Wednesday, August 25, 2021

July Climate Anomalies

Regrettably I've been hard-pressed to find time for posts lately, but I couldn't help noticing that July saw another discrepancy between competing estimates of the month's North Slope temperatures.  Last month I pointed out that the NOAA/NCEI climate division data showed below-normal temperatures in the North Slope division for June, whereas the ERA5 gridded reanalysis was distinctly warmer than normal.  The same thing happened for July - click to enlarge:

According to NCEI, the North Slope division saw the 6th coolest July among the prior 30 years, but most of the area was warmer than normal in the ERA5 data.  Rick Thoman's excellent summary of station data again supports the warmer view:

Judging from the published documentation of NCEI's methods, the only inland stations contributing to the North Slope climate division are the RAWS sites at Umiat and Noatak - the latter being in the broad upland Noatak River valley in the western Brooks Range.

Here's another way of looking at the rank of the July temperatures for these two sites, as well as Utqiaġvik and the ERA5 and NCEI regional averages.  The Umiat RAWS period of record only extends back to 2008, but even so it's clear that July was significantly warmer than normal there.  Utqiaġvik was near-normal.  However, the Noatak site was definitely cool: 8th coolest out of 28 complete July values since 1991.

The long-term trends in the NCEI data also look a bit suspect: the North Slope division shows no warming in July since about the mid-1980s, whereas Utqiaġvik has a considerable warming trend, and ERA5 also shows warming for the regional area average.  Interestingly Noatak has not warmed in July over the last 30 years, but we wouldn't expect that one site to dominate the NCEI divisional values.

In short, there seems to be something awry with the NCEI data, and it would be nice to diagnose and help fix the problem.

Below are the ERA5 July climate ranks for other variables, with NCEI's estimate thrown in for precipitation.  It was extremely wet, cloudy, and windy for the time of year in most of the west, but sunny and dry weather in the eastern interior led to significant soil moisture deficits.  It's a bit of a surprise that wildfire activity wasn't worse; I'll put together a seasonal summary on that in the coming days.

Tuesday, August 17, 2021

August Chill

Ten days ago I was writing about the hottest weather of the summer for interior and western Alaska, but a dramatic reversal has suddenly produced unseasonably chilly conditions instead.  Widespread significant rain occurred again from the west coast to the White Mountains, and the scene has turned wintry in the Brooks Range: Atigun Pass is now reporting about 6" of snow on the ground.

Here's the FAA webcam view from Chandalar Shelf (elevation 3285') this morning:

With cloud and rain holding daytime temperatures down, Fairbanks has seen the chilliest couple of days for this early in the season since August 2002.

Interestingly the late summer of 2002 evolved quite similarly, with some distinctly warm weather in the second half of July and the beginning of August, followed by below-normal temperatures.  Both years had a significant rain in Fairbanks around August 7-9, then heavy rain around the 16th.

With more than an inch of rain in Fairbanks since Sunday morning (and nearly continuous too), the summer's rainfall total is now above 5.5" and above the long-term normal, for the 8th consecutive year.  I wrote about this remarkable trend in a few posts last year, for example:

For a broader look at why August tends to be wet, see the 2018 post below.  The short answer is that the rapid loss of solar heating over the Arctic is already causing substantial strengthening of the jet stream, leading to more vigorous large-scale weather disturbances, and there's still a lot of moisture available in the atmosphere to produce copious rain across much of the high latitudes.

Friday, August 13, 2021

Rain on Permafrost

Reader Gary posted a comment about possible ocean impacts of excess water runoff from this summer's heavy rainfall over western Alaska, and while I don't have any insight on that question, it got me thinking about effects on permafrost.  Back in March I commented on a National Park Service study showing that permafrost warming has been extreme in northwestern Alaska, and it seems likely that excessive rainfall can only worsen the situation.  Here's a different study, also published this year, that reaches the striking conclusion that "precipitation is as important an environmental control on permafrost degradation as surface air temperature":

I'll be the first to admit I don't know much about permafrost physics, but a back-of-the-envelope estimate seems to confirm the significance of the issue.  For example, if 10 inches of rain falls at an air temperature of 10°C, and all the water percolates into the subsurface, this makes available about 10 MJ of energy per square meter for warming or thawing near-freezing soils (based on the heat capacity of rain water: 250 kg x 10°C x 4200 J/kg/°C).

If we're talking about simply melting ice, this amount of heat could melt 30 kg of ice per square meter, which is only about 3cm depth of pure ice.  However, if the heat goes only into warming the soil, then 1°C of warming could occur through as much as 5m or more of the ground - based on the heat capacity and density of "icy peat" that I found here.  I'll refrain from any further guesswork, but it seems the effect could be significant.

Here's a paper - apparently under review - that argues the same thing:

On the other hand, another study focused on Tibet finds the opposite, with enhanced rainfall being associated with a decrease in heat conduction (presumably because rainy weather is also cooler and cloudier).  Looks like there is scope for further research.

Saturday, August 7, 2021

Heat Wave

By and large, August typically brings cooler, cloudier, and wetter weather to interior and northern Alaska as summer fades quickly, but the start of this month has seen very unusual heat across the interior and west coast.  It has been the hottest spell of the summer.

Fairbanks reached 89°F at the airport on Tuesday, the highest August temperature since 2010, and the daily mean temperature was the highest on record for August, owing to a very warm daily minimum temperature of 64°F.  And it wasn't just a one-day heat wave: four consecutive days reached 84°F, and this too is unprecedented in August (looking at data since 1930).

Out on the west coast, the heat was even more unusual relative to climate normals: Kotzebue reached 80°F on Wednesday, which ties the record for the month of August.  Like Fairbanks, the peak daily mean temperature of 70°F set a new record for August.

Here's a map view that shows the evolving distribution of daily high temperatures: click to enlarge.  (But note that the thermometer at Kivalina had a problem - it wasn't 90°F there on Tuesday.)

Here's the mid-atmosphere (500mb) circulation that brought the amazing warmth to the west on Wednesday (4pm analysis courtesy of Environment Canada):

Tuesday, August 3, 2021

Extreme July Rainfall

Following up on last week's post, recent rainfall has been prodigious in western and northwestern Alaska, leading to the wettest calendar month of record in Kotzebue (5.32" versus 5.26" in August 1998) and easily the wettest July on record (old record 4.16" in 1931).  Nome surpassed the very wet July of 2012, with the wettest July since 1920 (6.41" vs 8.43").

Note, however, that the 30-day rainfall record in Kotzebue is 6.54", which occurred mostly in August 1946.  Remarkably, Nome has twice seen over 10" of rain in 30 days: in 1922 and 1954.

The rainfall since late June has been most excessive in the far western Brooks Range; check out this accumulated precipitation chart from the Red Dog CRN site, at about 1000' elevation and 80 miles north of Kotzebue.  The site has seen over 12" of rain since late June, and the June-July total was an astonishing 14.5".

The nearby Kelly Station SNOTEL instrument (~20 miles SE of the CRN site) has measured over 10" since late June:

Much farther to the south, the Rocky Point SNOTEL on the southern coast of the Seward Peninsula (not far from White Mountain) has seen 9" in the same period:

For the record, and following on from the last post, here are the NWS rainfall estimates from the past week:

Looking more broadly afield, the west-east contrast in July rainfall across Alaska was very striking; here is Rick Thoman's summary graphic, via Twitter:

The monthly-mean circulation pattern involved an unusually strong trough of low pressure from the Chukchi Sea down to the western Bering Sea, but a ridge over southern Alaska kept jet stream disturbances away from the eastern and southeastern parts of the state.

Not surprisingly, temperatures were quite cool in the west, but were generally warmer than normal in the Arctic, eastern interior, southeast, and Alaska Peninsula.