High Arctic Warmth and ENSO

The recent very unusual warmth in the Arctic has drawn considerable attention from various quarters, and justifiably so; the conditions have been quite extreme.  Here's an estimate of the temperature anomaly for the week ending November 20, based on NOAA's CFS reanalysis data.  Temperatures have been 15-20°C above even the relatively recent 2001-2010 normal over parts of the Arctic Ocean; but parts of Siberia have endured a correspondingly extreme cold anomaly.

Pressure and temperature analyses over the same period (see below) show that the recent Arctic warmth was connected to a circulation pattern involving low pressure in the Norwegian and Greenland Seas and high pressure over central Russia.  The flow associated with these features transported warm air into the Arctic from far to the south over western Europe and the North Atlantic Ocean.  Therefore an absence of Arctic sea ice is not the only reason for the warm conditions, but of course the mutual reinforcement of open water and warm air helped to amplify the anomaly.

One question that occurred to me is whether the Arctic warm anomaly may be a lingering result of last winter's strong El Niño episode.  Could a warm early winter in the Arctic be an amplified reflection of the jump in global temperatures that always accompanies El Niño?  The chart below shows that the answer is no, and in fact the reverse is true: on average, El Niño tends to be followed in the early part of the next winter by colder November-December conditions in the high Arctic (north of 80°N), whereas La Niña quite reliably brings unusual warmth in the subsequent winter.

The inverse correlation that's evident here is partly caused by twin long-term trends towards less frequent El Niño conditions and warmer Arctic conditions in recent years, as seen in the chart below.  In other words, recent years have been warmer and have also seen a preference for La Niña conditions, and this by itself explains part of the inverse correlation.

Nevertheless, a fairly robust correlation remains after removing the long-term trends - see below.  It's interesting, however, that the two strongest El Niño's (1982-83 and 1997-98) were followed by near-normal Arctic temperatures relative to the long-term trend.

Looking at a map of the correlation coefficient, we see that the region from the Barents and Kara Seas to the North Pole shows the largest inverse correlation between early winter temperatures and the ENSO conditions of the previous winter.  This region is similar to the area that has seen the most anomalous warmth recently, but this year's conditions are opposite to what the historical correlation shows!  If the historical connection were being reflected this year, then the high Arctic would be cold, not warm, at the moment.  This is puzzling, but it seems to show rather clearly that we can't blame El Niño for the current warmth.

Looking at the MSLP correlation to the previous winter's ENSO index, the map below shows that El Niño tends to be followed by high pressure over western Europe and low pressure over Siberia in November-December of the next year.  Again this is roughly the opposite of what we have seen this year.

Blog Changes

Long-time readers of Deep Cold will know that the blog's authorship has changed over time; I didn't start the blog, but I've been posting semi-regularly since mid-2013, and I've enjoyed just about every minute of it.  There's nothing more fascinating to me than Alaska's weather and climate, and I have no intention of stopping now.  However, I am probably going to post less often in the next couple of months or so, as I try to devote some spare time to writing an article for publication.

As I step back, however, I expect that a new author will begin to contribute occasionally; I've invited Mike Garrison, a long-time reader and commenter, to add his thoughts and analyses on interior and northern Alaska weather and climate.  Mike is an Alaska resident and avid weather enthusiast, and he works with weather and climate data in the course of his professional employment.  I expect we'll see a few words from Mike on the blog soon.

In the meantime, I was asked recently about a chart of Fairbanks temperature percentiles that I showed here last year.  I decided to clean up the chart and create parallel versions for maximum and minimum temperatures; so here they are.  They serve as a useful reference for the climate in Fairbanks.  Many comments could be made, but I'll just point out one feature that stood out to me: the greater variability of daily high temperatures in summer compared to daily low temperatures.  It's clear to even the casual observer that winter temperatures are much more variable than summer temperatures, but this difference is less significant for high temperatures than for low temperatures, and I hadn't quite appreciated this distinction before.  The reason, I believe, is that the presence or absence of clouds and rain in summer makes a bigger difference for high temperatures than for low temperatures.

Clearing and Cooler

You know it's winter in Fairbanks when clearing skies bring a temperature drop in the middle of the day.  Today at 11am it was mostly cloudy and +8°F with some very light snow, but by 1pm the clouds were dissipating and the temperature was down to +3°F.  It looks like mostly clear skies will prevail for the next several days, so it will be seasonably chilly.

Snow cover has finally arrived at all the major observing sites of the interior and north, with Nome finally getting a measurable snow cover yesterday - the first of the season.  McGrath appears to have begun its permanent winter snow cover at the weekend, which is of course much later than usual.  Judging from the Suomi NPP landcover image at 1pm today, there seems to be at least a thin snow cover throughout the interior, except in the northeastern Copper River Basin, including perhaps Slana.

Clear skies in the western interior allowed temperatures to drop to a crisp -19°F this morning at the Clear Creek HADS site near Hogatza; this is the coldest report from the interior this month, which shows how warm it has been.  Tanana dropped to -15°F this morning, and McGrath saw -13°F.

Sea Ice Update

We're half way through November, and sea ice is finally beginning to close in around the northernmost tip of Alaska, according to the National Weather Service.

Nevertheless, remaining open water in the vicinity of Barrow (soon to be renamed Utqiagvik) is still holding temperatures up whenever the wind direction is onshore.  Today is a case in point: with a light southerly breeze this morning the temperature was around 7-10°F, but it jumped up to 28°F when the wind went around to the west.  So far the lowest daily high temperature this season is 22°F, which shows a remarkable absence of cold; the most similar year in the past was 1998, when the coldest day through mid-November had a high temperature of 15°F.  According to 1981-2010 data, it's "normal" to see a high temperature below zero by this time of year, but this is difficult to achieve without sea ice in place and has rarely been observed in the past decade.

The chart below shows the accumulation of freezing degree days so far this season in comparison to the past 20 years.  The warmth this season is unprecedented in the modern historical era, although early winter 1998 was also very warm; and interestingly that too was a year following a very intense El Niño event.

Here's the long-range sea ice outlook from NWS Anchorage, issued today:

http://tgftp.nws.noaa.gov/data/raw/fz/fzak30.pafc.ico.afc.txt

Interestingly the outlook includes the following:

"NEAR SHORE NAVIGATIONAL WATERS AROUND POINT BARROW WILL LIKELY CLOSE
OFF MUCH EARLIER THAN IN RECENT YEARS...OWING TO THE SEA ICE
REMAINING INTO LATE SEPTEMBER AND EARLY OCTOBER. THESE NAVIGATIONAL
WATERS ARE ALREADY QUICKLY FILLING IN WITH NEW SEA ICE...AND WILL
LIKELY FILL IN WITH GREATER THAN 7 TENTHS NEW ICE BY THE FOURTH WEEK
OF NOVEMBER. OFFSHORE SEA ICE FROM ICY CAPE TO BARROW WEST TO 170W
HAS STARTED TO FILL IN WITH NEW SEA ICE MAINLY OVER THE HANNA SHOAL
REGION. SEA ICE CONCENTRATIONS GREATER THAN 7 TENTHS ARE LIKELY BY
THE FIRST WEEK OF DECEMBER."

This seems a little difficult to reconcile with the record warmth as measured in Barrow lately, so I'll try to obtain some clarification on this.

Update Nov 17: the same statement ("near shore navigational waters around Point Barrow will likely close off much earlier than in recent years") appeared in the September and October sea ice outlooks; I wonder if perhaps these words were left in the latest forecast by mistake.

North Pacific Temperature Update

I added a new post on the Alaska "Blob Tracker" blog this morning:

https://alaskapacificblob.wordpress.com/2016/11/14/dramatic-changes-in-recent-weeks/

The contrast between a strongly positive PDO phase and now strongly negative NPM phase is quite remarkable.  Looking at past years in which this combination was observed during winter, it is typical to see a stronger than usual Aleutian Low and a strong ridge over the western half of Canada.

Unfortunately for snow-lovers, this pattern is a distinctly dry one for interior Alaska and a distinctly warm one for southern and southeastern Alaska.  Of course, there's no guarantee that the North Pacific temperatures will remain in this pattern for the next 3 months, but I wouldn't want to bet against it.

When Will It Get Cold?

Temperatures in Fairbanks have returned to the "significantly above normal" category in the past couple of weeks, with a mean temperature so far this month of more than 5°F above normal.  Only 3 days this season have seen sub-zero temperatures at the airport, which is right on track with the past 3 years but well behind the usual pace.  Snowfall has been extremely meager, only 2.9" in total so far, and the second lowest on record for this point in the season; but at least now the ground is snow-covered and the scene looks somewhat wintry.

The latest long-range forecasts show no sign that significantly colder weather will develop any time soon for interior Alaska.  For example, here are the 8-14 day and 3-4 week forecasts from the Climate Prediction Center.

The raw computer model forecasts are similar, with nothing but warmth expected right through the next couple of months.  However, a look at the current behavior of the Madden-Julian Oscillation (MJO) provides a hint that cooler weather might be on the horizon for later this month.

For those who are unfamiliar, the MJO is a cycle of enhanced and suppressed rainfall that propagates eastward in the tropics, typically completing a circuit around the globe in about 30-60 days.  The latest MJO phase diagram, shown below, indicates that the MJO has moved into a period of strong activity (blue line moving farther away from the origin; the small "10" denotes the latest observation from yesterday, November 10), with the enhanced rainfall now moving across the Pacific.

The MJO is known to have significant impacts on weather patterns in extra-tropical regions, and so a period of strong and predictable MJO activity can provide useful long-range forecast information.  I looked at the impacts on Fairbanks conditions at this time of year by finding all days with significant MJO activity during November in each phase (1-8) and then looking up the subsequent temperature in Fairbanks out to 60 days in the future.  The figure below shows the results.

Notice the pattern when the MJO is in phase 7, as it currently is.  At about 8-15 days after the occurrence of phase 7, colder than normal temperatures are favored in Fairbanks, but warmer than normal conditions tend to return about 25-35 days later.  If the historical pattern holds true this year, then Fairbanks has a fair chance of turning significantly colder about 8-10 days from now.  Let's see if it works out.

Arctic-Wide Warmth

The extraordinary warmth across the Arctic basin in October prompted me to take a closer look at historical temperature data to get a sense for the magnitude of the recent anomalies.  I first did a search for stations with reasonably complete monthly data since 1971, and then I selected 19 sites that lie on or near the coast and also fall within 2000km of the Arctic Ocean's pole of inaccessibility (marked with an X below).  The spatial distribution of the 19 sites is reasonably even around the Arctic Ocean.

The chart below shows the mean temperature anomaly for these 19 stations in each October since 1971.  Remarkably, the 19-station mean temperature in October 2016 was 5.6°C above the 1981-2010 normal and more than 2°C above the 2012 record.  October 2016 was the second most anomalous calendar month in the data since 1971 - only January 2016 was warmer relative to normal, at +6.5°C.

Looking at recent daily temperature anomalies for the same 19 stations, it is amazing to see that the warmth has become even more pronounced since the end of October; the 19-station mean anomaly reached +9.6°C last Thursday (November 3), and on Friday the coolest of the stations was 5.4°C warmer than normal.  In terms of standard deviations, Thursday's mean anomaly was the highest of any day from 1971-present.

Here's the 19-station daily mean temperature for 2016 on an absolute scale rather than an anomaly scale.  Only 7 out of 311 days have been cooler than the 1981-2010 normal, and then only by a fraction of a degree.

New Record Low for Arctic Ice

Extraordinary warmth and a remarkably slow freeze-up in the Arctic Ocean in recent weeks have led to new records being set again for lowest observed sea ice extent for the time of year.  Today's write-up from the NSIDC describes the situation nicely:

http://nsidc.org/arcticseaicenews/2016/11/sluggish-ice-growth-in-the-arctic/

Based on the monthly "sea ice index" data from NSIDC, the standardized anomaly of October's sea ice extent was the lowest of any month in the satellite record since 1979.  The previous record was set in May of this year, as we discussed here.

Extremely Dry October

Last week I noted the very unusual lack of cloudiness in Fairbanks since the beginning of October, and now that the month's climate data is complete, the magnitude of the anomaly stands out more clearly.

First, it's remarkable to observe that the airport's ASOS instrument reported less than 50% cloud cover on 23 of 31 days, which ties a record for any month of the year: only March of 2002 and March of 2011 had this many mostly-clear days (based on ASOS data since 1998).  Of course, March is usually much less cloudy, with an average of 14 mostly-clear days compared to October's normal of 5 days.  The next closest October for mostly-clear days was 2009, with only 10 days.

The normal seasonal variation of cloudiness in Fairbanks is approximated by the normal relative humidity at 850mb, as measured by balloon soundings - see the chart below.  Based on soundings since 1958, October has the highest normal 850mb relative humidity of any month of the year.  Consider how remarkable it is, then, that October 2016 had the lowest 850mb relative humidity of any month in the historical record (1948-present).

Compared to past Octobers in Fairbanks, the low humidity aloft was really extreme: the monthly mean 850mb relative humidity was 4.9 standard deviations below the 1981-2010 normal.

As noted in the previous post, the clear weather has been caused by a very persistent high pressure ridge over the state, leading to subsidence and drying aloft together with blocking of normal moist westerly winds.  Despite a lack of snow cover for most of the month and windy conditions at times, the clear skies allowed surface temperatures to drop relatively low compared to the pronounced warmth aloft: the month's mean surface-850mb inversion was the strongest on record for October.

October's "blocking" circulation pattern was pronounced around the Northern Hemisphere, with high pressure across the Arctic Ocean and a strongly negative Arctic Oscillation.  It's interesting to note that the Arctic Oscillation and its cousin the North Atlantic Oscillation have tended to be negative during October in the past 15 years, and this seems likely to be related to the loss of Arctic sea ice at this time of year.

With clouds in short supply over Fairbanks in the past month, precipitation was nearly zero; the October total liquid equivalent was 0.02", the lowest for the month since 1914.  The meager snow cover from the 20th has mostly sublimated or melted away, and the airport's snow depth measurement has been down to only a trace for several days.  However, there's variation from place to place; a downtown webcam showed a thin snow cover today, but the ground is basically bare on UAF's West Ridge and up at Cleary Summit.

PDO Forecast Skill

Hot off the press today, I have calculated the skill of the PDO forecasts produced by some of the leading seasonal forecast models.  This is of interest because the PDO is a primary mode of climate variability, and seasonal forecasts - especially for Alaska - often depend on expectations for the PDO phase in the coming months.

I looked at each of the models in the North American Multi-Model Ensemble (NMME) as well as the ECMWF seasonal forecasts.  For each model, I calculated the PDO index from the monthly sea surface temperature forecasts from 1982-2010; these are retrospective forecasts that are provided to help with model bias correction, forecast calibration, and skill assessment.  The chart below shows results averaged over all months of the year, for 3 different lead times.

As we would expect, the ensemble mean of the NMME models (i.e. all but ECMWF) provides the most skillful prediction of the PDO index and is better than even the best individual model; this is why we look at multi-model ensembles.  However, the ECMWF is not far behind in terms of skill, and the CMC2 model is also good.  Interestingly, the CFSv2 (NOAA's climate model) is the worst model by some margin; this is a big surprise to me, because I have a high regard for CFSv2 on the whole.  Further investigation will be needed to see if there is a particular area of the North Pacific where the CFSv2 temperature forecasts are poor.

Looking at the breakdown of skill by month, the winter PDO index is relatively easy for the models to predict, and the early summer season is the most difficult - see below.  This is not too surprising because the winter PDO is linked to tropical Pacific conditions, which the models are quite good at predicting; but the models notoriously struggle with ENSO evolution through the "spring predictability barrier".

Here's the most recent PDO forecast from the same set of models.  NCAR's CESM model is an outlier with its very strongly positive PDO phase, but this is one of the less good models according to the skill statistics.

October Temperature Contrast

There will be much more to say about this once all the October numbers are in, but there's no doubt that Arctic Alaska and the wider Arctic region have seen extraordinary and record-breaking warmth this month.  The chart below shows the updated season-to-date accumulation of freezing degree days in Barrow: the total is now up to a meager 78 FDDs.  The previous record lowest through this date was 128 FDDs in 2012 (data from 1930-present).

The mean temperature so far this month in Barrow stands at 30.2°F, and with a chance of rain (plain, not freezing) in the forecast, that number isn't going to drop much by the end of the month.  Sea ice is nowhere near Barrow, and the Arctic-wide ice extent is at record low levels.

In contrast, clear skies and light winds in Fairbanks have produced strong inversion conditions as surface temperatures have turned rapidly cooler - we might say winter had a sudden onset this year.  Season-to-date FDDs are now higher than the past 3 years at this date and are close to the long-term normal.

North Pacific Blob Update

I posted an update on the Alaska "Blob Tracker" blog today:

https://alaskapacificblob.wordpress.com/2016/10/26/subsurface-conditions/

Clear and Colder

With a little snow now on the ground and nights continuing to lengthen rapidly, temperatures have dropped sharply under clear skies in the past few nights.  Last night was the coldest yet, with temperatures below -15°F in quite a number of locations in the eastern interior.  Here are a few of the coldest measurements so far:

-24°F  Bolio RAWS at Fort Greely
-22°F  Chicken COOP
-20°F  Tok #2 COOP
-17°F  Circle Hot Springs COOP
-16°F  Salcha RAWS
-12°F  Northway AP

The coldest spot around Fairbanks seems to have been the sensor near Smith Lake on UAF's North Campus, which reached -13°F yesterday and -12°F today.  Fairbanks airport dropped below 0°F for the first time yesterday; this is just slightly earlier than normal.

The cold is not particularly unusual for the time of year, but the persistence of clear skies this month certainly is very unusual.  In just the past 2 weeks, Fairbanks has seen sunny and very warm weather (55°F on the 12th), sunny and windy weather, and now sunny and calm, cold weather.  The absence of cloud is remarkable, because this is normally one of the cloudiest times of year in Fairbanks (only early August is comparable).  Including today, 19 of 24 days so far in October have had less than 50% cloud cover on average, and this is easily a record for the entire month - the previous highest total was 10 of 31 days in 2009 (based on ASOS data since 1998).

The reason for the clear skies is plain to see in a reanalysis estimate of 500mb heights so far this month: an axis of high pressure has extended from the northeastern Pacific to northwestern Alaska and the Chukchi Sea.

Compared to normal, upper-level pressure has been well above normal to the northwest, and so the usual westerly flow across interior Alaska has been blocked.  Obviously this also explains the lack of precipitation; the light snowfall last week produced a mere 0.02" of liquid equivalent precipitation - the month's only precipitation.  The driest October on record in Fairbanks was 1954, with 0.08".

Here's the October 1-22 average relative humidity at 850mb, according to the NCEP/NCAR reanalysis: there's a large zone of sub-45% values across the interior.

The departure from normal shows where the dry conditions aloft have been most unusual:

Snow Cover - Will It Last

A fair portion of the interior now has at least a little snow on the ground after yesterday's modest snowfall.  Half an inch of accumulation was reported at Fairbanks airport, but the snow depth was rounded up to 1".  It's little more than a dusting of snow, so what are the odds that it will last and prove to be the start of the winter snow pack?

An analysis from a couple of years ago (see here) indicates that we're now late enough in October that there is less than a 20% chance that a one-inch snow cover will melt out to a trace or less.  This suggests that this year is likely to be one of the 10% or so in which the first measurable snowfall doesn't melt out.  On the other hand, the forecast for next week looks warm, with major storminess in the Bering Sea importing warm air over interior Alaska.  About half of all years reach 40°F or warmer after October 25 in Fairbanks, so if it doesn't snow any more - which looks likely in the short-term - then I'd say there's a fair chance of seeing bare ground again.

The chart below shows the number of days with zero or trace snow cover occurring after the first measurable snowfall, versus the date of the first snowfall on the horizontal axis.  Obviously the earlier the first snow occurs, the more subsequent snow-free days there tend to be before winter sets in.  But it's interesting to see that very late first snows do tend to melt out - which makes sense if the weather pattern is unusually warm in those years.

The outlier on the lower left is from the infamous early winter onset of 1992 - the only time the winter snow pack was established in September, and mid-September at that.

MOS Forecast Bias

As Fairbanks awaits the much-delayed first accumulating snow of the season today (according to the NWS), I wanted to look back at the forecast discrepancy I noted a couple of weeks ago to see how things turned out.  The chart below shows the automated computer forecasts of daily high temperatures from the GFS MOS product (black line) and the actual outcome in red.  Note that I've taken the forecasts for "day 3", for example a forecast for Thursday's high temperature produced on Monday evening.

The results are amazing: the MOS forecast was about 18-20°F too cold for more than a week.  Given the usually strong performance of MOS, the error is very surprising.

I've done some digging on potential causes for the problem, and there seems to be a partial answer in the way the MOS equations are developed.  First, recall that the MOS (Model Output Statistics) technique uses multiple regression to estimate relationships between observed weather variables and predictors in the "raw" model forecasts (e.g. 850mb temperature).  To better capture differences in the relationships between summer and winter, the equations are developed separately for April through September and for October through March (reference here); so the regression switches on October 1 and April 1.

What does this mean for Fairbanks?  Well, the vast majority of days between October 1 and March 31 have snow on the ground, and so the "winter" model will reflect the physics of a snow-covered landscape; for example, when high pressure develops with low humidity and clear skies, the model will predict a substantial drop in temperatures.  However, in the minority of cases when snow is absent, then the model forecast will be too cold.  In other words, the model is effectively assuming snow cover to be present, although it does not use snow cover as a predictor.

This information partially explains the huge cold bias in early October this year, because the weather pattern was just the kind of set-up that is normally cold in winter - high pressure, clear skies, and low humidity.  However, if the snow-cover explanation were completely adequate, then we would expect that similar errors would have occurred in other years with no snow cover in October.  The charts below show the same analysis for selected years in the past.

First, in 2015 the MOS forecasts were too warm (as expected) during the period of unusual snow cover in late September and early October, but there was no obvious bias later in October after the early snow melted off.

October of 2013 was very unusual with its lack of snow, and MOS showed a modest cold bias then, but nothing like what we've seen recently.

Finally, I looked at two earlier years in which strong high pressure occurred in combination with zero snow on the ground during October.  In 2003, high pressure aloft peaked at the beginning of October, and there was a slight cold bias in the forecasts - but nothing too alarming.

In 2009, a ridge of high pressure developed on about the 7th of October, and once again the MOS forecasts were too cold, but not drastically so.  Bear in mind that the GFS model and the MOS equations have changed over time, so the comparison to 2016 isn't quite fair, but if anything the newer forecasts should be better.

In conclusion, it's still not clear what caused the enormous errors in the MOS forecasts recently.  Part of the problem is that the winter MOS equations aren't suitable for predicting temperatures during snow-free conditions, but in previous years this hasn't caused a major issue.  The good news is the NWS forecasters ably detected the recent bias in the MOS output and adjusted their forecasts accordingly.  As a meteorologist myself, it's comforting to know there's still room to improve on what the computers provide.