Sea Ice and Southerly Winds

Last month's look at ice loss in the Bering Sea provoked a comment from reader Eric about the impact of southerly winds on ice extent, so I thought it would be interesting to make a quick comparison of this winter to previous years in this respect.

The chart below shows some November-February average data from the north-central part of the Bering Sea.  Based on reanalysis data from a grid point near St. Lawrence Island, the black line shows mean temperature at 925 mb (about 2000' above sea level), and the blue line shows the southerly component of the wind at the same height - so negative wind values indicate northerly flow (the usual situation).  The red line shows the southerly wind component observed at the surface at Gambell and Savoonga, based on hourly observations since 1996.

It's interesting to note that this winter so far has produced the record highest mean temperature and southerly wind component at 925mb, according to the reanalysis; the average 925mb wind still had a northerly component, but barely.  The surface winds from the two observing sites on the island are highly correlated with the 925mb winds, and the surface data reveal a positive mean southerly component for the first time on record.  It's no wonder that ice extent has suffered with such an absence of cold northerly flow.

Here's an update of the chart from the last post, showing that the ice extent staged a modest recovery after its collapse in February, but in recent days it has turned down again.

Obviously the ice situation is now well outside the historical range from the satellite era, and this is partly because there was such little ice cover prior to the recent episode of ice loss.  The magnitude of the recent loss is not in itself unprecedented, as 6 previous years since 1979 saw at least as much ice loss over a 15-day period - although presumably it's "easier" to lose a lot of ice when it's being removed from warmer locations in the southern Bering Sea.

The maps below show sea-level pressure anomalies from the aforementioned 15-day periods in which ice loss was greater than the recent episode, and the last map shows this year's anomaly.  The common feature is obviously an enhanced southerly flow between high pressure over the Gulf of Alaska and strong low pressure over the western Bering Sea or far eastern Russia.

The unusual flow pattern appears to be partly related to the modest La Niña anomaly that continues to be evident in the tropical Pacific; the map below shows the average February-March sea-level pressure anomaly for 10 previous years when weak-moderate (but not strong) La Niña conditions were observed in late winter.  Strong La Niña episodes tend not to be as favorable for ice loss, because the PDO is more often negative in those winters and the Bering Sea tends to be more dominated by high pressure (and thus it's colder in Alaska).

Winter Winds

After suggesting last Saturday that Fairbanks' February snow total probably wouldn't match last year's number, another round of very stormy conditions moved in on Monday, delivering enough snow to beat last February by just 0.1".  The airport's snow depth of 32" is now exactly the same as last year's number on February 28th.

As in my last post, I want to focus on the very unusual winds that have occurred in conjunction with the recent repetitive storms.  Remarkably, the peak 2-minute wind speed at Fairbanks airport reached 20 mph or higher on 9 out of 15 days in the latter part of February, and this is easily a record for any time in the winter, including March (based on data since 1996).  In fact, this many "windy" days has not previously occurred in anything less than a 65-day period in the last ~20 winters in Fairbanks; it's just extremely unusual to have such a focused period of repeated "high winds".

The rarity of recent events raises the question of whether there's a long-term trend towards more wind in winter in Fairbanks.  Based on ASOS data from the past 20 years, the answer seems to be clearly yes.  The two charts below show the average wind speed (10m above ground) for November-December and January-February, along with the maximum values for both 2-minute and 5-second averages.

The sample size is obviously very small for the seasonal maximum wind gust values, so I dug a bit deeper by looking at frequencies of exceeding the 98th percentile of the climatological distribution.  I estimated the 98th percentiles separately for November-December and January-February and then combined the results for the following chart:

The results indicate that both 2013-2014 and this winter had 8 days with peak wind speed above the 98th percentile on both the 2-minute and 5-second time scales; the long-term expected value would be 2-3 days.  The suggestion of an increasing trend is rather striking, although 20 years is a short period, and if we take out the two big outliers the picture would be less clear.

Dividing the results into early and mid-late winter (see below) emphasizes the outliers even more.  The 98th percentile estimates are included on the legends; they're higher for January-February as a result of the normal annual cycle of wind speed in Fairbanks.

Looking farther back in Fairbanks' climate history is not simple, because the pre-ASOS wind measurements are not directly comparable with the modern data.  We can, however, gain a longer-term view from the winds reported by balloon soundings; the charts below show results for mean and maximum 850mb wind speed in November-December (top) and January-February (bottom).

The November-December data show some indication of an uptick in 850mb wind speeds in the past 20 years, but in a longer-term context there's no obvious trend, and recent conditions do not seem to be outside the range of historical norms.

The situation for January-February is a bit more interesting, as the 850mb wind speed seems quite stable, including in the past 2 decades, and there's little evidence of a recent increase as suggested by the surface data.  Could it be that surface winds have increased while winds at around 4000' above ground level have not?  This might be consistent with a change in preferred flow direction, or perhaps it's a reflection of weaker surface-based inversions that allow winds aloft to mix down more often.  It would be worth looking into this more closely, and it would also be interesting to see if the same thing has happened at other locations, including in the Arctic where recent warming has been so pronounced.

Snow and Wind

The weather of the past couple of weeks has been very unsettled across western and interior Alaska, with multiple stormy episodes, and the contrast to the first 10 days of the month could hardly be greater.  Fairbanks has seen accumulating snow on 12 of the past 14 days, which is the most for this late in the season since 1971; snowfall normally becomes less frequent and lighter as winter advances.

The recent snow has brought the seasonal total to 67.1 inches, or 20" above the long-term median for the date (see below, click to enlarge).  This is marginally more than last winter for the same date, and it's the most since 2000.  A few more inches in the next few days could push February into the top 10 for snowfall, but we probably won't catch last February's total of 23.3" (see here for last year's blog post).

Another symptom of the stormy pattern has been the recurrence of relatively high winds of late in Fairbanks.  Conditions have been notably breezy in 5 separate events of the past 2 weeks, and 5 calendar days have seen peak 2-minute wind speeds above 20 mph (an unusual occurrence in Fairbanks).

The history of 2-minute wind speeds in Fairbanks only goes back to 1996 (the beginning of the ASOS era), but 5 windy days in 2 weeks ties the winter-time record from both March 2003 and December 2013.  In both of those previous cases most of the wind was in one prolonged multi-day event (see here for a blog post on the December 2013 wind); whereas this month we've had repetitive events, and this is really unusual.

The 500mb and 850mb height maps from the past 2 weeks show a pattern that is quite typical for warm and snowy conditions in Fairbanks, with a long fetch of southwesterly flow entering western Alaska.  Westerly flow aloft is easily the most favorable wind direction for heavy precipitation in the cool season, as we saw here.  It's also most common for strong breezes in Fairbanks to come from the west, as they have this month; the last figure below shows that the vast majority of windy days occur with westerly flow in deep winter.  (But note that this is not true in March, when winds are typically stronger and much more often come from the northeast.)

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/

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.

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.

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

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.