Dawson Follow-Up

Following up on last week's post about Yukon River freeze-up at Dawson City (Yukon Territory), it's worth looking at whether excessive warm season precipitation may have been a factor in preventing a complete freeze-up in some recent years.  To recap, the Yukon failed to freeze over properly at Dawson in the winters of 2016-17 though 2018-19, and then again last winter; but freeze-up did succeed in 2019-20 through 2022-23, and again this winter.

Here are the May-September precipitation estimates from ERA5 for the months prior to the "no freeze" winters:

There's no evidence here that the southern half of the Yukon Territory (much of which is in the Yukon drainage) was consistently wetter than normal in these years, and in fact the summer of 2023 was somewhat dry.

As for recent "normal freeze" years, 2019 was dry, but others years tended to be more on the wet side; and this summer wasn't particularly unusual in either direction.

How about water levels at Dawson?  Here are the September mean water levels in the no-freeze years, for the Yukon River (left) and the tributary Klondike River (right), which meets the Yukon immediately upstream of downtown Dawson:

2016   2.02m   1.36m

2017   1.56m   1.23m

2018   1.85m   1.10m

2023   1.60m   1.30m

And the normal freeze-up years:

2019   1.03m   0.85m

2020   2.65m   1.39m

2021   2.30m   1.28m

2022   2.16m   1.65m

2024   2.12m   1.50m

With the exception of 2019, the normal freeze-up years all had higher Yukon River levels in September than the no-freeze years.  The tendency is the same for the Klondike River, with 2022 and 2024 both having higher levels than the no-freeze 2023.

I think we can therefore dispense with the idea that the freeze-up failures occurred because of higher flows - if anything, the reverse seems to be more likely - but I won't indulge in any more speculation now.  Previous posts and discussions in the blog comments can be accessed by searching the blog archives for "Dawson".

Frozen Over at Dawson

A couple of interesting items came to my attention today.  First, the Yukon appears to be properly frozen over at Dawson City in the Yukon:

This is big news over there, because the lack of freeze-up next to town has been a huge problem in recent years; freeze-up has often been much delayed and sometimes hasn't occurred all winter.  I've commented on it occasionally, most recently last March:

https://ak-wx.blogspot.com/2024/03/yukon-at-dawson-still-open.html

The change this year isn't caused by colder weather - it hasn't been unusually cold - and the lack of freeze-up in recent years was generally not caused by unusual warmth.  When I have a chance, it will be worth looking at regional streamflow and precipitation data from recent months to see if anything stands out.

Here's an animation of webcam images from today, courtesy of http://dawson.meteomac.com/

On a completely different note, I ran some calculations today to establish whether the Northern Hemisphere circulation patterns have been unusually volatile this month; it sure seems like it.  The answer is yes: the following chart shows a metric of 500mb volatility for 45-90°N around the globe.

To be precise, the figure shows the area-average of the standard deviation of 500mb heights for November 1-26, expressed as a fraction of the 1991-2020 normal (which differs from place to place).  Here's a map of the departure from normal: 500mb heights have been much more variable than normal over the eastern Bering Sea and southwestern Alaska, as well as near Baffin Island and over northern Europe.

The region of high variance near Alaska largely reflects the dramatic change from this a few weeks ago:

to this last week:

The big ridge over the eastern Aleutians and southwestern Alaska about a week ago was bumping up against records for the month of November.

What could explain the volatile patterns around the higher latitudes of the Northern Hemisphere?  I think it's the unusual combination of sea surface temperature anomalies across the tropics and the mid-latitude oceans.  We have a weak La Niña, which tends to favor meandering "blocking" patterns in the high latitudes in early winter (November in particular), but we also have tremendous warmth in the northwestern North Pacific, which favors mid-latitude ridging (high pressure) and a stronger, more west-east jet stream to the north of the ridge.  These two Pacific ocean anomalies therefore currently have opposing influences on the jet stream pattern, as I see it, and volatility has resulted.

Looking ahead, as winter settles in, La Niña's influence will very much shift towards favoring a stronger jet stream with a strong polar vortex, and that in turn will tend to favor unusual cold in Alaska, particularly in the south.  This is very consistent, of course, with a negative PDO phase, which is locked in because of the warmth to the east of Japan.  Here's the SST anomaly from the past month:

In conclusion - bundle up; and happy Thanksgiving to U.S. readers.

Yukon at Dawson Still Open

This has been a recurring theme in recent years, and a problem for residents of Dawson, YT: the Yukon River is reluctant to freeze over next to town.  As in some other recent winters, it hasn't been possible to build the usual ice bridge across to West Dawson.  Here's today's webcam view, suggesting that a complete freeze-up won't occur at all this winter:

And a video confirming the flow of water in the open channel:

November and December were significantly warmer than normal in Dawson, but January and February were both slightly colder than normal, so it seems unlikely that the lack of ice can be blamed on the weather.

Yukon Frozen at Dawson

Long-time readers will recall that in the past few winters I've drawn attention to the interesting failure of the Yukon River to freeze over properly at Dawson in the Yukon Territory.  For the past three years the Yukon government has been unable to construct an ice bridge across the river to West Dawson, and local residents have resorted to alternative routes to make the crossing.  Here are some posts from previous years:

https://ak-wx.blogspot.com/2017/02/yukon-river-at-dawson.html

https://ak-wx.blogspot.com/2018/02/yukon-river-at-dawson.html

https://ak-wx.blogspot.com/2019/02/dawson-ice-bridge-problems.html

Happily, this winter is a return to normal, as the government-sanctioned ice bridge was open by Christmas and was available for heavy traffic a month ago.

https://www.cbc.ca/news/canada/north/dawson-city-ice-bridge-opens-2019-1.5407399

https://www.whitehorsestar.com/News/ice-bridge-enters-wider-thicker-phase

One could be forgiven for thinking that colder weather this winter is the reason for the more normal freeze-up, but in fact it wasn't particularly cold at all during the freeze-up period.  The average temperature in November and December was -2°F, compared to -5°F and -7°F in 2016 and 2017, respectively; the 1995-2015 average was -5.1°F for these months.  Nor was there a pronounced cold spell; only 6 days before the turn of the year dropped to -30°F, compared to a 1995-2015 median of 10 such days.

The accumulation of freezing degree days shows the same thing: this winter was apparently no more favorable for freeze-up in terms of thermal conditions, although the January cold provided a good boost after freeze-up.  Click to enlarge the chart below.

This simply confirms what we noted in prior years: unusual warmth did not explain the persistent lack of freeze-up in the last 3 years, and unusual cold can't explain the river's return to normal behavior this winter.

As noted before, there are many possibilities for potential causes of the abnormality in recent years, but one new clue comes from a look at warm-season precipitation in the upstream Yukon drainage.  The chart below shows May-September precipitation at four sites for the past 5 years, and clearly last summer was drier than any of the preceding 4 years.

The 1981-2010 normal for Mayo and Dawson is about 195mm for May-September precipitation, so last summer's deficit was fairly substantial.  (Whitehorse was close to its normal of 155mm, and I don't have a normal value for Carmacks.)  This is a tiny sample across a very large area, of course, but the river level data from Dawson support the idea of reduced flow late last year.  Here are the September mean level values from the past few years (data obtained here):

2015   2.83m
2016   2.02m
2017   1.56m
2018   1.85m
2019   1.03m

To my mind it makes sense that lower flows would freeze over more easily, but I don't think this is all there is to it; I believe there was an ice bridge in 2015-16, but that year apparently had a very high flow rate going into autumn.  At any rate, it's nice to see something more normal this winter.  Here's the webcam view from a couple of weeks ago.

Dawson Ice Bridge Problems

Readers may recall that in the past couple of winters I've mentioned the odd reluctance of the Yukon River to freeze up properly at Dawson in the Yukon Territory.  This has created a problem for residents of West Dawson who in years past relied on an ice bridge for seasonal access to the main town on the east side of the river.

https://ak-wx.blogspot.com/2018/02/yukon-river-at-dawson.html

https://ak-wx.blogspot.com/2017/02/yukon-river-at-dawson.html

https://ak-wx.blogspot.com/2016/12/ice-cold-and-solstice-sun.html

Perhaps not surprisingly, the same problem has occurred this winter, and efforts to stimulate ice growth across the stubbornly open channel were called to a halt last week.  Here are a couple of news articles on this winter's lack of success:

https://www.cbc.ca/news/canada/north/gov-halts-dawson-city-ice-bridge-1.5001387

https://www.cbc.ca/news/canada/north/dawson-ice-bridge-hopes-1.4974031

Here's a view from the webcam in Dawson a few days ago, looking out across the river; as in past winters, a narrow strip of open water is visible.

The lack of an ice bridge has been deemed sufficiently important that Canada's National Research Council looked into the issue last year, and a report was issued in October; it makes for very interesting reading.  The report discusses a variety of hypotheses about what may have caused the change, but there are no definitive conclusions; there may be multiple factors at play, some of which we've speculated about before on this blog (including good comments from readers).

As we've noted before, the change in ice conditions can't simply be pinned on warmer winter weather, because temperature data from Dawson doesn't show substantially warmer conditions in the winters when freeze-up failed.  The accumulated total of freezing degree days has been slightly lower this winter than the two-decade normal, but 2016-17 and 2017-18 were both near normal through this point in the season (see below).  Of course it is possible that ground and/or river water temperatures have risen, as noted in the NRC report.

Yukon River at Dawson

Long-time readers may recall a few posts last winter that mentioned the exceptionally slow freeze-up of the Yukon River in Dawson (Yukon Territory).  It's interesting to note that the same thing has happened this winter - there is still a substantial gap of open water that has prevented the construction of the usual ice road to West Dawson.  This year an effort was launched to accelerate the freeze-up with a "slush cannon", but lack of progress led to the project being called off recently.

Here's how the river looked last Sunday.

Last winter we noted that an unusual ice jam upstream was the primary reason for the open water, rather than exceptionally warm weather.  It appears that the same may be true this year, because again the weather has not been particularly warm overall this winter in Dawson; the chart below shows that total freezing degree days through January 22 (when the ice-making project was canceled) were very close to the normal of the last 20 years.

The repetition of the slow freeze-up this winter suggests that it's not a random occurrence - "something" has changed - but the temperature data rules out a simple explanation based on excessive warmth.  I started to look for river flow data from the autumn to see if the river discharge has been unusually high or low, but I had difficulty acquiring suitable data.

One possibility (and this is mere speculation, being outside my field of knowledge) is that there could have been a subtle change in the river profile/cross-section upstream of Dawson, owing to deposition or erosion, that may have altered the hydrodynamics in a way that now favors the formation of an ice jam in a new location.  If any readers have comments on this idea, or other suggestions on how to identify the underlying cause of the open water (i.e. is it related to weather and climate), then I'd be glad to hear them.

New Blog

Regular visitors may be interested to learn that our own reader Eric has started his own blog on interior Alaska's weather and climate; I expect I will link to it occasionally or even repost some material.  Take a look here:

http://climateoftheboreal.blogspot.com/

On another note, the webcam from Dawson (Yukon) reveals that the Yukon River has at long last frozen over completely next to the town.  There's been no lack of cold weather this winter, but an unusual ice jam upstream allowed an open lead to remain in the middle of the river throughout the entire winter until now.  It looks like the ice road to West Dawson may finally have opened just in time for the onset of spring.

March 1:

March 7:

March 17: