Tuesday, February 21, 2017

Daily Minimum Temperatures

Here's a chart showing the lowest temperature reported statewide in Alaska each day so far this winter, along with the same analysis for last winter.  This is constructed with data from NOAA's ACIS tool, but I've excluded Snotel sites because of quality concerns.  Many, though not all, remote automated observing sites are included in the analysis.

The contrast with last year has become pronounced since the beginning of the year: from January 1 to present, only one day had a higher statewide minimum than the same date in 2016.  The Kanuti Lake SCAN site dropped below -50°F again this morning, so that makes the 4th separate episode with sub-minus 50°F observed in Alaska; and that's a lot more like normal than recent winters.

Saturday, February 18, 2017

Duration of Cold Spells - Part 2

A few weeks ago I showed some evidence that severely cold spells have become shorter in recent decades in Fairbanks - in other words, the coldest extremes of the modern climate (which are warmer than earlier cold extremes) have become less persistent as warmer conditions have tended to return more quickly.

This follow-up provides a quick look at the average 500mb height patterns associated with the most severe cold spells in two contrasting periods, 1931-1960 and 1981-2015.  To examine this I used data from the 20th Century Reanalysis, courtesy of NOAA/ESRL/PSD.  The 20th Century Reanalysis uses historical sea-level pressure and sea surface temperature observations to produce an evolving 3-D estimate of atmospheric conditions back to 1851.  Owing to the strong correlation between sea-level pressure and upper-level heights, the historical 500mb height estimates should be broadly correct back to the early 20th century or before; by 1925 there were already well over 1000 daily pressure observations north of 20°N (although not at all evenly distributed around the Northern Hemisphere).

The two sets of maps below show the mean height anomaly (departure from normal) for 1931-1960 cold spells on the left (as defined in the earlier post) and 1981-2015 on the right.  The first pair of maps is for 10 days prior to the first day exceeding the cold threshold, and then pairs of maps are shown at 5-day intervals until 15 days after the cold spell onset.  Click to enlarge the images.

-10 Days

-5 Days

0 Days

+5 Days

+10 Days

+15 Days

With respect to the main question of interest here, it's immediately clear why cold spells in the earlier period tended to last longer than more recent cold spells.  In 1931-1960 the average flow pattern remained quite similar from the date of onset through 10 days later, with high pressure over the Bering Sea and low pressure over western Canada; this pattern funnels cold air southward across the interior.  In contrast, the 1981-2015 pattern is completely different by +10 days, with low pressure over the Bering Sea and high pressure over western Canada; this pattern brings warmth to Fairbanks.

It's also interesting to note that the precursor flow patterns were quite similar between the two periods at a lead time of 10 days, with high pressure over and south of the Aleutians and low pressure over the Arctic Ocean.  However, at 5 days in advance, the modern pattern is much more amplified, with a very strong northerly flow anomaly over the Bering Sea (rather than northwesterly in 1931-1960).  It seems as if modern severe cold spells tend to arrive with greater ferocity, but the high-amplitude pattern continues to evolve rapidly so that the cold blast is followed quickly by warmth.  In the "old days" the cold-advection pattern gives the impression of being more stable and long-lived, at least as far as the Alaska-centric flow anomalies are concerned.

A possible next step would be to investigate historical sea surface temperature anomalies to see if we can identify a reason for the preferential patterns shown above.

Tuesday, February 14, 2017

Extreme Warm-Up

I was going to write a few words on the brief but notable cold spell over the weekend, but the cold has already been eclipsed by an incredibly rapid warm-up over the eastern interior.  The map below show 24-hour high temperatures through 5pm today in the wider Fairbanks area: most places above freezing, and +40's in the hills to the east.

A broader view shows clearly where the southerly chinook flow reached the surface today and, conversely, where it did not; the Yukon Flats have remained cool, and locations west of Fairbanks have been less warm; even Nenana was far enough west to stay 10°F cooler than Fairbanks.  Interestingly the upper Tanana Valley was cooler as well.

The rate of warming at certain spots has been really remarkable: the Salcha RAWS, for example, warmed from -45°F at 9am on Sunday to +45°F at 1pm today.  This is extreme even by interior Alaska standards.

At Fairbanks the low on Sunday morning was -41°F, and today's high so far is +35°F; this is the warmest it has ever been within two calendar days of a -40°F or colder reading.  The warmest it's ever been within a single day of -40°F is +21°F (in 2012), and the warmest within 3 days is +44°F (2009).  So this is one for the history books.

Here's a sequence of 500mb charts showing the flow adjustment that has brought such a dramatic change of conditions.  The maps are from 3am on Sunday, Monday, and today, respectively; notice the strong upper-level low dropping south over western Alaska, allowing the the flow to pivot quickly to the south across the Alaska Range.

The readiness of the low to drop south appears to have been aided by the deep trough that was already in place over the North Pacific at around 160-180°W (evident in the first map above).  This in turn was favored by widespread below-normal sea surface temperatures in a west-east band across the North Pacific, reflecting the negative NPM phase that has become entrenched this winter.  Here's a recent SST analysis:

And here's the typical 500mb height pattern when the NPM is strongly negative in February; notice the similarity of the trough position to where the low ended up this morning.  This is a nice example of a single weather event reflecting the favored mode based on long-term (seasonal-scale) forcing mechanisms.

Thursday, February 9, 2017

Arctic Update

It's time for another update on temperatures at selected surface observing sites adjacent to the Arctic Ocean.  For background on the choice of sites, see here.  January was another very warm month, relative to the average conditions of earlier decades, with a mean January temperature more than 4°C above the 1981-2010 normal.  This is down a little from the incredible warmth of last January, but it was still the 5th warmest anomaly of any calendar month in this data set (1971-present).  The top 3 months were November, January, and October of 2016.

Daily mean temperatures remained well above normal throughout January, and the 19-station mean has pushed back up to nearly +10°C in recent days (see the chart below).  The chart also shows a recent spike up to nearly +30°C in the red line, which shows the warmest of the individual stations on any given day.  This spike reflects extraordinary warmth a week ago at Cape Billings on the northern coast of Chukotka (176°E); the temperature reached +4.6°C or 40°F, at a time of year when the 1981-2010 normal mean daily temperature is about -17°F.  The departure from normal of the daily mean temperature last Friday was +27.7°C or +50°F, and this sets a new record for the most extreme daily warm anomaly at any of the 19 stations since 1971 (narrowly beating the extreme daily warm anomaly from two months ago - noted here).

The continued drumbeat of extreme warmth in the Arctic is of course reflected in sluggish growth of sea ice and record low sea ice extent for the time of year, according to the NSIDC's recent update.  January's update from the University of Washington's PIOMAS model indicates that Arctic sea ice volume is likely also at record lows and well below anything observed on this date even in recent warm winters.

Saturday, February 4, 2017

Satellite Temperature Estimates

Back in December I mentioned that I had found a way to begin working with land surface temperature estimates from the Suomi NPP satellite (launched in 2011); the modern radiometer on the satellite measures infrared radiation at a spatial resolution of 750m, which allows for nice high-resolution estimates of surface temperatures when clouds are absent.  I've recently also decoded similar data from the MODIS instrument on the TERRA and AQUA satellites, launched in 1999 and 2002 respectively.  The MODIS resolution is about 1km.

Here are a couple of images showing estimates of the minimum surface temperature observed by these satellites on January 18 over the Fairbanks area and points eastward into the hills; this is the day that temperatures bottomed out during the cold snap.  I've interpolated the data to a 750m grid in both cases for consistency (click to enlarge the images).  The similarities are striking, but greater spatial detail is evident in the S-NPP (lower) image, as expected.

It's gratifying to see that both maps show the cold conditions on the Salcha River in the bottom right; the Salcha River RAWS (SLRA2) reported -59°F, and the satellite data support this very cold reading.  On the other hand, the -65°F reported by the Upper Chena River HADS site (UCHA2) is not supported and was almost certainly in error.

Below is a comparison of the minimum temperatures (in °F) reported at each of the sites marked on the maps.  The agreement is, in my view, quite impressive, with only the Stuart Creek RAWS seeing a substantial difference if we exclude the Upper Chena HADS site.  At both Stuart Creek and at the Salcha RAWS, the estimates are better for the S-NPP, which probably reflects the superior resolution of the S-NPP instrument.


Tuesday, January 31, 2017

Winter So Far

January has been another relatively snowy month in Fairbanks, with a total accumulation of 17" - the highest January total since 2005.  The combined December-January total was almost exactly 50", which is the 5th highest on record for the two-month period.  Temperatures have averaged slightly below the 1981-2010 normal and close to the long-term (1930-present) normal, but with alternating cold and warm spells.  Snowy winters are typically colder than dry winters in Fairbanks.

As noted in earlier posts (e.g. here), the cool and snowy conditions are surprising in view of this winter's North Pacific sea surface temperature patterns.  The positive PDO phase and negative NPM phase, which continue to be evident in the latest SST data, would normally promote warm and dry conditions in the interior; we discussed this back in November.  Here's another map showing the average 500mb height anomaly (departure from normal) in November through January for 6 winters with a significantly positive PDO and significantly negative NPM:

Compare this pattern to what has actually transpired in the last 3 months:

The patterns are fairly close to the inverse of each other, and this of course explains the weather in Fairbanks: the flow pattern has produced enhanced westerly flow across western Alaska, thereby transporting moisture to the interior, and the tendency for higher heights in the Bering Sea has allowed cold air to drop south across Alaska at times.

The reason for the discrepancy appears to be that the weak La Niña in the tropical Pacific has had a much more dominant influence on the circulation pattern than expected (at least by me).  The map below shows the height pattern for the 10 strongest La Niña winters since 1950-51; this is a better match to what has happened this winter, although admittedly the La Niña flow is a colder pattern for Alaska than we've seen this winter.

The most intriguing part of the situation, I think, is how the North Pacific SST patterns have remained much more reminiscent of El Niño than of La Niña, despite the evidently profound influence of the La Niña episode on the atmospheric circulation.  A positive PDO and negative NPM are generally observed in association with El Niño, like last winter - but not this time.  But regardless of the cause, this winter's outcome will serve as a cautionary tale about the danger of relying too heavily on the North Pacific SST patterns in isolation for making a forecast.

Update Feb 2: This nice graphic from NWS Fairbanks shows the complete contrast between last winter and this winter in terms of early versus mid-winter snowfall accumulation; the totals before and after December 1 are almost perfectly reversed.

Friday, January 27, 2017

Duration of Cold Spells

Last week's cold snap turned out to be rather short-lived in Fairbanks, as there were only 3 days with a midnight-to-midnight minimum temperature below -40°F.  The cold persisted considerably longer in the western interior, and a relatively small distance made a big difference: Tanana saw daily minimum temperatures below -40°F for 9 consecutive days, including 5 days below -50°F.  And not far to the north, the SCAN site in the Kanuti NWR reported minimum temperatures between -56°F and -61°F for 6 consecutive days.

Prompted by a question from reader Gary, I looked into the duration of cold snaps in Fairbanks to see if there's evidence that periods of intensely cold weather have become shorter over time.  This is not a question of how much warming has occurred in the mean - we know Fairbanks now doesn't see the extreme cold that used to occur - but rather we are asking how long the coldest conditions stick around during periods of relatively cold weather.  Here's a chart of each winter's lowest daily mean temperature since 1930.

I began by looking at the evolution of daily mean temperatures during the most severe cold spells relative to the climate of two periods: 1931-1960 and 1981-2015.  It turns out that in the first of these two periods, 20% of the winters produced at least one day with a mean temperature below -50°F, and in the more recent period, 20% produced at least one day below -45°F.  We'll consider these the thresholds for a severe cold spell in the two periods respectively.

The two charts below show the evolution of daily mean temperatures for each of the cold spells within these two periods, as defined by having at least one day at or below the threshold temperature.  The events are labeled with dates indicating the first day that met the criterion (day 0 on the horizontal axis).  Notice that there were two such spells in the winter of 1933-34, but the temperature rose above freezing in between, so I would consider them as separate cold spells rather than a single long one.

The most striking feature of the comparison is that several of the cold spells in the earlier period were lengthy, and half of the events returned to -50°F or lower more than 10 days after the initial onset.  In contrast, only 2 of the 7 cold spells in 1981-2015 returned to a similar degree of cold after a week, and then only for a brief period.

A comparison of the multi-event means for the two periods (see below) shows a divergence after 5-6 days, with the modern climate showing a tendency for continued warming after one week, while in earlier decades the mean remained lower for a few days as cold sometimes returned for an extended period.  Based on this comparison, it certainly appears that cold spells have become shorter on average in recent decades, although admittedly the sample size is rather small.

Another way of looking at the change is to track the number of days spent below the cold threshold for overlapping 30-year periods.  The columns in the chart below show these numbers, and the line and markers above indicate the threshold for cold in each period.  The overall warming trend is reflected by the rising threshold for cold snaps, but the interesting result is the marked decrease in the number of days spent within cold snap territory; in recent decades, cold snaps (as defined here) have lasted only 1 or 2 days on average, whereas 5 or more days was typical in some earlier periods of Fairbanks' history.  So not only has the climate warmed in the mean, but considerably fewer days are spent at the bottom end of the temperature distribution in the modern climate. 

The obvious next question is why has the average length of cold spells become so much smaller over time?  A detailed answer would require analysis of the circulation patterns during the coldest episodes in Fairbanks - a project for another time.  For now I'll simply suggest that the climate of recent decades seems to have precluded the kind of persistent trough over eastern Alaska and western Canada that would be required to maintain very cold conditions in Fairbanks.  Even in last week's cold spell we saw that the ridge over western Canada was reluctant to budge, and as a consequence the trough had a difficult time progressing eastward; and so for much of the time the flow over Fairbanks had a southerly component, and moisture and clouds made their way up from the Gulf of Alaska.  Cloud cover was much more sparse over the western interior, and so of course those areas were much colder.  It would be interesting to look at historical cold spell durations in, say, McGrath, to see how they compare to Fairbanks.