Calendar of Cold

It's back to cold(er than normal) in Alaska again, with a brisk -31°F observed at Fairbanks airport this morning, and -40°F in colder spots to the north, including Bettles.  Wind chills are unpleasant on the North Slope too, but as noted by reader Mike, Arctic Alaska is very much still locked in winter.  The seasonal minimum in temperature occurs a lot later for the North Slope - and indeed for many of Alaska's coastal locations - than for the interior.

For a visual perspective on the timing of the seasonal minimum, here's a map based on ERA5 1991-2020 data.

According to this analysis, the seasonal temperature cycle bottoms out before the turn of the year for a few areas in western and interior Alaska, but most of the interior sees its coldest "normals" in the first half of January.  Cold peaks later for the North Slope - the first half of February for the eastern North Slope - and the highly maritime climate of the North Pacific sees a seasonal minimum even later, in late February or even early March.  The pronounced lag over the ice-free Pacific is related to the very large heat capacity of the ocean's upper layers, as well as the ample cloud cover that prevents a strengthening sun from bringing warmth to the ocean surface.

I'm a little surprised by the early seasonal minimum for coastal western Alaska, where I would have expected sea ice to keep temperatures suppressed to a later date.  The ERA5 results are confirmed by the NCEI 1991-2020 daily normals: Nome has its lowest normal temperature around January 12, and Bethel around January 9.  For comparison, Fairbanks also bottoms out around January 12.  I don't know why the west coast doesn't have more of a maritime influence in this aspect of the climate.

But a couple of caveats are worth noting in this analysis.  First, I'm using harmonic functions to smooth the seasonal temperature cycle through the year, and the details of the method (e.g. the number of functions) can affect the timing of the estimated seasonal minimum.  Second, the sample size of 30 years also implies some uncertainty in the date of the minimum, because the random timing of major historical cold and warm episodes will affect the details of the calculated cycle.  It would be interesting to quantify the uncertainty with some statistical experiments.

On a broader scale, the map below shows that the European side of the Arctic Ocean has a later seasonal minimum than the Pacific side: it even extends past March 15 for a small area near Iceland.

And expanding the domain into the mid-latitudes, we see that the eastern North Pacific has the latest seasonal minimum of anywhere in the hemisphere: as late as March 29 according to this analysis.

Another question I can't answer is why the eastern North Pacific has so much more lag than the western North Pacific.  Perhaps it's as simple as the western ocean being influenced by continental air from the vast land mass of Asia, which warms up quickly in late winter.