The late freeze-up is at least partially explained by the slow accumulation of freezing degree days so far this winter. As seen in the chart below, the freezing degree days so far this season are barely above the level of the past two winters, which were notoriously mild. There have been other years with lower FDDs through this date, but the only other time 3 such mild freeze-up seasons occurred back to back was in 1979-1981.
Slow freeze-up this year was also noted in a recent article here about ice fishing conditions near Kotzebue. The article states that the timing of freeze-up was "once relatively consistent" but has "become sporadic over the past decade or so". I thought it would be interesting to see if there's any evidence of this in the history of early winter freezing degree days in Kotzebue; the chart below shows the annual number of FDDs in September, October, and November. Note that I show a data point only if 2 or less days are missing in the month, and for any missing days I fill in the values with normal temperatures to avoid a bias.
It's plain to see that the October FDDs have decreased rather substantially since 2002, and the November FDDs were also very low last year and this year. We would therefore expect that freeze-up has become later in Kotzebue since 2002, but I'm not sure we can say anything about changing variability of freeze-up dates. If anything I would guess that freeze-up has been more consistently late recently, whereas in earlier decades it appears that it was sometimes late and sometimes very early. A more sophisticated freeze-up model using daily temperatures might reveal more about the nature of the changes.
One other point of interest on the chart is the warmer conditions that are evident in October in the early decades, particularly the 1940s and early 1950s; a very substantial cooling shift occurred in the mid-1950s, and the next 45 years were generally colder than the decades on either end.
Out of curiosity I also created the charts for Barrow and Fairbanks - see below. The recent warming trend is very striking in all 3 months in Barrow, with the 10-year means dropping well below anything observed before; and here we can certainly say that interannual variability has decreased. This year and last year brought slightly higher FDDs in October in Barrow (as we noted here), but there's still some way to go before Barrow might see a "normal" October.
The chart for Fairbanks shows relatively little change over the long-term, although the period since 2002 has been relatively mild on average, and the last three freeze-up seasons have certainly been delayed in comparison to normal.
I've been watching the Nenana Ice Classic cam daily, as well. What I've noticed from observing Alaskan rivers for many years is that once a substantial open hole survives the initial ice formation on a river, it is likely to take a long time to freeze over, even in below zero temperatures. The holes get a vertical ice edge, often quite thick, and slowly fill in horizontally as water freezes to the edge.ReplyDelete
I'm sure the Tanana River has been classified as "frozen" for weeks now, but perhaps with more open holes than usual due to the relatively brevity of the cold snap in November.
Interesting observation, Andy. Thanks. To me it highlights the risk of winter river travel.Delete
This was linked before but it may be worthwhile to renew in the spirit of safe traveling on frozen rivers:ReplyDelete