Cold Descends - Thank La Niña

It's been a very cold week for the time of year in western and southern Alaska, and the eastern interior is now seeing its first real cold of the season, with an impressive -44°F at Northway this Saturday morning.  Chicken also saw -44°F last night.  Here are a few of the coldest readings in the past several days:

Tuesday Nov 16

Bettles   -35°F

Wiseman   -35°F

Denali Visitor Center   -28°F

Wednesday Nov 17

Norutak Lake RAWS   -32°F

Kaltag   -31°F

Unalakleet -31°F

Thursday Nov 18

Selawik 28E  -31°F

Ruby 44ESE  -29°F

Friday Nov 19

North Pole 1N  -34°F

Tok #2  -34°F

Wednesday morning's -31°F at Unalakleet on the Bering Sea coast is really impressive, with an easterly breeze making it feel even colder.  Unalakleet climate data is patchy after 1977, and non-existent after 1997, but there is good data from 1944-1977, and the earliest -30°F in all those years was November 22.

The upper-air observations from Fairbanks reported an 850mb temperature of -25.3°C on Wednesday morning, which is the earliest -25°C at 850mb since the remarkable pre-Thanksgiving cold snap of 2011.  On that occasion, Fairbanks saw -41°F on the 17th, and the 850mb temperature reached -30°C on the 21st.  Read about it here and in other posts from that month.

In terms of departure from normal, the greatest temperature anomalies have been in the southwest.  Here's a simple animation of approximate daily departures from normal in the last week (based on model data):

The southwestern mainland is where cold is most reliably seen in Alaska during strong La Niña winters, as illustrated by the map below; this shows that all of the 10 strongest La Niña's since 1950 produced below-trend December-February temperatures in southwestern mainland Alaska.

The intensity of the current La Niña is admittedly up for debate, because it depends on how intensity is defined, but these days I prefer to watch the Multivariate ENSO Index, which uses five different variables to measure the phase of ENSO.  The map above shows results for the 10 most negative MEI values in November through March, and the current MEI is running at about 7th most negative since 1950.

Why does La Niña tend to bring cold to Alaska?  The answer is that La Niña involves a westward shift in the semi-permanent zone of frequent heavy rainfall over the western equatorial Pacific and the Maritime Continent, and this has predictable impacts on the atmospheric circulation at higher latitudes over the North Pacific.

The two maps below show the 200mb (jet stream level) wind during strong La Niña winters (top) and strong El Niño winters (bottom).  While the jet stream maximum near Japan remains basically the same - the basic north-south temperature gradient doesn't change that much - there's a major difference in how far east the jet extends, and this is linked to the longitude of peak tropical rainfall (westward for La Niña, eastward for El Niño).  In particular, the jet stream is stronger across the central and eastern North Pacific during El Niño.

The jet stream differences are connected to subtle but important shifts in the average circulation near Alaska: compare the 500mb maps below (mid-atmosphere level, La Niña top, El Niño bottom).  There's always a trough over northeastern Asia on average during winter because of the basic temperature gradient (it's always cold in winter in Siberia), but La Niña keeps the trough focused near the Sea of Okhotsk, while El Niño brings the trough farther east and strengthens it in association with the stronger North Pacific jet stream.  

The difference between the two states is easier to see if we look at anomaly maps, i.e. departure from normal:

During La Niña, the tendency for above-normal height (pressure) near the Aleutians - which is equivalent to a weakening of the usual trough - means that cold northerly flow has a better chance of reaching Alaska.  The average flow aloft still has a southerly component (which is why Alaska isn't as cold as it could be in winter, considering its latitude), but the southerly component is lessened compared to normal.  The maps below show average wind at 500mb (La Niña top, El Niño bottom)

Also, and I believe this is significant, La Niña's weaker trough means a higher frequency of clear skies and calm winds, which strongly promotes surface cooling; I suspect Alaska gets more cooling out of La Niña that we would expect from upper-air temperatures alone.  It would be worth examining this idea a bit more for a subsequent post.  Here's the average MSLP in the two contrasting patterns: lots more storminess from the Aleutians to the Gulf of Alaska in El Niño winters (bottom).

As an aside, El Niño's impacts on Alaska aren't the opposite of La Niña's effects; the climate response is non-linear and asymmetrical.  Unusual warmth in El Niño is traditionally confined to the southeastern half of Alaska (see below), although the last major El Niño in 2015-16 was an exception, with extraordinary warmth across most of the state.