Monday, December 28, 2020

Decadal Warming

The other day Rick Thoman published an interesting comparison between average temperatures in the past decade and the 1981-2010 normals, for Anchorage, Fairbanks, and Utqiaġvik.  On average, the decade has been warmer than the prior 30-year baseline throughout the year for all three cities, but late autumn really stands out as having seen the most exceptional warming at all three sites - and especially in Utqiaġvik, as we've documented here many times before.


An additional aspect that stood out to me is how warm the decade has been in Anchorage, given that Anchorage has the smallest temperature variance of the three locations.  In a climate that's inherently less variable, a relatively small temperature change takes on greater statistical significance.

One way of leveling the playing field is simply to divide each day's decadal temperature anomaly by the 1981-2010 standard deviation of daily mean temperature on that day.  With a little help from Rick, I managed to come close to reproducing his results, and the charts below show the comparison of the absolute (top) and standardized (bottom) temperature anomalies.

The second chart illustrates my point about Anchorage, particularly for summer and autumn: the last decade has been really exceptionally warm in Anchorage when we factor in the low variance of temperatures in the prior decades.  I typically think of Utqiaġvik's autumn warming as being "unrivaled" and "off the charts" in terms of statistical significance - and it is - but this simple analysis suggests that Anchorage is arguably not that far behind in terms of how persistently warm it's been in late summer and autumn.

If we focus on temperatures for August and September in Anchorage, we find a similar result for conditions averaged over the two month period.  In the past 10 years, the Aug-Sep mean temperature has been 1.3 standard deviations above the 1981-2010 mean of Aug-Sep temperatures.  This is actually greater than the October standardized anomaly in Utqiaġvik: the October mean temperature in Utqiaġvik has been 1.2 standard deviations above the 1981-2010 baseline.

When we compare the two time series in absolute terms (see below), Utqiaġvik's change is obviously more eye-catching, not least because of the amazing collapse in variance nearly 20 years ago.  However, the last 5 years of early autumn conditions have certainly been very exceptional in a local sense in Anchorage; and this is connected to persistent and remarkable warmth across the entire North Pacific - a topic for another time.

Saturday, December 19, 2020

More on Wind and Cold

Wind chill has been a concern across much of interior and northern Alaska in recent days, with a notable combination of wind and cold in many locations.  The more exposed locations of course had the worst of it; Eagle Summit saw a wind chill of -66°F on Thursday morning (-30°F temperature with 28mph wind), and Brooks Range passes have been very cold in the past couple of days.  The Arctic coast has had several rounds of strong winds in the past week, but residual warmth from the ocean kept temperatures a little higher, at around -10°F or so.

As usual, the "winner" in the wind chill department is Howard Pass, with a wind chill down to -71°F yesterday evening.

This latest episode of extreme weather at Howard Pass, and comments from reader Gary on last week's post, led me to do a comparison of wind and temperature data from relatively nearby stations in the western Brooks Range.  The results illustrate nicely the unique and highly localized winter climate of the Howard Pass site.  Here's the location we're talking about, for those who may not be familiar (click to enlarge);

First, the chart below shows the joint distribution of daily mean temperature and wind speed at Howard Pass in the winter months.  There's a huge amount of missing data, because the instrumentation often fails to make it through the winter owing to the harsh conditions; the last reasonably complete winter of data was 2016-2017.  Nevertheless, there's plenty of evidence to show the remarkable inverse correlation of temperature and wind in winter: the lowest temperatures always come with wind, and high winds almost always bring serious cold.  This is because cold air from the North Slope is funneled up and through the pass from a NNE direction, as in today's observations above.  The strongest pressure gradient across the Brooks Range tends to accompany the coldest conditions on the North Slope, so this is a recipe for extreme wind chills at favored pass locations.

The same chart for the Imelyak RAWS, about 40 miles to the south and 1500 feet higher in elevation, is completely different:

The absence of cold, let alone cold plus wind, is quite remarkable at Imelyak; with nearly complete data since summer 2012, the lowest minimum temperature recorded is only -37°F.

Here's the joint distribution of winter wind chill for these two sites.  Here I've calculated wind chill from the daily mean temperature and wind speed, rather than starting from hourly data, so it's not a true daily mean wind chill (because the formula is non-linear).


A histogram format shows another view of the same thing: Howard Pass completely dominates on the low wind chill side of the distribution, even though Imelyak is 1500 feet higher in elevation.

Another relatively nearby observing site is the top-quality Ivotuk CRN installation, about 37 miles to the northeast of Howard Pass as the crow files.  Despite a similar elevation, Ivotuk has a wind-temperature relationship that looks much more like the interior North Slope: not much wind, and when it does get breezy, it's almost always warm.  The contrast from Howard Pass is really striking.


Here's a chart from the classic interior North Slope site: Umiat, another 100+ miles to the northeast.

Note that winter temperatures are highly correlated between Ivotuk and Howard Pass; they usually experience the same air mass, but the wind speed behavior is opposite.  (The coldest days at Howard Pass are missing on this chart because Ivotuk didn't start reporting until 2014.)

In summary - this is simply more documentation of the unique and fascinating local winter climate at Howard Pass.  As always, thanks to Ken Hill and Pam Sousanes (and probably others) for their persistent efforts over the years to maintain the Howard Pass site.

Wednesday, December 9, 2020

Wind and Cold

After last week's extreme rainfall in Southeast Alaska, a noteworthy North Pacific storm brought a lengthy episode of strong winds to southwestern parts of the mainland earlier this week.  Here's a surface analysis from 3am Sunday, courtesy of Environment Canada:

Bethel had two unusually windy days on Sunday and Monday - with snow and temperatures down near 0°F to start - and the wind was really howling at the more exposed locations.  For instance, Cape Romanzof (admittedly a very windy place) reported sustained winds around 60-70 mph for a full 24 hours, with gusts of 80 mph.

Our old friend Howard Pass, up in the western Brooks Range, also saw a modest blow as the pressure gradient worked its way north this week.  In characteristic fashion, the temperature dropped as the wind picked up at the pass, and bottomed out at -20°F with 40-45 mph winds.  Nasty, but nowhere near the conditions that have been observed there on some past occasions.

The fact that the strongest winds in western Alaska often seem to prevail from a northerly direction got me thinking about whether there is in fact an inverse correlation between temperature and wind speed in these areas.  In the interior lowlands we would generally expect the opposite in winter: wind brings warmth, but what does the relationship look like on a map?

With the ERA5 reanalysis, we can answer this question with a fair degree of confidence.  The sequence of maps below shows the correlation coefficient of daily mean 2m temperature and daily mean 10m wind speed, by month, from 41 years of data.  Click the images to enlarge.

Interesting features to me are:

- Inverse correlation (cold when windy) in the highest elevations

- Positive correlation (warm when windy) in winter across the interior North Slope, the Kobuk valley, the Yukon Flats, the mid-Tanana valley, and elsewhere in the lower elevations of the west.

- Inverse correlation in summer across western Alaska and the oceans (warmth occurs in conjunction with high pressure and reduced winds)

- Very high positive correlation in the warm half of the year across northwestern Canada and parts of eastern Alaska - but apparently not directly corresponding to elevation.  I'm not sure what to make of this.

If readers have any comments or insight on the patterns, I'd be glad to hear them.

Thursday, December 3, 2020

Southeast Deluge

I don't usually comment on weather in Southeast Alaska, but the past few days brought an extremely intense rain storm that broke longstanding records in the northern part of the region.  All-time records for calendar-day precipitation fell at Juneau, Skagway, and Haines, and unfortunately damage from flooding and a major landslide occurred in Haines.  Here's a summary from NWS Juneau as of 11am yesterday, and more rain fell after that.

The two animations below show precipitable water (the total vertical atmospheric water content) during two 24-hour periods, illustrating the tropical moisture source and the remarkable persistence of the "atmospheric river" directed into the Southeast.


Here's another really nice animation:


If we look at past episodes of extreme rainfall in Juneau in early winter (October through December), it's interesting to note that 5 of the top 6 (since 1950) highest 2-day precipitation totals occurred during La Niña conditions, and 3 of these were rather strong La Niña episodes similar to the one we're currently experiencing.  Those 3 events were Oct 19-20, 1998, Nov 29-30, 1988, and Dec 27-28, 1999.

This is an interesting point because La Niña more often brings drier, not wetter, conditions to Southeast Alaska for the second half of the rainy season as a whole (i.e. early winter).  Since 1950, 10 of the 15 strongest La Niña's in October-December were drier than normal overall in Juneau, and the map below shows the spatial pattern of early winter precipitation for strong La Niña years.  More investigation would be needed to be sure, but this may be an example of weather extremes having a different sensitivity to climate forcing than seasonal weather averages.

I mentioned last month that Fairbanks also has an enhanced frequency of heavy precipitation events during La Niña winters.  The chart below illustrates this for 2-day snowfall totals of 6-9" (blue) and over 9" (purple).


The same relationship (warmer ENSO -> lower chance of heavy snow) is evident in both early winter and late winter.

Update: NWS Juneau provided a list of sites that recorded all-time record daily rainfall amounts.

Tuesday, November 24, 2020

Inversion Season

Fairbanks has seen some notably strong temperature inversions lately, thanks to an abundance of clear skies and windless conditions with high pressure aloft.  This is, of course, the time of year when valley level weather tends to become persistently colder than conditions aloft, even during the middle of the day; the sun provides very little daytime heating now.

Here's a chart showing the seasonal cycle of afternoon temperatures at the surface and at the 925mb level, which is about 1800-2000' above valley level (but varies).  I've used data from 1992-2019 for the calculation, because the Fairbanks balloon soundings didn't regularly report 925mb conditions before 1992.  Click to enlarge the figure.

Fully one-third of the year (November through February) sees a temperature inversion, on average, at the 3pm observation time.

At 3am, on the other hand, an inversion is typically present between these two levels throughout the year.

Notice that the mid-winter temperature profiles look about the same at 3am and 3pm; there's not much systematic diurnal temperature range, especially above the surface.

It's interesting to consider how these curves vary with El Niño and La Niña; we're in a La Niña this winter, so might this explain the recent strong inversions?  The answer seems to be no; La Niña winters are often colder, but perhaps counter-intuitively, the inversion is not stronger - see below.

Here's the corresponding chart for El Niño: slightly warmer than normal temperatures, but the departure from normal is not as large as during La Niña.

A direct comparison of the surface-925mb temperature differences reveals that the inversion actually tends to be stronger in El Niño winters - see below.  It seems this is because El Niño winters tend to be drier, with more southerly flow over the Alaska Range owing to a strong Aleutian low; so with more clear skies over Fairbanks, inversions are more often pronounced in the El Niño pattern.  In contrast, La Niña tends to bring colder air but also more cloud cover from the west and northwest; temperatures aloft are considerably colder, but there's not as much opportunity as one might expect for inversions to amplify the cold.

Finally, here are the recent 3am observations in comparison to normal.

Last week's surface-925mb temperature difference of 39°F is an elite-level inversion; this is only observed on about 1% of days in November through February, and the earliest it's ever been observed is just a couple of days earlier, on November 16, 2014 (with data back to 1992).  The 1992-present record is 53°F on February 10, 2018; see this post for discussion of that event and a climatology of inversion strength at all of Alaska's upper-air observation sites.

Thursday, November 19, 2020

Arctic Impacts Controversy

Work and life have been a little too busy for posts lately, but earlier this week I saw a brief new article published in Nature Climate Change on the topic of Arctic warming and its possible impacts on mid-latitude climate.  Here's a link for reading (it's very brief):

Weakened evidence for mid-latitude impacts of Arctic warming 

The authors (Blackport and Screen) are in the camp that has pushed back against the much-publicized idea that rapid Arctic warming may be significantly affecting weather patterns at lower latitudes; the hypothesis is that rapid Arctic warming is producing greater "waviness" in the jet stream and more frequent mid-latitude cold outbreaks in winter as Arctic air is discharged to the south.  My impression is that this hypothesis gained a lot of credence until quite recently, but it seems the pendulum is now swinging back the other way.  This new piece, and a review article earlier this year by Cohen et al. that argues in the other direction, add fuel to the fire of controversy.  A pdf copy of Cohen et al. can be found here.

It may be a little presumptuous to weigh in on a topic that is attracting so much attention from very competent researchers, but I've been looking at this for work, and a couple of issues do seem worthy of comment.  First, there's no question in my mind that Arctic "blocking" has simply not increased in parallel with the dramatic loss of Arctic sea ice.  In contrast, there's a clear case to be made that the winter jet stream has often been stronger and more uniformly westerly in recent decades than before the era of strong Arctic warming.  The chart below shows the Arctic Oscillation index since 1950 for winter (blue) and summer (red), and it's clear that the December-February index values have been generally more positive since 1988 than before.  If we focus only on 1988-present, as Cohen et al. did, then we might claim a downward trend through about 2013, but the larger context tells a different story.

The North Atlantic Oscillation, which is closely related to the AO but focuses on the jet stream behavior over the North Atlantic sector, shows a similar picture; it's been 10 years since there was a significantly negative NAO phase in winter (see below).  I see no evidence of increased winter blocking (more negative NAO), but rather the reverse is true for the full period since 1950.  However, it certainly is interesting to see that the summer NAO has frequently been significantly negative in recent years.  This reflects an association between summer weather patterns and sea ice loss - but which causes the other is difficult to say.  I discussed the summer ice/weather connection back in September.

If we look at the strength of the winter-time westerly winds at 60°N (a latitude that's commonly used to monitor the status of the stratospheric polar vortex), there is zero trend from 1960-present in the lower stratosphere (100mb).  At the mid-stratosphere level of 10mb, the ERA5 reanalysis data suggest a slight decrease in winter westerly flow, but the trend is nowhere near statistically significant.  At 500mb, in the mid-troposphere, there's a slight increasing trend, but again not significant.

In view of this data, I agree with Blackport and Screen that the evidence doesn't support the purported mechanism for increased mid-latitude volatility; the winter-time jet stream and polar vortex have not weakened, and if anything the large-scale circulation modes (AO and NAO) have become less favorable for mid-latitude cold outbreaks in winter.

Second, I also concur with Blackport and Screen that there's very little reason to believe that mid-latitude land areas have seen "almost no warming" (to quote Cohen et al.) in winter during the era of Arctic amplification (i.e. the last 30 years or so).  Here's a chart showing mid-latitude land area temperature trends for December-March from various sources (December-March was used by Cohen et al.); click to enlarge.


Here I've calculated trends for 1993-2016, which is the period for which model forecasts are available from the seasonal dynamical models included in the EU's Copernicus program.  I'll comment on these models below.  For now, notice that the smallest warming trend is found in the NCEP global reanalysis ("R1"), and this is the source that was used by Cohen at al. to support their statement that "the observations show that temperatures across the midlatitude continents have remained nearly constant".  Unfortunately, the NCEP reanalysis is a 25-year-old model that runs at a very coarse resolution and simply doesn't represent the state of the art for climate reanalysis.

In contrast, the modern ERA5 and JRA-55 data sets show greater warming trends and agree closely with each other.  These also agree quite closely with data from NOAA's surface temperature analysis, which is derived from surface observations rather than estimated from a model.  It's clear that ERA5 has tended to be warmer in recent years, and cooler in earlier years, than NCEP R1 (with both series having the same zero-anomaly baseline here).  If we had measured the temperature trends ending in 2013, we might have tentatively concluded that the data was hinting at more cold air discharge to the mid-latitudes, but the last several years have greatly weakened that hypothesis, in my view.

Having said all that, it is interesting to see that the observed mid-latitude warming has not been as great as the Copernicus models expected in their November-issued forecasts for December-March (as illustrated by the green trend line above).  Compare the two maps below, showing the spatial distribution of trends in the Copernicus and ERA5 data.  The Copernicus forecast models show considerably more warming in central and eastern Canada and the contiguous U.S., and there's also a much broader zone of warming in western and central Russia.  ERA5 even shows a few areas of cooling, but these aren't captured in the Copernicus forecasts.

I think it's fair to say, then, that mid-latitude temperature trends in the past couple of decades have not conformed to model expectations, as warming has not been as widespread and pronounced as the models predicted.  This much is consistent with Cohen et al., and so I think we can't yet rule out the possibility that Arctic amplification has a systematic effect that dampens warming in the mid-latitudes.  However, if this is happening, it's not because Arctic blocking has increased or the polar vortex has weakened, so I believe the fundamental mechanism proposed by Cohen et al. is not correct.

In my view, the most likely explanation for the relatively small mid-latitude warming is that natural variability of weather patterns has produced a trend on the low side of what might be expected.  With only about 30 years of data to work with, sample size is obviously a huge problem for assessing whether the models are "right".  Time will tell, of course; but I suspect we will eventually dismiss the counter-intuitive idea that Arctic warming causes mid-latitude cooling.

For the sake of completeness, here are the temperature trend maps for NCEP R1 (very unrealistic cooling over Asia), JRA-55, and NOAA.

And here's a chart of high-latitude trends, for both land and ocean:

In this case the reanalysis products show more rapid warming than the Copernicus models - the opposite of the situation in the mid-latitudes.  As noted by Cohen et al., the models show warming "more equitably distributed between the Arctic and midlatitudes"; they argue that this is because Arctic amplification favors greater "meridional exchange of air masses" than expected by the models, but I'd argue that the lack of increased blocking means this cannot be the case.

Saturday, November 7, 2020

Cold Then Snow

It's been a dramatic week for weather in central Alaska, with near-record-breaking early cold followed by a new November record for 24-hourly snowfall yesterday in Fairbanks.

The cold snap deepened further after I posted on Monday, and on Wednesday morning the temperature fell to a remarkable -29°F at Fairbanks airport; this level of cold so early in the season was only exceeded in 1975, which reached -30°F on the same date.  The Smith Lake sensor at UAF recorded -35°F, and the North Pole 1N co-op site apparently reached -41°F (although this seems a little low; the other North Pole co-op only reported -32°F).

Here's a 7-day temperature trace from Smith Lake.  With cloud, snow, and a massive influx of warm and moist air aloft, the temperature rose more than 65°F in two and a half days.


The mid-atmosphere map from early Friday morning shows a long fetch of strong flow from the west-southwest, which is the classic direction for sustained heavy precipitation in the Fairbanks area.

Up on Munson Ridge, the SNOTEL instrument measured 2.3" of new snow water content over two days, and the liquid equivalent in Fairbanks was a hefty 0.93".  Snow depth at valley level is now up to 19".  Here's some context for the snowfall; this was a big one.


I'll add some more on this soon, but heavy snows in Fairbanks are more common during La Niña; the frequency of 8" snow storms is more than twice as high compared to El Niño.

Monday, November 2, 2020

Cold Snap

Winter has suddenly made its presence known with sharply lower temperatures across much of the interior in the past few days, and last night saw widespread -20s Fahrenheit for the first time this season.  Click to enlarge:

The perennial cold spot of Chicken saw the state's first -40° this morning, and this is notably earlier than usual.  In Chicken itself it is the earliest -40° on record, but data only extends back to 1997; and in 2008 it was -39°F on October 28.

It's interesting to note that the first -20°F in Alaska was only 4 days ago (in Wiseman), and the first -30°F was just yesterday at Chicken.

In Fairbanks-land the usual cold spots did their thing (-28° near Goldstream Creek, -26° at Smith Lake, and -31° over at Salcha), but it was surprisingly cold in the hills too: for instance, -14° at 2150' elevation to the east of Eielson AFB.

Despite the cold, the Tanana River is not quite frozen up at Nenana; patchy ice was still flowing past the webcam today.

However, the Yukon froze up yesterday at Dawson City: