Cold Snaps in El Niño

The cold snap appears to be reaching its nadir this weekend for the western and northern interior.  This morning's temperatures bottomed out in the -50s Fahrenheit for many locations from as far south as Nikolai up to the Kobuk River valley and eastward to the Yukon Flats.  The coldest spots in Fairbanks-land dropped to near or just below -50°F, although the airport only made it to -40°.

With more than a month having passed since the solstice, afternoon sunshine allowed for temperatures to recover somewhat this afternoon, but less so in the northern interior, and Bettles saw a high temperature of -47°F.  It's fairly late on the calendar for this: the latest date in Bettles with such a cold high temperature is February 3rd (1993).

This morning's 500mb chart from Environment Canada shows the mid-atmosphere situation, with a deep, cold trough over central and northern Alaska.

This afternoon's 500mb height of 4950m on the Fairbanks sounding is the lowest in 7 years.

I mentioned the other day (prompted by Rick Thoman's comment) that really significant cold snaps are relatively uncommon during El Niño winters.  Here's a look at some data to illustrate that point: the chart below shows each winter's lowest weekly (7-day) mean temperature for Fairbanks and Bettles combined (i.e. the average of the two sites).  The colors indicate El Niño or La Niña winters, as defined by the December-January Multivariate ENSO Index being above +0.5 or below -0.5 respectively.

It's clear that for the seven decades as a whole, La Niña winters have tended to produce colder cold spells, although admittedly the evidence is only compelling from about 1980 through 2010.  In the 1950s, 60s, and 70s there were some nasty cold spells with El Niño, and in recent years La Niña hasn't produced anything like the cold that it used to.

The result is similar if we classify years based on the Oceanic Nino Index, which uses only Niño3.4 SSTs as an index of ENSO strength.

Either way, we find a difference of about 5-6°F on average between the coldest weeks in El Niño versus La Niña winters; but there is obviously a lot of variability between winters, and from decade to decade.

The disappearance of the most severe cold in recent decades is remarkable.  Here's a chart showing all winters:

The current cold snap will break the sharp uptrend, but perhaps only marginally; we'll see.

The Fairbanks urban heat island is certainly a factor here, but the trend is only about 20% greater for Fairbanks than it is for Bettles (+1.8°F/decade vs +1.5°F/decade for the coldest weekly temperatures).