Wednesday, February 21, 2018

North Pacific Blog Post

I added a new post this evening on the Alaska/North Pacific "Blob Tracker" Blog, discussing the potential for extraordinary warmth this summer in the North Pacific:

https://alaskapacificblob.wordpress.com/2018/02/21/north-pacific-forecast/



Friday, February 16, 2018

Bering Sea Ice Loss

[Update Feb 17: the Bering Sea ice extent fell another 10,000 km2 yesterday; I've updated the chart.  Also, I added an animation of February 15 ice extent maps and a comment on St. Lawrence Island.]

In Tuesday's post I highlighted the strong ridge over Alaska that produced remarkable temperature inversions across the interior a week ago.  Another effect of the amplified circulation pattern is that the Bering Sea has been subject to strong and persistent southerly flow, and this has really done a number on the sea ice.

Bering Sea ice was already running at record lows for most of this year so far, and now the ice extent has dropped farther below the long-term normal than at any other time in the modern era.  The chart below illustrates this: the gray shading indicates the historical range of daily ice extent anomalies (departure from a 1979-2016 mean), and the red curve is this year's anomaly; as of yesterday it was more than 400,000 km2 below the mean for the first time.


I've annotated four previous years in which sea ice extent also dropped to very low levels in late winter and early spring, and it turns out that each of these was a La Niña winter (like this winter).  I'll do some more analysis later, but it seems clear that La Niña favors the kind of high-amplitude trough-ridge pattern that can bring disruptive southerly flow to the Bering Sea ice.

As an aside, it's interesting to note that the current ice loss episode has produced a decrease of 133,000 km2 in ice extent, or over 30%, in just 8 days.  As extreme as this seems, especially for mid-winter, it's not unprecedented; in the early February 1985 episode, the Bering Sea lost 161,000 km2 of ice in 10 days - but then gained it back, and more, in the next 10 days.  The difference this time, of course, is that we're starting from a very low point owing to the season-long shortfall in ice.

Here are the daily ice analyses from February 7 (left) and today (right), courtesy of NOAA.



Here's a simple animation of the February 15/16 sea ice extent maps from NSIDC (clipped to a suitable domain).  This gives a bit of context on interannual variability.  The maps for 1985, 1989, 2001, and 2018 are included separately below.







Note that St. Lawrence Island was only partially surrounded by ice in mid-February 2001, and this is similar to the current situation; webcam images from Savoonga and Gambell today both clearly showed open water.



Tuesday, February 13, 2018

Extreme Inversions

This is a few days late but still worthwhile as an interesting note, I think: late last week some very extreme temperature inversions were observed over Alaska's interior as a strong upper-level ridge moved eastward.  On Friday morning the balloon sounding at McGrath measured a temperature difference of 62°F between the surface (-21°F) and 3500' above ground (+41°F); this is the strongest inversion measured at McGrath since March 2008.

At Fairbanks the inversion strength peaked on Saturday morning, right under the ridge axis, with a temperature difference of 60°F between the surface (-19°F) and 3000' above ground (+41°F).  This is essentially a tie with the strongest inversion ever measured by radiosonde in Fairbanks; the record inversion was just 0.1°C stronger in December 1956.  Here's the so-called skew-T diagram.


Here are the 500mb maps from Friday and Saturday mornings (top and bottom respectively), showing the slow progress of the ridge eastward; the peak inversions were closely aligned with the central axis of the ridge.




Rick Thoman posted a very remarkable map of Saturday morning temperatures around Fairbanks, showing a full 60°F of temperature difference at the surface in a distance of under 10 miles.  Locations in the hills were above freezing while some valley locations were in the -20s.


Looking at historical sounding data from Alaska's upper-air observing sites, the state record for inversion strength is 72°F at McGrath on January 13, 1966 (-41°F to +31°F).  McGrath has seen quite a number of occasions with inversions of more than 65°F, but on a typical winter day the inversion strength is greater in Fairbanks.

The chart below shows the all-time records (blue columns) for 13 sounding sites in Alaska that have been continuously active since around 1950, and the red columns indicate the 1981-2017 median inversion strength for morning (3 am) soundings in winter (December through February).  Only Annette Island has a median of zero, i.e. the average winter 3am sounding does not show an inversion.


How about the depth of inversions?  In general, stronger inversions are deeper, and so Fairbanks, McGrath, and Utqiaġvik (Barrow) are the winners for both strong and deep winter inversions.  The chart below shows the record and median values for inversion depth.



An interesting side note is that upper air observations were made at Northway from 1948 to 1955 and at Barter Island from 1953 through 1988, and both of these sites observed some very strong inversions.  In just a few years at Northway a number of 60+°F inversions were observed, with the record being 68°F of inversion on January 24, 1952.  The record at Barter Island was 64°F on January 25, 1983.

Wednesday, February 7, 2018

Warmth to Return

The southern and eastern interior of Alaska has seen a decent spell of cold weather in the past couple of weeks, but it's on the way out now, and unseasonable warmth looks likely to return in the near future.  The chart below puts the recent cold in perspective relative to the anomalous warmth earlier in the winter; notice how few days have been more than 1 standard deviation below normal, compared to the many days of 1SD or more above normal.

As chilly as the recent spell may have seemed, the departure from normal for the past 2 weeks is "only" 14°F, which is quite modest for interior Alaska; the 2 weeks ending December 19 were over 23°F above normal.


The impending shift back to warmth is partly related to a very dramatic weather event that is unfolding in the stratosphere: a "sudden stratospheric warming" (SSW).  In these events, which occur a few times a decade, there is a weakening and disruption of the winter-time vortex of westerly winds that usually prevails in the stratosphere above the Arctic.

In some SSW events the vortex is merely weakened and displaced away from the pole, but in the more dramatic cases the vortex splits or breaks down completely and the flow reverses to easterly around the pole for a time.  The upcoming SSW will be of the latter variety; the two maps below, courtesy of tropicaltidbits.com, show the change over the course of a week beginning last Sunday.  The second map shows that the vortex will split into two daughter vortices by this Sunday, with high pressure and anticyclonic flow in the middle.



These events are followed very closely in the long-range forecast community, because the disruption of the vortex usually works its way down to the troposphere and leads to a weakening of the westerly flow closer to the surface in the subsequent weeks.  This in turn often allows blocking high pressure to set up and cold air to spill south into the mid-latitudes, and in particular Europe and western Asia have a strong tendency to be cold in the weeks following a SSW.  Conversely, the blocking pattern tends to favor unusual warmth in southern and interior Alaska as low pressure sets up over the Bering Sea.

Of course the SSW isn't the only driver of the current circulation pattern, but the latest long-range forecasts are certainly quite consistent with the expected SSW impacts.  Here are the latest MSLP and temperature forecasts for the North Pacific sector over the next 6 weeks from NOAA's CFSv2 model; the maps show the ensemble mean forecast anomaly of MSLP (left) and temperature (right) relative to a 2000-2016 normal.  Based on this forecast, it wouldn't be a surprise to see some record breaking warmth in Alaska in the next several weeks.

Week 1


Week 2


Week 3


Week 4


Week 5


Week 6




Friday, February 2, 2018

Yukon River at Dawson

Long-time readers may recall a few posts last winter that mentioned the exceptionally slow freeze-up of the Yukon River in Dawson (Yukon Territory).  It's interesting to note that the same thing has happened this winter - there is still a substantial gap of open water that has prevented the construction of the usual ice road to West Dawson.  This year an effort was launched to accelerate the freeze-up with a "slush cannon", but lack of progress led to the project being called off recently.

Here's how the river looked last Sunday.


Last winter we noted that an unusual ice jam upstream was the primary reason for the open water, rather than exceptionally warm weather.  It appears that the same may be true this year, because again the weather has not been particularly warm overall this winter in Dawson; the chart below shows that total freezing degree days through January 22 (when the ice-making project was canceled) were very close to the normal of the last 20 years.



The repetition of the slow freeze-up this winter suggests that it's not a random occurrence - "something" has changed - but the temperature data rules out a simple explanation based on excessive warmth.  I started to look for river flow data from the autumn to see if the river discharge has been unusually high or low, but I had difficulty acquiring suitable data.

One possibility (and this is mere speculation, being outside my field of knowledge) is that there could have been a subtle change in the river profile/cross-section upstream of Dawson, owing to deposition or erosion, that may have altered the hydrodynamics in a way that now favors the formation of an ice jam in a new location.  If any readers have comments on this idea, or other suggestions on how to identify the underlying cause of the open water (i.e. is it related to weather and climate), then I'd be glad to hear them.

Tuesday, January 30, 2018

Record Chukotka Warmth

Unseasonably cold conditions are hanging on in Fairbanks-land, with temperatures around 10-20°F below normal for the time of year, but some really exceptional warmth occurred yesterday in Russia's Chukotka district, on the other side of the Bering Strait.  Temperatures rose above freezing in a zone stretching from the northern Sea of Okhotsk and the Kamchatka Peninsula up to the Arctic Ocean, and the town of Ostrovnoe at 68°N saw a high of 42°F.

The location of Ostrovnoe (see the map below) means that it is usually a very cold location in winter; it's in northeastern Siberia, above the Arctic Circle, 100 miles south of the Arctic Ocean, and in a river valley.  The normal high temperature in January is well below -20°F and so it's considerably colder than any inhabited place in Alaska.  According to my calculations, yesterday's high temperature was 64°F above the 1981-2010 normal, and the low temperature of 33°F was a remarkable 69°F above normal.  The GHCN daily data from 1936-present (which is a bit patchy in some years) indicates that this is the first time Ostrovnoe has seen a daily minimum above freezing in January or February.



To put these daily anomalies in context, the largest positive daily temperature anomalies on record in Fairbanks are just over 50°F (for example, a high of +52°F on January 16, 2009, compared to normal of about 0°F).

Here's a chart of daily mean temperatures in Ostrovnoe since November 1; the daily mean temperature was 67°F above the 1981-2010 smoothed normal.  It should be noted that this is actually not unprecedented: December 21, 1955, was 68°F above the 1981-2010 normal (and presumably even farther above the then-colder normal).



The profound temperature gradient across the Bering Sea is being caused by an intense ridge of high pressure centered right over the date line - see this morning's 500mb height analysis below.  Powerful southerly flow to the west of the ridge is pulling warm air up into eastern Siberia and the high Arctic, but northerly flow is keeping Alaska rather cold.


The map below shows the average 500mb height anomaly in January and February, for 10 winters with strong La Niña conditions; a Bering Sea ridge is one of the main elements of the mid-high latitude atmospheric response to La Niña.  Although the current La Niña episode is not particularly strong, the current flow anomaly clearly fits the pattern rather well.


Thursday, January 25, 2018

Deep Cold and Persistent Snow

[Update 5am Jan 26:  Snow ended before 2pm yesterday in Fairbanks; one more hour would have tied the record for most hours of snow in a 7-day period.  As for consecutive hours with snow, this ended at 69 hours - nowhere near a record, although 45 hours of snow with temperatures below -20°F does tie a record from 1999 (thanks to Brian Brettschneider for that one).]

Very cold air associated with the upper-level low over the western interior has spread east in the past 36 hours, and as a result temperatures have dropped markedly in the Fairbanks area despite plentiful cloud cover and continuous light snow.  Normally the higher elevations in the hills provide some refuge from the valley-level cold, but this morning's sounding shows no low-level temperature inversion; however, there's a very deep inversion extending up to 700mb, and this is more characteristic of a true Arctic air mass.


Temperatures at the standard levels of 925mb (~2000' above ground) and 850mb (~4000') are very low this morning; the 850mb temperature of -30.3°C is the lowest since January 2013, and the 925 mb temperature of -32.1°C is the coldest since late January 2012.

Here are the minimum temperatures (°F) at the surface from the last 24 hours; colder conditions occurred just to the west of Fairbanks where the sky cleared out for a time yesterday.



Also of note is the persistence of the light snowfall in Fairbanks in recent days, caused by the lingering zone of ascent aloft to the east of the upper-level low.  As of the latest hour (6am AKST), snow has been falling continuously for 61 hours.  This is nowhere close to the record from late December 1984, when snow was reported every hour for 5 and a half days (133 hours).

However, if we look at the number of hours of snow in the past 7 days, we are getting close to a record: starting last Friday, there have only been 13 observations at the top of the hour that did not report snow falling - it has been snowing for more than 90% of the time in the past 6+ days.  Based on data since 1950, the lowest number of no-snow hours in a period of 7 calendar days (midnight to midnight) is 22 (in January 1996), so this record will be broken if Fairbanks gets another 9 hours of snow before midnight tonight.

For reference, here's this morning's 500mb analysis.  It's interesting to note the very cold air aloft to the east of the low; the temperature of -41°C at 500mb above Fairbanks is accompanied by wind from the south-southeast.