Sunday, January 23, 2022

Warmest and Wettest in Winter

Today's post is mostly to share some new results pertaining to the Christmas storm, but first a note on recent cold in northwestern Alaska.

Kotzebue reached -40°F or lower on three consecutive days from Tuesday through Thursday last week, with -44°F on the first two nights.  This might not seem too remarkable, but it's the coldest there since February 2012; Kotzebue has had nine straight winters without reaching -40°, and two of them (2013-14 and 2015-16) didn't even reach -30°F.  Prior to 2000, -40° was reached in more than half of all winters.

The recent cold is a brief reprieve from the extraordinary warming that has affected northwestern Alaska in the last decade, particularly in winter.  The change is documented in last year's paper by David Swanson, Pam Sousanes and Ken Hill, as discussed in this post:

And now for some more follow-up on the extreme conditions that occurred in southwestern and interior Alaska just after Christmas.  As a direct result of the extraordinarily powerful ridge to the south of Alaska, incredible warmth and moisture were pumped up into the state.  Rick Thoman highlighted some of the extremes that were observed in the balloon sounding data, such as a freezing level of 9600 feet at Bethel and extremely high precipitable water (i.e. total column water vapor) at Fairbanks.  Many records were broken for the time of year; but how widespread was the record warmth and moisture?

To answer this, I used ERA5 reanalysis data back to 1950 and calculated the area over which the December storm brought the highest 1000-500mb thickness on record for deep winter (December-February).  The 1000-500mb thickness measures the average temperature of the bottom half of the atmosphere, and is an excellent indication of unusual warmth or cold over that deep layer.  Here's the result of the calculation for December 26, when record warmth was most widespread:

The darkest red indicates that the maximum 1000-500mb thickness on December 26 exceeded the peak value observed in December through February prior to this event (by "observed", I mean estimated by the reanalysis, which is very reliable for things like this).  The medium red shading indicates the highest thickness since at least 1979, and the light red shows where the thickness was the highest since at least 2000.

By this objective measure, then, the post-Christmas air mass that reached southwestern Alaska was the warmest on record (1950-present) for the deep winter months of December-February.  As a check on this result, I looked at the sounding data from Cold Bay (data since 1946), and indeed the 1000-500mb thickness on December 26 was well above the previous winter record from, coincidentally, Christmas Eve 1989.  Record warmth did not make it far into the interior, however.  For example, the highest Dec-Feb thickness at Fairbanks still stands, from January 26, 2014.

Here's the sequence of maps showing the extent of record high 1000-500mb thickness by day.

How about deep-layer moisture, i.e. precipitable water?  I did the same analysis using ERA5.  Again the most dramatic extent of record conditions was on December 26, and here records certainly were broken in the interior:

Looking again at Fairbanks sounding data, the December-February precipitable water record was broken by nearly 10%, with the previous record being on February 3, 1982.  Here's the daily sequence of record extent:

It would be interesting to take a look at other reanalysis products that extend farther back in time, including for example January 1937, although of course the uncertainty associated with those earlier estimates is very large indeed because of the lack of upper-air observations.

If readers have suggestions for any other ways to slice and dice the ERA5 data to analyze the recent extremes, I'm all ears.

And by the way, it's worth noting that the extreme cold that developed over the eastern interior at the beginning of this month did not set any records for lowest 1000-500mb thickness; it wasn't even the coldest since 2000, according to ERA5.  The cold outbreak did, however, bring the coldest 850mb temperatures since February 1999 in some areas:

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