Thursday, January 11, 2024

December Climate Data

Looking back at December, the dominant circulation feature near Alaska was a pronounced Bering Sea trough, with 500mb heights significantly below normal over western Alaska.  And so it was another wet month for the state - the 8th wettest December on record (1925-present), according to NOAA/NCEI data.  By my count, every month in 2023 except October was wetter than the long-term median for Alaska as a whole, and it was the wettest year statewide since... last year.

December's mid-atmosphere flow pattern was quite reminiscent of a typical El Niño outcome.  The map below shows the average 500mb height anomaly for 10 Decembers with strong El Niño conditions.

Interestingly, this occurred despite dramatically different North Pacific SST patterns - compare the two maps below.  This highlights the fact that it's tropical SSTs (and associated tropical convection) that tend to drive higher latitude circulation anomalies.  Extratropical SSTs have an important influence in terms of setting a baseline for air temperatures locally and downstream, and they can reinforce prevailing weather patterns (e.g. a trough over locally colder ocean regions), but they can't compete with the likes of a strong El Niño in terms of overall circulation forcing.

Having noted El Niño's large-scale signature in the December circulation pattern, however, there were subtle differences.  The Bering Sea trough was a bit farther north than usual, and that created less warm southerly flow and more cold westerly flow into western and southern Alaska.  Compare the air temperature maps below; the southwestern quadrant of Alaska was colder than would be typical of El Niño.

Here are my usual climate rank maps based on ERA5 and NOAA data:

The Panhandle was exceptionally warm, with only a handful of Decembers being warmer in the modern climate history.  The Southeast warmth was merely the western margin of a vast area of extreme warmth across Canada - the warmest December on record in many places.  Canada is used to relatively warm winters during El Niño, but this was off the charts.

Here's the estimated distribution of precipitation anomalies: much wetter than normal in the Southeast and eastern interior, and either wet or dry elsewhere depending on which data source you prefer.  Given the abysmal state of ground-truth precipitation measurements these days in Alaska, the ERA5 model data is probably preferable. 

Wind was below normal for a large part of the state, which is a bit surprising given the proximity of the trough; but it was evidently a rather stagnant pattern, with the storm track and higher winds taking aim at the Panhandle rather than running up into western Alaska.

As for the Arctic, warmth was extreme on the Canadian side.  For 3 of the sites noted below - Resolute, Cambridge Bay, and Kugluktuk - it was the warmest December on record, and these all have more than 50 years of data.  At Cambridge Bay, with 87 years of data, the previous December record was broken by more than 2°C (-20.1°C vs -22.2°C in 1987).

And as for Utqiaġvik, it was the 3rd warmest December on record, behind only 2017 and 2022.

For more details, Rick Thoman's December summary is well worth a read:

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