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: