Saturday, January 28, 2023

Wind Chill

The past week has seen some unpleasant wind chill in many of the typical cold spots of western and Arctic Alaska.  The conditions haven't been particularly extreme by historical standards, but this serves as an opportunity to look at what is unusual or extreme according to local climatology.

Let's start with Bethel, where strong cold outflow from the interior quite often brings notable wind chill.  On Monday morning the temperature was -15°F with a sustained northeast wind of 35mph (see below), which is good for a wind chill of -47°F.  This is just a few degrees shy of what's typical for the lowest wind chill of the winter (around -50°F).  The average wind chill for the duration of Monday in Bethel was -43°F, and a day like this has occurred in about two-thirds of winters in recent decades (although not at all from winters 2012/13 through 2015/16).

Based on the 1991-2020 history, a wind chill of -40° occurs about 5% of the time in Bethel in December through February, and the wind chill is below -50°F about 1% of the time on average.  For its latitude and (sea level) elevation, Bethel is remarkably prone to very low wind chill.

The Arctic coast had similar wind chill yesterday, although there it was much less unusual.  Barter Island measured -55°F for wind chill in the afternoon on a stiff easterly breeze:

Up at Barter Island, the winter's worst wind chill is typically a full 10°F lower, at -63°F, based on the 1991-2020 history.  Remarkably, since 1991 the location has spent about 2% of the time in January with a wind chill below -60°F, although the majority of this time occurred in just a few severe and prolonged events; some winters do not see a wind chill of -60°F.

To summarize the low wind chill climate of the exposed west and north coast of Alaska, the figures below show the 10% and 1% levels of hourly wind chill for Bethel, Nome, Kotzebue, Utqiaġvik, and Barter Island.  The 10% level might be considered a threshold of "unusual" low wind chill, and occurs nearly every winter, whereas the 1% level represents a more extreme event that occurs in less than half of winters.

It's interesting to note that Nome is relatively sheltered from the northerly winds that would otherwise bring the lowest wind chill, and so it has easily the least extreme wind chill climate of these locations.  Also, the prolonged winter of the Arctic coast is evident, as low wind chill hangs on into March and even April for Utqiaġvik and Barter Island.  Readers will no doubt spot other interesting aspects - feel free to leave a comment.

Tuesday, January 24, 2023

Cloud Cover Trends

Continuing with the topic of cloud cover in Alaska, reader Gary asked about the 1976/77 Pacific climate regime change, noting that Hartmann and Wendler (hereafter "HW") showed an increase in average cloudiness for Alaska, particularly in winter.  The results in that study were derived from ground-level observations at 19 climate measuring sites across the state, and with data used only through 2001, this was mostly pre-ASOS and therefore presumably quite reliable (i.e. manual human observations).

Here's the key table from the study, showing the percent change in mean cloudiness from 1951-75 to 1977-2001.


It's worth doing the same calculation with ERA5 data, to see if the reanalysis captures broadly similar changes.  We would hope so, because I've been relying on the ERA5 reanalysis data for its depiction of Alaska's cloud climate.

Here are the results for a few of the NOAA/NCEI climate divisions; I didn't attempt to reproduce HW's six climate regions, which are larger than the 13 NCEI climate divisions.

There is considerable similarity in the results for winter, with ERA5 also showing a large increase in cloud cover, and particularly so for south-central and western Alaska.  However, for the rest of the year, the more modest changes do not align well between ERA5 and the station data, and in particular ERA5 shows an increase in summer cloud cover (except the North Slope) versus HW's decrease.  ERA5 also suggests more of an increase in autumn cloudiness.

Can we identify the 1976 regime shift in the ERA5 time series?  Looking at the statewide average for DJF, the answer is actually no - we see very high cloud cover in the winter of 1976-77, but then it dropped back down for several more years.  However, ERA5 does seem to show a regime shift in 1984-85.

The West Coast division has the most statistically significant 1950-2022 trend in DJF cloud cover, and the mid-80s regime change seems quite striking here:

Interestingly, HW identified the sudden 1976 increase in Alaska's temperatures and the simultaneous PDO phase shift from negative to positive, but they didn't document a sudden change in cloud cover - they just looked at the pre/post 1976 differences.  According to the ERA5 data, there is more going on with cloud cover than just the PDO phase change.

A possible explanation for the discrepancy between ERA5 and the HW results is the (perhaps likely) possibility that ERA5 "total cloud cover" doesn't correspond closely to what human observers report from the ground.  Also, there are considerable differences in the trends for ERA5 "low cloud" (below about 2km above ground) versus "high cloud" (above about 6km); for example, the North Slope has a decrease in summer low cloud but an increase in summer high cloud.

Most of the state has less of an increase in "low cloud" cover, so perhaps the human observations correspond more to the low-level clouds in the model.

The difference in low versus high cloud trends is really striking for spring, with a big decrease in low cloud for the interior according to ERA5:

It seems possible that this is related to earlier snowpack meltout in the spring, perhaps because low clouds are easier to disperse over bare ground (more solar absorption at the ground and stronger daytime warming in the boundary layer).

On an annual basis, both HW and ERA5 point to a modest increase in cloud cover, driven largely by the winter changes, and concentrated in south-central and western Alaska.

Here are the "total cloud cover" trend maps for the four seasons.

Tuesday, January 17, 2023

Arctic Temperature Update

As promised, here's some follow-up information on Arctic temperatures last year.  For the 32 sites that I regularly look at, the 2022 annual average temperature was slightly less than 1°C above the 1991-2020 reference period.  The warmth was widespread, but was most pronounced in the western half of the Russian Arctic, with some sites more than 2°C above normal for the year.

Only two of the 32 sites had an annual temperature below the modern normal, with Kotzebue being the coolest - and that's partly because 2014-2019 were so incredibly warm there (thus contributing to a higher "normal").

According to this data, 2022 was warmer than 2021 overall in the Arctic, but not as warm as 2019 and 2020; and 2016 remains the warmest year on record.  Below are the temperature anomaly maps from prior years since 2015.


The jump up from 2015 to 2016 is attributable to the super-El Niño of winter 2015-2016, as global average temperatures also jumped up at that time.  Remarkably, every one of the 32 sites was warmer than the modern normal in 2016, the only year that's happened.  Here's the chart view of monthly anomalies since 2010.

The slight reduction in overall warmth in the last two years is probably also related to ENSO: the tropical Pacific has been in the grip of La Niña for two and a half years now.  However, a standardized view of the monthly anomalies shows that there have been some extreme spikes in monthly temperature at some sites in the past two years - see below.

 A longer-term view:

It's going to be "interesting" to see what happens when the next El Niño arrives; it could be later this year, judging from the latest seasonal model guidance and expert opinion at NOAA.

Friday, January 13, 2023

December and Annual Climate Data

Taking a look back at December now that climate data is in, we see that it was an extremely warm month relative to normal for the North Slope and northwestern Alaska.  According to NCEI data, the North Slope climate division was essentially tied with several other years for third warmest December on record, trailing only 2002 and the incredibly warm December of 2017.  Utqiaġvik was actually within 1°F of the 2017 record, and no other December comes close to these two years (2017 and 2022).  It's no surprise after seeing the all-time winter high temperature record broken on the 5th of the month in Utqiaġvik.

The Aleutians were also very warm compared to normal, but widespread cold occurred in southeastern Alaska.  Interestingly the same contrast occurred across southern Alaska last December, and it was even more pronounced then, with the Aleutians being even warmer and the Panhandle being colder.

There are some substantial discrepancies between the NCEI and ERA5 precipitation data for the month, so take your pick on which seems more realistic below.  Unusually wet/snowy in the eastern interior and unusually dry in the Panhandle seem to be the only features with strong agreement.

All in all, the month's departures from normal were broadly similar to those of November, and that's because the upper-level circulation pattern was somewhat alike.  As in November, a mid-atmosphere ridge of high pressure prevailed over the Bering Sea and western Alaska, but this time it extended up into the Arctic in connection with a strongly negative Arctic Oscillation.  This high-latitude "blocking" pattern allowed unusual cold to drop south into the mid-latitudes, and northern Alaska paid the price, so to speak, with an excess of counterbalancing warmth.

The rather pronounced temperature gradient from west to east across the state was associated with above-normal wind for the west coast and much of the interior, according to ERA5.

On a statewide annual basis, 2022 was warmer than both 2020 and 2021, but quite a bit cooler than some recent years like 2016, 2018, and (the record holder) 2019.  The most unusual month for temperature was June (sixth warmest on record), and none of the months was notably cold.

There was much more of a story with precipitation, as discussed here, for example.  Overall it was the 4th wettest year in the NCEI data for Alaska (1925-present), and the wettest since 1943.  For the Bristol Bay and Southeast Interior divisions, it was the wettest year on record - and by some margin for the Southeast Interior.

It's interesting, then, that Fairbanks airport actually had less than normal precipitation in 2022, and much less than any of the prior 8 years.  The moisture contrast was very striking from south to north across eastern Alaska during the summer (see here).

For completeness, here's the annual wind speed ranking: much above normal for western Alaska, and few places below normal.

Looking farther afield at temperatures across the Arctic, December saw some big contrasts.  The northern North Atlantic near and to the east and north of Iceland saw exceptionally cold conditions because of the high pressure over Greenland; Iceland had its coldest December since 1973 (coldest since 1916 in Reykjavik!), and Jan Mayen was the coldest since 1996.

On the other hand, December was the warmest on record at Russia's Anderma site (data since 1934) - it was a whopping 9.4°C above the 1991-2020 normal.  And as already noted, Utqiaġvik had its second warmest December on record.

Here's the map in terms of standard deviations.

I'm out of time for this post, but I'll follow up later on the annual temperature for these Arctic sites.

Sunday, January 8, 2023

More on Cloud Cover Climate

Following up on the climate of cloud cover in Alaska - in response to a reader's question - I created some maps to illustrate the relationship between the large-scale weather patterns and cloud cover.  I'm relying on the gridded ERA5 reanalysis data for its ease of use, and I make no claims about accuracy, but it's considered a state-of-the-art system that should be suitable for this kind of broad-brush analysis.

With Alaska being a big place and having tremendously different cloud climates across the state, I extracted the ERA5 cloud cover data for the NOAA/NCEI climate divisions (shown below); this allows us to look at the different regions separately.

For example, here's the 500mb height pattern that tends to give the cloudiest conditions for the Southeast Interior region in January.  This is simply a map of the average departure from normal of January 500mb height in the 8 years since 1950 that had the highest January cloud cover.

For those who are not familiar, the 500mb height is equivalent to the pressure at middle levels of the atmosphere, so relatively high heights represent a mid-level high pressure system (a "ridge"), whereas low heights correspond to a low pressure system (a "trough").  Alternatively we could look at sea-level pressure maps, but I find the 500mb height maps usually show more coherent patterns.

In this example, the map is telling us that a stronger-than-normal trough over the Aleutians and a stronger-than-normal ridge over British Columbia is a favorable combination for high cloudiness in the southeast interior of Alaska in January.  With a tendency for counter-clockwise circulation around the trough, and clockwise around the ridge, this pattern tends to bring warm, moist, and therefore cloudy air up from the south into interior Alaska.  The pattern also reflects a jet stream that is directed into Alaska from the southwest, importing lots of Pacific cloud.

The years with the least January cloud cover tend to have a flow pattern that is nearly opposite, with a trough over British Columbia and a ridge from the Aleutians northward into the Arctic.  The associated circulation brings cold, dry, and therefore relatively cloudless air down from the Arctic into the southeast interior.

The cloud cover variations are not just about humidity, however.  It's important to note that the circulation also generates patterns of rising or sinking air that generate or suppress cloud cover respectively.  In the "least cloud cover" example above, the ridge produces sinking air to its east, which lowers the relative humidity of the air and suppresses cloud cover over eastern Alaska.

The map below shows the 500mb height pattern for above-normal cloud cover in July in the Southeast Interior division.  It's interesting to see that the cloud-producing trough is much farther north than in the winter.  In summer, the southeast interior sees its cloudiest weather with strong westerly flow and frequent rainy disturbances, whereas cloud in the winter is more about the orientation of the jet stream over the North Pacific.


As for low cloud cover in July, it's commonly associated with high pressure centered to the north, producing a tendency for dry, easterly flow from Canada into Alaska; and the ridge of high pressure itself suppresses cloud and rain.  This would be not only a very sunny pattern, but a very dry one.

Let's look at some other regions.  The results are quite similar for the West Coast climate division as for the Southeast Interior - see below.  The main differences are that the winter high-cloud pattern tends to involve a trough extending farther north into the Chukchi Sea, and in summer high cloud cover is associated with a Chukchi Sea trough rather than a broad trough over northern Alaska.

How about the North Slope climate division?  Not surprisingly, the winter circulation signals are shifted farther north again.  But unlike in the regions farther south, low cloud cover in winter is associated with a ridge to the north rather than a ridge to the west.  It's also interesting to note that the winter and summer low-cloud-cover patterns are similar to each other for the North Slope; and the summer "most cloudy" pattern is nearly the same as that for the Southeast Interior, although only 2 of the 8 cloudiest years respectively are the same.


And one more, results for the Central Panhandle:

The winter patterns are akin to those in the other regions, but summer cloud cover variations are more closely linked to the height pattern in the local vicinity - over and just to the west of the Panhandle - than for other regions.  Clearly the spatial scale of the mid-atmosphere flow anomalies is smaller in association with summer cloud than for winter cloud, and while this is also true for the other regions, it seems most notable for the Panhandle.

Finally, it's worth remembering that these patterns are focused on the ERA5 total cloud cover, which includes cloud at any height above ground.  The data set also includes cloud at low, medium, and high levels, and of course these are all influenced differently by the circulation anomalies.

Just as one example, here are the January patterns that typically produce the least cloud at low levels (below about 2km above ground) and high levels (above about 6km) respectively for the Southeast Interior.  According to ERA5, the Bering Sea ridge that we noted before is more associated with reduced high cloud than reduced low cloud.


There are equally significant differences in the summer "least cloud" patterns for low versus high levels of the atmosphere:


But interestingly the cloudy-sky patterns are much more similar to each other for low and high clouds, in both summer and winter.  I suppose that the cloudiest weeks and months tend to be cloudy at all levels of the atmosphere, owing to unusually disturbed weather, whereas it's quite possible to have low-level clear skies marred by high cloud, or vice versa.  This is just for the Southeast Interior, however.

If anyone is interested in acquiring the ERA5 monthly cloud data, I'd be happy to pass it on.  And if there are any suggestions for further analysis, feel free to leave them in the comments.