Summer Climate Anomalies

Looking back at climate anomalies for June through August, the summer as a whole was wetter and slightly cooler than the 1991-2020 normal statewide - but with lots of variability in both time and space.  Here are my usual percentile rank maps based on ERA5 and NCEI data:

The contrast between east and west was substantial, especially for precipitation, and of course there was a dramatic difference between June and the rest of summer: June was very warm and dry, but it was followed by the wettest July on record statewide.  August continued the wet theme in the west and north, and was very cool in the southwest, but it was dry, sunny, and quite warm in Southeast Alaska:

All in all, it's notable that the North Slope climate division had its second wettest summer on record (1963 was wetter), and the West Coast and Central Interior divisions were third wettest; this is more remarkable after the dry start in June.

June-August temperatures were not especially notable anywhere owing to the big sub-seasonal contrasts.  The Bristol Bay climate division did have its coolest summer since 2012, but that's not saying a whole lot, because the past decade has been so warm:

It's quite interesting to inspect the time variability of temperatures during the season, courtesy of UAF's statewide temperature index.  Back in June, which ended up being much warmer than normal, the statewide temperature varied on a time scale of a few days, and without a great deal of volatility; but the generally cool pattern in July and August was punctuated by some enormous swings to the warm side: 

The big reversals into highly anomalous warmth around July 20-25 and August 4-8 prevented those months, or late summer as a whole, from being truly notable for a statewide chill.  The average temperature for July and August combined was again the coolest since 2012, but it wasn't a whole lot different from 2021 and 2022, and prior to 2013 it wouldn't have been unusual at all.

To round out the picture for the summer climate anomalies, here are the wind and sunshine maps:

The wind anomaly is really remarkable: according to the ERA5 reanalysis, the June-August average 10m wind speed was the highest since at least 1940 (the start of the dataset) over quite a wide area, including much of Norton Sound and inland to places like Kaltag and the lower Yukon valley down to the coast.  I'll see if I can illustrate this in more detail in a later post.

Cool Bering/Chukchi Sea

I've mentioned the PDO (Pacific Decadal Oscillation) a few times in recent months, and late August brought a fresh dive into the strongly negative phase.  This reflects an expansion of below-normal sea surface temperature anomalies across the Bering Sea in response to the strong and persistent trough near the Date Line in late summer.

The very pronounced trough delivered very cool Arctic air from Chukotka across the northern Bering Sea to western Alaska, and very cloudy and windy conditions across the eastern Bering Sea really reinforced the cool ocean anomaly.  Here are some climate anomaly maps for the month of August:

As a result, the sea surface waters in the eastern Bering Sea and the Chukchi Sea became the most anomalously cool of any ocean area in the world by the end of August:

Here's a time series of SST anomalies since 2018 for the Bering/Chukchi area east of the Date Line and north of 56°N (i.e. Bristol Bay northward).

While the cool anomaly is now quite notable, it is nowhere near as pronounced as the extreme warmth in 2019, at least when assessing conditions relative to a prior multi-decadal normal.  Notice that most of the major anomalies occur in the summer and autumn, when sea ice is absent and the near-surface ocean layer can become much cooler or (especially) much warmer than normal.

A longer-term chart puts the last few years in context.  The current cool anomaly is among the most pronounced in the modern (satellite sensing) era, although we haven't quite reached the level of the 2011-2012 negative PDO extreme.

Follow-up on Sleet

First things first: overnight freezes are becoming more widespread across the northern interior, as befits the season.  Here is this morning's list of some observed low temperatures, courtesy of the NWS in Fairbanks:

Webcam views show a delightful scene in many areas today:

Cleary Summit above Fairbanks

Central

Fort Yukon

Bettles

On another topic, Mike from Kaktovik posted a few weeks ago about sleet (ice pellets) falling in late July at his location on Barter Island.  I was curious to see what the ERA5 reanalysis has to say about the frequency and distribution of this kind of "wintry" precipitation.  According to the model, it is not particularly unusual for sleet to be the dominant precipitation type across the Arctic waters to the north of Alaska in climatological summer (June through August):

(It should be noted that the model diagnosis of a certain precipitation type does not imply that such precipitation actually occurred; the modeled precipitation rate might be extremely small at times.  A more comprehensive analysis would look at the precipitation rate as well.)

Not surprisingly, the estimated frequency of summertime sleet is drastically lower over land, but a close look at the image above reveals that the "offshore" region of higher frequency also extends over Barter Island (on the north side of the river delta region protruding north on the far northeastern coast).  ERA5 shows a 1991-2020 mean of about 50 hours per summer with sleet as the dominant precipitation type (subject to precipitation rate) at Kaktovik.

Here's a chart of the annual June-August frequency of the six ERA5 precipitation type categories at a grid cell location very close to Kaktovik:

The long-term increase in the dominance of rain reflects the multi-decadal warming trend.  Similar trends are seen if we take the area-average over a box encompassing the northern North Slope and nearby waters: all "wintry" precipitation types have decreased in frequency in recent years.

Interestingly, the same is not true over the Arctic Ocean farther to the north of Alaska: the ERA5 data suggests that snow and other icy precipitation types have become more common at the expense of rain in recent years:

Here are the annual values for a 74-78°N area-average well to the north of Alaska:

There seems to have been a notable change in 2007, which was a very big melt year for Arctic sea ice.  It's not clear to me why more open ocean would produce more, not less, wintry precipitation in summer; more investigation of the model temperature profiles would be required to explain what's going on here.