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:
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.
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):
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.