Fairbanks saw its first snowflakes of the season today, just a few days earlier than normal. Temperatures were above freezing and no accumulation occurred in town, but the scene looked a lot like "late autumn" in the hills. Here are a couple of webcam views from Cleary Summit on the Steese Highway shortly after 9am today:
On another topic, reader AlexG asked whether this summer's extreme North Pacific warmth is likely to have an effect on winter in Alaska. It just so happens that I gave a talk related to this question 10 years ago at UAF, and here's a link to the slides from that occasion:
https://s2s.worldclimateservice.com/wcs/uaf_sep2015_north_pacific_sst_blob.pdf
The winters of 2013-14 and 2014-15 both saw a tremendous amount of warmth in the northeastern Pacific, and that sea surface temperature anomaly became known colloquially as the North Pacific "blob". In a 2015 paper, Dennis Hartmann of the University of Washington discussed a mode of natural variability - the North Pacific Mode - that resembled the "blob" pattern, and my 2015 talk looked into its connection to Alaska winter climate.
The bottom line from this work of a decade ago is that a positive North Pacific Mode (warm ocean) is linked to unusual winter warmth in most of Alaska except the Southeast, and it also favors wet rather than dry conditions in most areas. The link with winter precipitation is strong in interior Alaska, because a positive NPM is associated with stronger westerly flow and less chinook flow from the south.
The other major mode of North Pacific temperature variability is of course the Pacific Decadal Oscillation (PDO), which is also closely linked to precipitation across interior Alaska and to temperatures statewide. A negative PDO phase, which is what we have at the moment, tends to bring wetter winters to the interior (like the +NPM) but colder temperatures statewide (generally opposite to the +NPM).
Here are the canonical sea surface temperature (SST) patterns associated with positive NPM and PDO phases:
Note that the PDO and NPM are independent of each other, so a positive NPM can readily coincide with a negative PDO, as this year. Also, a negative PDO doesn't necessarily imply cooler than normal SSTs in the northeastern Pacific; the mode expresses the spatial contrasts/gradients in SST rather than the absolute values. This year's negative PDO phase has been very much driven by extreme warmth near and east of Japan, rather than cool water along the coast of North America.
If we look at previous winters with the +NPM/-PDO combination in the North Pacific, we see - as expected - a strong indication of above-normal precipitation across most of Alaska to the north of the southern coastal regions (where it tends to be dry), and there's also a fairly robust cold signal that apparently reflects the dominance of the PDO influence. The fundamental circulation anomaly involves unusual high pressure (or equivalently, less low pressure) over the Aleutians, producing a stronger component of westerly flow across central Alaska.
Of course we don't yet know whether the North Pacific SST patterns will remain similar until winter and throughout winter, and indeed there's a good chance they won't. It's therefore worth examining past years when the +NPM/-PDO combination occurred in summer, rather than assuming it will persist unto winter. Here's the result:
Interestingly, the precipitation signal is very similar, but the cold signal is much weaker in the summer "analog". This seems to be because several of the summer matching years ended up with El Niño and/or a near-neutral PDO phase during the subsequent winter, thereby changing the remote influences on the circulation patterns near Alaska. In contrast, most of the winter +NPM/-PDO matches involved La Niña during winter, reinforcing the cold signal.
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