Sea Ice Follow-Up

Following up on Saturday's post, I searched the NSIDC sea ice index data (since late 1978) for historical dates with comparable ice extent in the southeastern Bering Sea, and specifically reaching the north shore of Unimak Island.  The only date that came up was in early March 1984.  Compare the ice coverage back then with yesterday's map:

The latest NWS analysis shows a higher-resolution estimate of the state of affairs (click to enlarge):

Here's a satellite view from yesterday, courtesy of UAF.  Notice that dense ice is packed along the north shore of Unimak Island, but there is more open water just to the north.

The previous day's image (Sunday) shows a magnificent clear-sky view of the Bering Sea ice cover:

For more comments, check out Rick Thoman's latest blog post, where he shows that 1976 likely had higher ice extent in the southeastern Bering Sea, based on ERA5 estimates.  It will be interesting to compare the ERA5 data between the two years once the reanalysis is available for recent days.

https://alaskaclimate.substack.com/p/alaska-style-march-madness

Record Sea Ice

The remarkable cold spell of the past few weeks has produced a major southward expansion of sea ice in the eastern Bering Sea.  Here's the latest NWS sea ice analysis (click to enlarge):

The analysis shows that the north side of the Alaska Peninsula is entirely iced in, and some ice has reached Unimak Island.  Perhaps not surprisingly, Cold Bay saw a daily record low temperature on Thursday: -3°F.  Only two other dates in Cold Bay (since 1950) saw temperatures this low so late in the season (in 2012 and 1976).

Looking at NSIDC sea ice data back to 1978, the ice extent to the east of 170°W is the greatest on record for any date, exceeding the March 2012 record as of yesterday's analysis.  The recovery from the last 12 seasons of relatively low maximum ice extent is sudden and impressive.  (But note that Arctic-wide sea ice remains far below normal - actually the lowest on record at this time!)

It's an interesting question as to the last time sea ice was observed along the north shore of Unimak Island.  Ice is not too uncommon historically in Isanotski Strait (False Pass) to the east of the island, and the following site indicates that ice "rarely" reaches Unimak Pass to the west, but I'm not sure that has happened since 1978.  I'll dig into the NSIDC data a bit more to look for a firm answer.

https://unimak.us/climate.shtml

Cool Bering/Chukchi Sea

I've mentioned the PDO (Pacific Decadal Oscillation) a few times in recent months, and late August brought a fresh dive into the strongly negative phase.  This reflects an expansion of below-normal sea surface temperature anomalies across the Bering Sea in response to the strong and persistent trough near the Date Line in late summer.

The very pronounced trough delivered very cool Arctic air from Chukotka across the northern Bering Sea to western Alaska, and very cloudy and windy conditions across the eastern Bering Sea really reinforced the cool ocean anomaly.  Here are some climate anomaly maps for the month of August:

As a result, the sea surface waters in the eastern Bering Sea and the Chukchi Sea became the most anomalously cool of any ocean area in the world by the end of August:

Here's a time series of SST anomalies since 2018 for the Bering/Chukchi area east of the Date Line and north of 56°N (i.e. Bristol Bay northward).

While the cool anomaly is now quite notable, it is nowhere near as pronounced as the extreme warmth in 2019, at least when assessing conditions relative to a prior multi-decadal normal.  Notice that most of the major anomalies occur in the summer and autumn, when sea ice is absent and the near-surface ocean layer can become much cooler or (especially) much warmer than normal.

A longer-term chart puts the last few years in context.  The current cool anomaly is among the most pronounced in the modern (satellite sensing) era, although we haven't quite reached the level of the 2011-2012 negative PDO extreme.

Strong Bering Sea Storm

Rick Thoman pointed out that an unusually strong storm made its way across the northwestern Bering Sea earlier this week.  Here's the surface analysis from 4pm AKDT Wednesday, courtesy of Environment Canada, with a red circle indicating the location of St Lawrence Island.  Click to enlarge.

The storm's minimum MSLP was estimated to be 974mb as it passed west of St Lawrence Island, and it "made landfall" in eastern Chukotka at 977mb.  This is unusually low for the time of year - in fact, it's the lowest MSLP on record (1950-present) anywhere in the northern Bering Sea for this late in the season.

To illustrate this, the map below shows the 1950-2023 minimum MSLP for May 11th through the 31st, based on ERA5 data.  The record low values from the northern Gulf of Anadyr across to the northeastern Bering Sea and Norton Sound are 980mb or above; so this was a record strong storm for this late in the season (the June record values are higher). 

Are strong storms becoming more common at this time of year in the Bering Sea?  Not at all.  Interestingly, the trend is the opposite: here's the annual series of May minimum MSLP in a box that approximates the northern Bering Sea.  Note that this is for all of May: there have been stronger storms than this week's event in the first week of the month.

The trend is statistically significant at a fairly high level (R=+0.28, 74 years), and it makes this week's event all the more notable.  But admittedly there is some uncertainty in the reanalysis data, especially for the earliest years (pre-1958, in particular).

Looking at trends in monthly minima through the year, spring stands out as a time with positive trends, i.e. less extreme storms over time, whereas February shows a trend for increasingly strong storms.

The time series for February again has a significant trend, and indeed the change around 1990 does look real.  I don't have an explanation for these trends.

Sea Ice Update

I'm in a busy stretch for travel and work at the moment, so posts may be light and sporadic for a few weeks.

But here's a quick update on Bering Sea ice, which appears to have reached its seasonal maximum on March 19, according to NSIDC ice extent data.  The peak of 734,000 km² is about 14% below the 1991-2020 median value for the seasonal peak, although the daily values have been generally quite close to the climatological daily median for the latter part of the winter: compare the black and orange lines in the figure below.

The shortfall of the seasonal peak value reflects the fact that we never saw a significant surge above normal, whereas a typical winter would tend to see such an event as a result of subseasonal variability.

Given that January-February temperatures were above the 1991-2020 normal across most of the Bering Sea (at least according to ERA5), it's actually surprising that the sea ice managed to reach near-normal levels by the end of February.

Here's the NWS ice analysis for March 18, when low-concentration ice just about reached St Paul Island:

Last winter the ice did not reach St Paul Island, but it did in 2022, when ice coverage was the most abundant since 2013.

30 Years Ago

Today's (and tomorrow's) Bering Sea storm is a very big deal, and there will be more to say about it as reports roll in from hard-hit west coast communities.  NOAA's Ocean Prediction Center analyzed the storm at a remarkable 937mb this morning, and the Canadians had it at 944mb.  Either way, it's incredibly strong for the time of year, and it may be the lowest MSLP for any time of year at its location (1950-present).  We'll see what ERA5 reanalysis has to say in a few day's time.

Thirty years ago, another major weather anomaly was affecting Alaska: extreme early-season cold.  The September 1992 cold outbreak was outstandingly anomalous, and I'll have more to say about this too in the coming days.  For now, here's a chart of Fairbanks daily mean temperatures compared to the modern normal.

By this point in the month (the 16th), Fairbanks was only a week into the extreme cold anomalies that lasted until early October.  There were 10 inches of snow on the ground, and the low temperature was 12°F on the morning of the 16th (and +4°F in North Pole).  Essentially, winter had arrived, because the weekly average temperature did not reach 32°F again until spring.

The scene is entirely different today.  Here are a couple of glorious Twitter photos that caught my eye in the past few week - one from the Chena River State Recreation Area, and the other from our friend Brian Brettschneider near Anchorage.  Spectacular!

Trail running in Interior Alaska during Fall is pretty friggin amazing! pic.twitter.com/4EgVywbDIe

— Chris Butcher (@AKtrailrunner) September 12, 2022

All the colors. pic.twitter.com/xp0yaYnL5t

— Brian Brettschneider (@Climatologist49) September 10, 2022

West Coast Storm Inbound

Just the other day I mentioned that this summer was one of the windiest on record for the northeastern Bering Sea, and an impending storm is about to ensure that September 2022 goes down in the records book as well.

A typhoon of very modest strength ("Merbok") is currently heading north in the western Pacific, passing the latitude of Japan this evening, and reaching the Aleutians by this time tomorrow.  It has already begun the process of transition from a tropical cyclone to an extratropical cyclone by interacting with the mid-latitude jet stream, and the storm will strengthen and expand dramatically as that process unfolds in the next couple of days.

The west coast of Alaska is going to be hard hit.  Here are forecast maps from the ECMWF model, valid for Friday morning, Friday evening, and Saturday morning.

Rick Thoman, who would know better than anyone what wind and flood risk looks like on the west coast, suggests that the storm's impacts will rival that of November 2011, which has its own Wikipedia page.

The NWS discussion is grim reading:

"The track and strength of this storm moving into the Bering Sea on Friday and Saturday has to the potential to produce major coastal flooding and will likely push significant storm surge into the Yukon River Delta. Water levels with this storm are expected to rival the November 2011 storm. Water levels are expected to rise 5 to 8 feet above normal high tide level along the coast in YK Delta and will push well inland along the Yukon River and surrounding YK delta areas Friday night and Saturday. Water levels in Norton Sound are expected to rise 8 to 12 feet above the normal high tide level and are expected to rise 7 to 10 feet above normal high tide levels along and near the Bering Strait. Hurricane force wind gusts are likely along much of the West Coast as this storm passes."

Here's an animation showing the tropical -> extratropical transition, from Levi Cowan's wonderful site:

For historical perspective, the expected sub-960mb MSLP in the northern Bering Sea will, if it happens, be the lowest MSLP north of about St. Paul Island since at least 1950, according to ERA5 data:

Sea Ice Update

First a quick comment again on northern Alaska, where winter is holding on.  Temperatures have dropped into the -20s °F widely across the North Slope the last 3 nights, and not because it's been calm; strong winds have pushed wind chills down into the -40s and even -50s in some spots.

According to airport ASOS data, Point Lay on the west coast has seen nearly continuous blizzard conditions (blowing snow, presumably) since late Tuesday morning.  Winds have been sustained at 30-40mph or greater the whole time, with a temperature between -10°F and -20°F.  Not bad for April.

But of course we're nowhere near record cold for the time of year across the region.  Daily record lows at Umiat are still in the -40s, and just last year it was -38°F on April 3.  The Umiat thermometer notched -50°F on April 5, 1986, and that stands as the latest observed -50°F in the state of Alaska.

And now for an update on sea ice, a follow-up to this post from two months ago.  Bering Sea ice has remained relatively abundant in terms of areal extent, at least compared to the recent history.  Ice extent has mostly been well above last year and it has been far above the extreme lows of a few years ago.

The chart below shows the strong recovery from 2018, with this year's January-March extent reaching 6% above the 1991-2020 median.

The map below shows today's analysis from the NWS.  On the plus side, full ice coverage is found as far south as Saint Paul Island, but on the flip side Norton Sound ice is already in poor shape, with a lot of open water visible on satellite (per Rick Thoman's Twitter comments).

Northern Bering Sea #seaice is in poor condition as seen in this false color NOAA-20 image courtesy @uafgina. The exposed water will warm with the increasing solar heating. Thicker ice Bering Strait & on the east side of St. Lawrence Island. #akwx @UAFGI @KNOMnews @nomenugget_ak pic.twitter.com/DysQlkpRXM

— Rick Thoman (@AlaskaWx) April 2, 2022

As for other seasonally ice-covered basins of the Northern Hemisphere, the major anomaly has been in the Sea of Okhotsk, where a January-March shortfall of nearly 200,000 km² (over 20% of normal) has far exceeded the small Bering Sea surplus relative to normal.  The Greenland Sea has also been running a deficit of about the same size as the Bering Sea surplus.

How to Eliminate Cold

The reprieve won't last long, but a very powerful Bering Sea storm has dramatically swept away the cold air that dominated the state in the past few weeks.  It was an extraordinary transformation: check out the sequence of daily temperature maps below, showing Friday through yesterday.

Rick Thoman notes the extraordinarily rapid change in the statewide daily temperature index:

How fast did the weather change over Alaska? The Alaska Statewide Temperature Index, one of our ACCAP tools, showed its largest one day change (Saturday to Sunday) in over a decade. Details about the index at https://t.co/eph70nSIeM. #akwx @IARC_Alaska H/T @Climatologist49 pic.twitter.com/k0ZlaUNkN6

— Rick Thoman (@AlaskaWx) December 6, 2021

The storm was a nasty one along the Bering Sea coast, with very high winds.  Here are some of the peak gusts, courtesy of NWS:

The following article describes some of the impacts on coastal communities.  Rick notes that sea ice was sufficient to prevent serious coastal flooding, which illustrates a major upside of the recent cold weather.

http://www.thearcticsounder.com/article/2149wind_gusts_in_northwest_alaska_pushed_away

Here's the NWS sea ice analysis from Friday.

Below are the 500mb and surface analysis maps from 3 am yesterday morning (December 6), courtesy of Environment Canada.  MSLP of 958 mb was reported on Sunday afternoon on St. Lawrence Island (at both Gambell and Savoonga), and that appears to be close to the all-time record for the area (about 955 mb according to 1950-present ERA5 data).

This powerful storm may come as a bit of a surprise in a strong La Niña, because just a few weeks ago I discussed the fact that the Aleutian Low tends to be weaker during La Niña.  However, another very interesting aspect of La Niña winters is that daily variability tends to be higher, so the weather patterns near Alaska tend to be more volatile.  I explored this topic a few times in past years, so search for "ENSO variance" if you're interested; or check out the following post.

https://ak-wx.blogspot.com/2015/09/enso-height-patterns.html

Rainfall Trends

As a follow-up to last week's post on increased rainfall in Fairbanks, I did a broader analysis with a couple of gridded data sets to examine the spatial extent of the changes.  This provides a quicker view of broad trends than digging through data from individual observation sites, which often suffer from missing days or other quality issues.

Looking first at a gridded analysis of observed precipitation (courtesy of Germany's Global Precipitation Climatology Centre), I calculated the percentile of the June-September precipitation in each of the six years from 2014-2019, relative to the distribution of June-September precipitation from 1979-2013.  In other words, if (for example) June-September 2014 precipitation was higher than any year from 1979-2013, the percentile would be 100% (the top of the distribution), but if it was equal to the median of the 35-year history, the percentile would be 50%.

I then took the average of the percentiles from the six years; see the map below.  The analysis shows an area centered just to the west of Fairbanks where the 6-year average percentile is around 90%, and this is consistent with the Fairbanks data; the last 6 years have all produced June-September precipitation near or above the 90th percentile in Fairbanks.

The gridded analysis shows unusually wet conditions extending northwestward from the central interior all the way up to Utqiaġvik, and there's another area of sustained high precipitation near the north end of the Alaska Peninsula.  We should regard these results with caution, however; the number of Alaska sites with consistent long-term precipitation data is small, and the gridded analysis is only as good as the data that goes into it.

For an independent view, the map below shows the same calculation from the ERA5 reanalysis, which is simply a long-term sequence of short-range model forecasts; actual precipitation data is not included as an input.  ERA5 confirms that Fairbanks is not the only place with very unusual precipitation during June-September in recent years, and the wet signal is particularly striking along the eastern North Slope.

Out of curiosity, I also looked at the average percentile of June-September shortwave radiation, which of course is mostly tied to cloudiness in June and July.  This shows reduced sunshine in the eastern interior and much more so over the eastern North Slope and the Beaufort Sea.  But over the Bering Sea summer sunshine has been persistently high, according to ERA5.

The percentile of sea-level pressure illustrates the weather pattern that has tended to dominate mid-late summer in recent years: a ridge of high pressure over the Bering Sea.

A global view of average 500mb height anomalies since 2014 at the same time of year reveals that the Bering Sea ridge has been one of the largest flow anomalies worldwide - see below.  (Note that heights are higher almost everywhere because of rising global mean temperatures, but the spatial distribution of the changes is particularly interesting.)

The Bering ridge seems to be part of a pattern of stronger high pressure ridges in the upper mid-latitudes of the Northern Hemisphere (in contrast to smaller height increases in the tropics and the high Arctic).  I believe this is also connected to persistently recurring high ocean temperatures across the North Pacific; I'll write more on this another day.

Modest Sea Ice Growth

Another wave of Arctic air has affected northern and western Alaska in the past few days, leading to extremely low wind chills in some places (-70°F and below in a few northern spots), and cementing this month into the solidly below-normal category for temperature in many locations statewide.

The pattern has remained remarkably persistent, and the latest cold blast has affected southwestern Alaska with a similar set-up to what happened a month ago; in particular, a strong trough over the southwest allowed a powerful low pressure system to move into south-central Alaska yesterday.  Here are the 500mb and surface analysis maps from Environment Canada for 3am yesterday (click to enlarge):

Here are the surface observations from 6am yesterday morning; note the cold and wind in the southwest.  The wind chill in Bethel got close to -50°F for the second time this winter, and yesterday was the 6th day this winter with an average wind chill below -40°.

The daily temperature anomaly chart shows the fresh surge of cold in Bethel:

As cold as this month may have seemed for many, however, it pales in comparison to some of the colder spells of the past, even as recently as 2012.  Bethel was more than 10°F colder in January 2012, with 17 days of average wind chill below -40°.  And as persistent as the chill has been in Fairbanks, January 2012 was more than 5°F colder.

Additional perspective comes from the Bering Sea ice extent data, which is showing a very modest recovery in ice for the basin as a whole (see below).  Remarkably, Bering Sea ice extent is below where it was at this date last year; but of course last year the ice coverage was decimated in February and never recovered.

Here's a side-by-side comparison of ice maps from last year (left) and this year (right).  With cold focused on southwestern Alaska this winter, the eastern Bering Sea is in good shape for ice, but there are still significant shortfalls farther west; there is mostly open water as far north as 64°N in the Gulf of Anadyr.