Tuesday, September 25, 2018

Permafrost Data - a First Look

Prompted by a question from reader Tracy on permafrost depth in Fairbanks, I recently tracked down some soil temperature data at the UAF Geophysical Institute's Permafrost Laboratory, headed by Professor Romanovksy.  A wealth of data is available, so I chose a site more or less at random: Smith Lake #1, located close to Smith Lake on UAF's North Campus.  Daily temperature data are provided from 2007 through May 2017, which is long enough to provide a few interesting results.

First, the chart below shows the sequence of annual mean temperatures at depths down to 3m (click to enlarge).  Immediately we're struck by a remarkable trend towards higher temperatures in the past decade or so, with the top 50cm seeing an annual mean temperature above freezing in 2016.  There was a big jump between 2013 and 2014, which is not surprising as Fairbanks' mean air temperature was more than 7°F warmer in 2014 than in 2012.  And ever since the remarkable transition from cold to warm in May of 2013, Fairbanks has seen persistent anomalous warmth with few significant excursions to the cold side of the climate.  The integrated effect of this warmth is clearly visible now in the sub-surface warming trend.

Secondly, as a partial answer to Tracy's question about seasonal thaw of the active layer (i.e. the top layer of permafrost that thaws and refreezes each year), the chart below shows annual maximum temperatures at each depth.  (Note that the 2m depth is missing from 2014-2016).  From 2007 through 2013, the seasonal thaw did not generally extend down below 50cm, but more recently the active layer has deepened to something like 1m.

Looking at annual minimum temperatures, it is really remarkable to see that by 2016 temperatures below 50cm depth were barely below freezing even at the coldest time of year.  It doesn't take too much imagination to envision a complete loss of the permafrost in this particular profile, which would upend the ice/water profile: as it stands now, seasonal thaw sits atop permanent ice, but Smith Lake #1 may soon see seasonal ice atop permanent thaw.

For a bit more perspective on the recent changes, here's the temperature time series from 0.525m depth.  March 2017, which was the one really cold month in recent years, produced a minor recovery compared to previous years, but presumably last winter's warmth (not yet reflected in the available data) will have reinforced the warming trend as strongly as ever.

Wednesday, September 19, 2018

Historical Context for Recent Extreme

Following up on Saturday's post, I did a bit more work with the NCEP/NCAR global reanalysis data to get a better sense of just how unusual the recent circulation anomaly has been.  How many times has a comparable two-week anomaly been observed in the past, not just over Alaska but anywhere in the Northern Hemisphere?

I addressed this question with the following series of calculations:

(a) Obtain a smoothed daily climatology (normal) of 500mb heights for the 1958-2017 period.  Note that the NCEP/NCAR reanalysis starts in 1948, but the quality is lower before 1958 owing to the paucity of upper-air observations.
(b) Calculate mean height anomalies for 15-day periods ending on every day since 1958.
(c) Extract a smoothed daily climatology of the standard deviation of the 15-day anomalies.
(d) Find the daily maximum and minimum of standardized 15-day anomalies across the entire Northern Hemisphere.

Here are the results: the red line shows the daily hemispheric maxima of standardized 15-day height anomalies, and the blue line indicates the hemispheric minima.  (Click to enlarge)

Remarkably, the 15-day anomaly ending just 3 days ago at a grid point over the Bering Sea was the largest of any in the entire Northern Hemisphere history since 1958.  It was also larger in magnitude than any 15-day negative anomaly.  The closest competitor on the positive side was an extreme blocking ridge over northern Greenland in November 1965; here's a pair of maps showing a comparison of the two events:

Of course the long-term global rising trend in 500mb heights gives a slight "advantage" to the recent anomaly in comparison to a fixed climatology, so I re-did steps (b) - (d) above after removing the 1958-2017 linear trend (calculated for each day of the year and each grid point).  In this case the 1965 event moves into first place as the most extreme positive 15-day height anomaly at 500mb (in terms of standard deviations).

It would be interesting to see how other atmospheric variables like temperature and moisture reflect the nearly unprecedented nature of this month's anomaly, and it would also be worth looking at other, more modern data sets.

But based on these results, it seems safe to say that the recent high pressure ridge over the Bering Sea and western Alaska has been one of the most extreme 15-day weather anomalies in recent decades anywhere in the Northern Hemisphere.

Saturday, September 15, 2018

Record Bering Ridge

Much of Alaska has enjoyed a most remarkable extended spell of clear, dry, calm, and warm autumn weather in the past couple of weeks, thanks to an extremely strong and persistent ridge of high pressure centered over the Bering Sea.  Here's a map (click to enlarge) showing the 500mb height anomaly (departure from normal) in the first two weeks of this month, expressed in terms of standard deviations.  Near St. Lawrence Island the 500mb height has averaged more than 2.5 standard deviations above normal for a period of two weeks.  This is a remarkable anomaly.

Courtesy of Environment Canada, here's the 500mb analysis from last Saturday afternoon when the blocking ridge was in full swing.

As of yesterday afternoon the ridge was much weaker but still quite sufficient to dominate the weather over much of Alaska.

The chart below shows the 500mb height measurements from the sounding site on St. Paul Island in the southeastern Bering Sea.  The gray lines represent the one-standard deviation range either side of the 1981-2010 normal.  The persistence of the very strong positive height anomalies is notable, to say the least.

Another perspective on just how unusual this is can be gleaned from the history of September 1-14 measurements at St. Paul Island - see below.  I'm not sure that I've ever seen a multi-decadal climate chart with such an extreme anomaly in only one year that lies so far outside the envelope of observed climate.  It's quite astonishing.

Unfortunately Alaska's sounding sites farther north have not been reporting regularly this month, but I pulled out the equivalent data from the NCEP/NCAR Reanalysis at a grid point near the Bering Strait - see below.  The result is similar but actually even more extreme relative to the climate of recent decades.

Compared to the 1981-2010 climate, the mean 500mb height of the past 14 days has been 5.1 standard deviations above normal.  If the reanalysis is correct (and it should be quite accurate for 500mb height), and if the 500mb climate is Gaussian and stationary (unchanging), then we'd expect this kind of anomaly to occur less than once every million years, on average (for this particular date window).  Obviously one or more of the assumptions is false, but this illustrates the magnitude of the extreme.

What might have caused such a remarkable departure from normal?  I suggest a couple of possible causes.  First, it seems like the chain of events to set up the ridge started in late August when a burst of tropical cyclone activity occurred in the West Pacific.  Typhoon Cimaron (Category 3) hit Japan on August 23 and then the remnants headed up to the Aleutians, and on the same day Typhoon Soulik (also Category 3) hit South Korea and then moved into the Bering Sea.  Then about 10 days later Typhoon Jebi (Category 5 at one point) hit Japan (September 4) and headed for northeastern Russia.  The upper-level circulation of the latter storm, in particular, moved up the west side of the Bering Sea ridge on September 6 and seems to have provided a reinforcing boost for the ridge by transporting huge quantities of warm, moist air up to the region.

Here are the paths of the 3 storms - maps courtesy of Wikipedia.




A second contributing factor may be that sea surface temperatures have been far warmer than normal in the northern and western Bering Sea in recent weeks, and this unusual ocean warmth probably helped boost the overall warmth of the atmospheric column and build the ridge in the same area.  Here's an SST anomaly map that Rick Thoman posted on Twitter recently:

Finally, to illustrate the impacts of the weather pattern, here are a few nice shots of the lovely autumn scene across the state, courtesy of the FAA webcam views on Thursday evening.

Kivalina (far northwest Alaska)

Chandalar Shelf (in the central Brooks Range)


Teller (western Seward Peninsula)

Grayling (on the lower Yukon River)


Denali National Park



Yakutat (one of the wettest places in Alaska, especially at this time of year normally)

And here's a true-color shot from the Suomi polar orbiter on Thursday afternoon (click to enlarge).  The reddish-brown tundra colors of autumn are quite distinct in the north as well as in higher altitude areas throughout the state.

Saturday, September 8, 2018

Temperature Trends

In last week's post I mentioned that the abundant cloud cover over Fairbanks in August produced much lower than normal diurnal temperature variations; the average day/night temperature difference was almost the smallest on record.  Below is a chart (click to enlarge) showing the average diurnal temperature range in August, beginning in 1930 for the university's Experiment Farm and 1952 for the official Fairbanks climate record (when observations moved to the airport).  The downward trend is highly statistically significant at both locations; late summer now tends to see less difference between daily high and low temperatures than in the past.

It's of interest to see how the trends in daily high and low temperatures contribute to the diurnal range trend, and to see how this varies through the year.  Accordingly, the chart below shows smoothed daily values of the 1930-2017 linear trends in each quantity at the Experiment Station.  The relative positions of the blue and red lines show that daily low temperatures have warmed more than daily high temperatures at all times of the year, and therefore the diurnal range has been reduced throughout the year.  Note that both high and low temperatures have cooled slightly in mid-autumn, but the diurnal range has still been reduced.

The largest change in diurnal range has occurred in mid-summer.  Daily low temperatures have risen by more than +0.7 °F/decade from mid-May through the end of July, but high temperatures have not changed significantly in high summer, and so the diurnal range trend peaks at a rather remarkable -0.85 °F/decade (8.5 °F/century!) around July 1st.  It's clear therefore that the reduction in diurnal range in summer is attributable to nighttime warming rather than daytime cooling; as a simple example, the Experiment Farm has seen only four days with a freeze in June in the past 30 years, but a summer freeze was not uncommon in the earlier years (31 freezes in June and July of the 1930s and 1940s).

Using Fairbanks airport data since 1952, there are some differences as we would expect, but some of the same signals are evident: the winter peak in overall warming, the autumn lack of warming, and the summer trend (albeit less pronounced) towards smaller diurnal range.

If we re-do the calculations for Experiment Station over the shorter time period, there is a lot more similarity between the results for the two sites.

Finally, a chart of the June-July diurnal temperature range illustrates the changes at the two sites in the high summer months.  I'm not sure what was going on at the airport in the 1990s, and one does wonder if the transition to ASOS instruments in 1998 could have affected the numbers.  But the long history of consistent data from the Experiment Farm tells a clear story of remarkable change in this aspect of Fairbanks climate.