter - see below.

Does the presence of a strong stratospheric vortex alter or magnify the AO temperature signal over Alaska?  Not really; the map below shows winters with both a positive AO phase AND a significantly stronger than normal stratospheric vortex, and it's a very similar signal to the AO pattern by itself.

So are there any winters in the historical record with a similar strong polar vortex/AO phase and more widespread and pronounced cold in Alaska?  The answer is yes; the winter of 1989-1990 was notably cold in Alaska, and the AO became increasingly positive (indeed extremely so) as winter wore on, similar to this winter.  The previous winter, 1988-1989, is probably more memorable to Alaskans because of the incredible cold snap in January 1989, but the subsequent winter of 1989-90 was actually colder overall, and February 1990 was bitterly cold (easily the coldest February on record for the state).

Here are the 500mb and surface temperature maps for 1989-1990: the match is by no means perfect, but it's a good deal better over the North Pacific and Alaska domains than with the positive AO composites.  Notice in particular the relative absence of cold in southeastern Alaska and the Aleutians, and the warm region over the northeastern North Pacific to the west of Oregon - all similar to this winter.

The similarities between these two winters, separated by exactly 30 years, are sufficiently intriguing - especially in regard to the dramatic strengthening of the AO phase in late winter - that I'll have to dig deeper and try to understand what was going on.  And that means readers can be on the lookout for Part 4 of this discussion in due course.

Friday, March 20, 2020

CRN Data Visualization

Spare time for new blog posts has been in short supply lately, but I'll mention a little project I've been working on that may be of some interest.  As many readers know, NOAA's Climate Reference Network of top-quality climate observing instruments has been gradually adding sites in Alaska over the past several years, and the state's network is now up to 22 sites.  Here's a map (click to enlarge):

All of the data is readily available from NCEI, but options to visualize the data appear to be very limited.  I'm aiming, therefore, to put together a simple interface that provides some charting and perhaps mapping capabilities to summarize the wealth of climate monitoring data that's flowing from the CRN instruments.

Here are a few prototype charts showing departures from normal of climate variables observed at the CRN site near Fairbanks over the past several months.  The seasonal normals are calculated from the full period of record, which is already nearly 18 years long for temperature, precipitation, and solar radiation, but only 10 years for wind speed and humidity.  Click to enlarge the images.

Here's precipitation since the beginning of the year.

Look for more posts on the CRN data in (hopefully) the not-too-distant future.

Wednesday, March 11, 2020

Why Cold Now - Part 2

Back in late January I raised the question of why this winter produced persistent cold over Alaska; it's such a dramatic change and stark contrast to other recent winters that it begs for attempts to explain it.  After all, if we can't explain these things even in hindsight, what hope do we have (speaking personally) of predicting them?

To confirm the point that this winter was indeed remarkably cold in Alaska, the chart below shows the December-February statewide average temperature according to NOAA's climate division data.  In absolute terms, the winter was the coldest since 1998-99, and compared to the trailing 30-year average it was the coldest in 50 years (since 1969-70).  One could argue that this winter was about a once-in-a-generation cold winter relative to the modern warmer climate; and if background warming continues (as seems inevitable), it may be a very long time before it's this cold again.  Click to enlarge:

In my earlier post I discussed the apparent lack of connection of the winter pattern with tropical ocean temperature signals.  Tropical oceanic and atmospheric phenomena are often closely followed by long-range forecasters because of their slow and predictable progression, together with known (or supposed) mechanisms for influencing higher latitude weather.

In addition to sea surface temperature (SST) patterns, forecasters often track the behavior of tropical convection, i.e. regions of disturbed weather, including large clusters of thunderstorms.  One tool for such tracking is the so-called velocity potential of the upper troposphere; the velocity potential simply isolates the divergent part of the flow, which reveals where convection is relatively active (divergence aloft) or inactive (convergence aloft).

To illustrate, here's the departure from normal of this winter's velocity potential (VP) at 200mb.  Blue shading (negative VP) indicates unusual divergence aloft, implying more rain than normal (hence the locust outbreaks in East Africa), and yellow/orange indicates unusual convergence aloft (less rain than normal).  Note that VP is only useful in this sense in the tropics, because large-scale weather disturbances outside the tropics are not typically dominated by convective processes.

Now to the point of this discussion: it's worthwhile examining past cold winters in Alaska to see if there are similarities in the VP patterns.  If there are, then we might argue that the tropical convection was linked to the outcome in Alaska; and such a finding would not be controversial.

Here are the VP anomaly maps for Alaska's three coldest winters relative to trend since 1980; the lack of good satellite data prior to 1980 makes me reluctant to look at the VP analyses from earlier years.

Unfortunately there are no obvious similarities between the VP patterns in these years.  1989-90 perhaps comes closest to 2019-20, with enhanced rainfall in the Indian Ocean and suppressed convection over the Maritime Continent, but 1989-90 did not have enhanced activity over the central Pacific or over most of Africa.

Here's what a good match looks like: the winter of 2002-03 had similar tropical VP patterns to 2019-20, but it was one of the very warmest winters on record in Alaska.

To pursue a more objective approach, I calculated the similarity to 2019-20 of the VP anomalies along the equator, and none of the 3 cold years is in the top 10 matches among 40 years of data (although 1989-90 comes closest).  And just to put a nail in the coffin of this hypothesis, here is a map of winter patterns in those top 10 matches, i.e. the 10 winters with the most similar VP to 2019-20.

The pattern is about as opposite as it could be for northwestern North America; contrast the 2019-20 maps below.  So even if we had known the VP patterns perfectly in advance, an analog approach like this would have predicted another warm winter for Alaska.

In summary, this cursory analysis provides absolutely no evidence that Alaska's cold winter was directly linked to patterns of tropical rainfall activity, and this is a disappointment.  It's not entirely surprising, though, because the SST patterns provided no help either, and there are very close links between SST anomalies and tropical convective regimes.

In Part 3 of this analysis, I'll look at another aspect of the winter that became increasingly unusual as the winter progressed, and that's the strongly positive Arctic Oscillation and increasingly intense stratospheric polar vortex.  In this case there is definitely some demonstrable correlation to Alaska's winter weather, but I think we'll find that it still leaves many questions unresolved.

Wednesday, March 4, 2020

Winter Hangs On

Unusual cold is hanging on with remarkable tenacity in Alaska this winter, and last night saw temperatures worthy of the depths of winter in many locations.  Here are a few examples of notable low temperatures today:

-50°F  Bettles
-45°F  Tanana
-44°F  Chalkyitsik RAWS
-42°F  Nenana

And in and around Fairbanks:
-46°F  Salcha RAWS
-41°F  North Pole
-39°F  Goldstream Creek
-39°F  Smith Lake at UAF
-38°F  Fairbanks airport

Here's the temperature trace from the past two weeks at Smith Lake (UAF's North Campus).  The temperature did not break -10°F today, which is pretty impressive for the time of year.

Of course clear skies at this time of year produce large diurnal temperature ranges, as seen in the Smith Lake data.  The Chalkyitsik RAWS (about 20 miles east of Fort Yukon) often produces spectacular examples, and just the other day this site saw temperatures vary from a low of -49°F on two consecutive nights to a high of 0°F in the intervening day.  When looking at days with a high temperature of 0°F or below, this is the largest diurnal range on record for the site (data since 1997).

The chart below shows daily Chalkyitsik temperatures so far this winter, as compared to seasonal normals (obtained from the 1997-2018 history).

Two features are striking: the persistence of unusual cold since mid-December, and the expansion of the daily temperature range since early February.  The latter is entirely in keeping with climatology, which shows a pronounced rise in the average diurnal range in late winter; in fact it's remarkable to see the normal high temperature bottom out very soon after winter solstice, while the normal low temperature doesn't show any meaningful rise until well into February.