Thursday, March 21, 2019

Winter Temperature Attribution

Looking back at the mid-winter period of December-February, it was another warm one for Alaska as a whole, although much less so in the southeast of the state than in the west and north, as the graphic below illustrates (courtesy of ACCAP and Rick Thoman, via Twitter).

The southeast was relatively cooler because it was located on the east side of the unusual high pressure (ridge axis) that tended to prevail to the south of Alaska; so the flow had a more northerly component than usual in the southeast, but there was a pronounced southerly wind anomaly in the Bering Sea and western Alaska.  The map below shows the departure from normal of the mean 500mb height.

It's interesting to observe that for the second consecutive winter the state has seen significantly higher than normal temperatures (on average) without a significantly positive PDO phase.  Here's a scatter plot of Dec-Feb statewide mean temperature versus PDO index, with the last 6 winters highlighted (click to enlarge).

The correlation between PDO index and temperature over the period since 1950 is significant but not very high (R2 = +0.42), and this is partly because of the shifting temperature baseline; Alaska's winter temperatures have become warmer than they used to be for the same PDO pattern.  If we create a simple linear regression model based on temperatures and PDO from 1950-1999, we find that every one of the last 20 winters has been warmer than the pre-2000 regression would suggest (see below).  This is compelling evidence that changes in the PDO don't explain recent warmth (unlike for example the sudden winter warming that occurred in Alaska in 1976 in tandem with a major PDO regime shift).

In light of these results, I thought it would be interesting to explore the "attribution" of winter warmth a bit more using 500mb circulation patterns.  The idea here is that seasonal mean temperatures are strongly affected by changes in atmospheric flow patterns, and so the question arises as to how much of (a) interannual temperature variance, and (b) long-term trend, can be explained just by flow patterns?

Essentially the same topic was explored in some detail by UAF's John Walsh, along with Brian Brettschneider, Rick Thoman, and others, in a 2017 paper that is well worth a read:

In the paper, Walsh et al used historical pattern analogs to estimate the contribution of the flow pattern to temperature variations in one particular winter (2015-16).  I've taken a different tack by calculating the correlation of Alaska Dec-Feb temperature to 500mb heights on a latitude-longitude grid; the result is below.  Note that I removed the linear trend from both temperatures and heights prior to doing the calculation.

The peak correlation (R2 = +0.70) is found over northwestern Canada, where higher 500mb heights are strongly connected to warmer Alaska temperatures in winter.  Of course this is simply because a ridge over western Canada, and a trough near the Aleutians, produce unusual southerly flow across most of the state.

Using this correlation result, I constructed an index to describe the temperature-relevant flow pattern by projecting each winter's 500mb height anomaly onto the R2 field across the Northern Hemisphere north of 45°N.  The resulting index is correlated with (detrended) Alaska temperatures at R2 = +0.74, so we can say that the flow pattern explains about 75% of the winter-to-winter variance in temperature.  Here's a scatter plot of observed (detrended) Dec-Feb temperature versus the prediction from linear regression with the 500mb circulation index.  As in the PDO plot, the last 6 winters are highlighted, and again we see that recent winters have been consistently warmer than the regression would suggest.

Now here comes the interesting part.  If we calculate each winter's circulation index with the non-detrended 500mb height pattern, we obtain an estimate of the actual (non-detrended) contribution of the circulation to temperature variations throughout the history.  It's no surprise to find that the circulation index has become more favorable to warmth over time, so the modeled temperature has a rising trend - but not nearly as much as the actual temperature, see below.  Once again it's clear that the last 6 winters have been consistently warmer - and often much warmer - than the 500mb circulation would suggest.

Subtracting out the circulation's influence from each winter's temperature results in an estimate of what the temperature history might have looked like if there had been no variation at all in 500mb flow patterns (taken in isolation, of course; the ocean-atmosphere system is all interconnected in reality).

Using this simple method, it seems that the circulation anomalies explain about one-third of the warming trend that has occurred since 1950; the rest can be attributed to broader warming trends, especially in the high latitudes, including higher sea surface temperatures near Alaska and of course reduced sea ice extent.

Going back to the 2017 paper by Walsh et al, they concluded that less than half of the anomalous warmth in winter 2015-16 can be explained by the atmospheric circulation, and my results here are consistent with this.  The 3 months of Dec 2015 - Feb 2016 were 9.7°F warmer than the 1950-2018 mean, but the 500mb index regression suggests only 3.0°F of anomalous warmth that winter.  Like this winter, the flow pattern of 2015-16 really wasn't all that favorable for extreme warmth in Alaska, with not much of a ridge over northwestern Canada - see below - although the PDO was much more positive than it is now.

Sunday, March 17, 2019

Nenana Tripod Hooked Up

With temperatures in the interior rising well into the 40s Fahrenheit today - far above normal high temperatures in the 20s - I thought I'd take a look at the Nenana webcam to see how the ice looks on the Tanana River.  The answer is: not good, with a lot of standing water and a dark, rotten appearance that suggests breakup may not be far away despite the incredibly early date.

It seems the folks at the Nenana Ice Classic feel the same way, because the tripod appears to have been hooked up to the contest's clock; the cables that are evident in the webcam view were set up today (click to enlarge the image).

I've watched the Ice Classic's webcam with interest for years, and I'd say the condition of the ice is at least 3-4 weeks ahead of the earliest I've seen it look like this before.  The earliest breakup on record is April 20 - with data back to 1917 - and that's more than a month away from now.  The contest doesn't even close to new entries until April 5th, but I think there's at least a fair chance the ice goes out before then.

Here's a webcam view from more than a month later (April 22) in 2017; that year (and last year) the ice went out on May 1st, which was in line with the long-term normal.

And here's the view during my first time on the ice last year on March 18th.  The rightmost figure is me!

Friday, March 8, 2019

Bering Sea Meltout

The big story in Alaska climate at the moment is the near-complete loss of sea ice in the Bering Sea in recent weeks.  Unrelenting warm southerly flow has reduced Bering Sea ice extent to a record low level for the date, dropping below even last winter's remarkable ice shortfall.  The climatological peak in Bering ice cover occurs at the end of March, but the latest daily ice extent of only 150,000 km2 is more typical of about June 1st, according to the 1981-2010 normal.

With only about 20% of normal ice cover for the date, the current anomaly is the greatest percentage shortfall (relative to normal) between about mid-December and mid-April (mid-May prior to last winter), so it's safe to say that such an extreme absence of winter ice has not been observed previously in the modern satellite record.

It's also of interest to note that the last 40 days have seen a decrease of 417,000 km2 in sea ice extent, and a 40-day loss of this magnitude has not previously been observed this early in the season.  The earliest was between mid-February and late March of 2002, but that was starting from over 900,000 km2.

Here's today's ice analysis from the National Weather Service (click to enlarge).

A few days ago the NWS map was also showing considerable ice loss north of the Seward Peninsula and in Kotzebue Sound, and today's satellite imagery suggests (to me at least) that the remaining ice in this area is broken and insubstantial.  The image below (3pm AKST today) is rather obscured by clouds, but the Seward Peninsula and a dark-looking Kotzbue Sound are visible just to the right of center.

A webcam image from Kivalina this afternoon also suggests there is a lot of open water in the southeastern Chukchi Sea; note the obvious dark band along the horizon.

The Iditarod mushers will be reaching the Bering Sea coast tomorrow, but unless the teams need to cool off, it seems they'll be confined to land as they travel up to Nome.  Here's the ocean view from Shaktoolik this afternoon.  Air temperature: 37°F.

Thursday, February 28, 2019

Radiation Climate Trends

Following up on a couple of previous posts about the balance of infrared radiation (both shortwave and longwave) that shapes Alaska's climate (see here and here), it's interesting to look at trends since 2001, when the CERES data begins.  Let's start with annual shortwave radiation data from near Utqiaġvik (formerly Barrow) - see below.  The blue line in the chart below shows the measured average flux of incoming shortwave energy according to the ground-truth CRN instrument, and the gray line indicates the estimated value from the CERES satellite data.  As we noted before, the CERES numbers are biased a bit high.  The green line shows the estimated amount of solar radiation that is reflected at the surface.

The CERES and CRN data are not well correlated year by year, but both series show a statistically significant downward trend in average incoming solar radiation.  This is consistent with increasing cloud cover associated with a warming Arctic in which reduced ice cover and a warmer ice-free ocean surface tend to produce more evaporation and higher atmospheric moisture content.

The CERES estimate of reflected solar radiation is more variable from year to year than the downward flux, but it too suggests a downward trend.  If annual average albedo is decreasing (owing to a shorter ice/snow season), then we would expect reflected solar radiation to decrease even more rapidly than incoming radiation, and in fact the linear trends do support this.  According to this short period of record, the surface is absorbing slightly more solar energy even though cloudiness has increased markedly.

The situation in Fairbanks is of course much different.  There are no significant trends in the shortwave radiation data, although it's interesting to note that 2018 was the least sunny year in the CRN data set by some margin.  2013 stands out as being a sunny year, but that's mostly because of the extraordinary weather of May and June that year.

How about longwave radiation?  First I'll emphasize how closely tied the longwave energy transfer is to the surface temperature; in both Fairbanks and Utqiaġvik, the annual average emission of infrared radiation upward from the surface is correlated with the annual mean temperature at about R=0.9; this is just the Stefan-Boltzmann law in action.

With longwave radiation being closely tied to temperature, it's no surprise that (a) there is not much variation in absolute terms from year to year, but (b) values have been noticeably higher since 2016 at Utqiaġvik - see below.  This means that the surface is losing energy upward at a greater rate; it's a natural consequence of warming and it's a negative feedback, although the downward longwave has also increased nearly the same amount.  The CERES data hint at a small increase in the net energy loss from the surface at these wavelengths.

As with the shortwave data, the trends are smaller in Fairbanks; it appears there has been a small increase in net energy loss - similar to Utqiaġvik - but it's very marginal in terms of statistical significance (p~0.1).

What happens when we add up the net shortwave and longwave contributions?  See the chart below.  For Fairbanks, there is a hint of a downward trend in the overall annual radiation gain owing to the longwave changes (increased upward emission), but again it's not really significant.

Utqiaġvik's total radiation gain is noisy, but here the interesting result is the absence of a trend; it turns out that the small gain in shortwave radiation (lower albedo) is almost exactly offset by a small increase in net energy loss via longwave.  Does this suggest that the ice-albedo effect is less of a concern than we might have thought?  I don't think so, but it would be worth examining more closely for other parts of the Arctic; there is always more to learn.

One caveat associated with all of this is of course that the surface CERES data is computed with a radiative transfer model that depends on all sorts of inputs, both measured and estimated.  The model undoubtedly has some deficiencies, so the results - and especially the trend analyses - could be sensitive to details of the model; in other words, I frankly have no idea what the error distributions might be on the CERES data.

Saturday, February 23, 2019

Return to Warmth

After a couple of cold spells in mid-winter, most of Alaska has returned to unusual warmth over the past month, although it hasn't been as persistently extreme as in early winter.  The graphic below, courtesy of Brian Brettschneider, shows the state-wide index devised by Rick Thoman and described here:

The culprit behind the recent warmth has been an unusual ridge directly to the south of Alaska - see below.

This is quite reminiscent of the persistent ridge that fostered the northeast Pacific's "warm blob" of a few years ago (for example see here for discussion), although the recent anomaly has been centered farther west, allowing cold air to affect southeast Alaska and western states to the south.  Here's the recent temperature departure from normal according to gridded reanalysis data, with western and northern Alaska showing the greatest warm anomaly.

The recent warmth has been particularly persistent in Bethel, where it's now been a month since the daily mean temperature was less than 5°F above normal.

Nome, Kotzebue, and Utqiaġvik have also been exceptionally warm compared to normal of late.

Looking at the interior, the recent warm anomaly diminishes from west to east as expected.

Finally, the warm anomalies at southern (not southeast) sites (e.g. Anchorage, Cold Bay) are not as large in amplitude, but in terms of standard deviations these two sites have seen more significant warmth since October 1st than any of the other 8 sites shown above.  Here's the mean departure from normal in terms of standard deviation of the daily anomalies for October 1 - February 21:

Anchorage  +0.72
Cold Bay  +0.68
Kotzebue +0.66
Bethel +0.65
Utqiaġvik +0.63
McGrath +0.56
Nome +0.53
Bettles +0.53
Fairbanks  +0.53
Northway +0.40

The chart for Juneau is refreshingly different, although it has still been a warm winter on average; the average anomaly since October 1 is +0.27 standard deviations.

Thursday, February 14, 2019

West Coast Storm

Earlier this week a strong storm moved through the northern Bering Sea and the Bering Strait, bringing strong winds to western Alaska and breaking up sea ice that was already weakened from unseasonably warm weather in the past few weeks.  According to NSIDC analysis, Bering Sea ice extent peaked at 567,000 km2 just a few days after my last update and has dropped by one-third in the 18 days since.  Here's a comparison of ice maps from the peak (Jan 26th) and today.

The most recent storm seemed like a strong one, with sustained wind speeds of over 50mph on St. Lawrence Island; and the balloon sounding from Nome on Tuesday morning reported a wind speed of 49mph at ground level.  However, the minimum central pressure of the storm was only about 980mb when it reached Alaska, which isn't particularly low by Bering Sea standards.

The chart below shows the typical monthly extremes in MSLP for a latitude/longitude box centered on the west coast of Alaska (depicted on the map underneath).  The months of October through February all typically bring at least one storm with minimum MSLP of 975mb or lower somewhere in this region; so the latest instance was a fairly run-of-the-mill storm.  What is not normal, however, is the lack of sea ice; only last year had significantly less ice on this date in the modern satellite record (but mid-February 1985 had about the same amount).

Thursday, February 7, 2019

Dawson Ice Bridge Problems

Readers may recall that in the past couple of winters I've mentioned the odd reluctance of the Yukon River to freeze up properly at Dawson in the Yukon Territory.  This has created a problem for residents of West Dawson who in years past relied on an ice bridge for seasonal access to the main town on the east side of the river.

Perhaps not surprisingly, the same problem has occurred this winter, and efforts to stimulate ice growth across the stubbornly open channel were called to a halt last week.  Here are a couple of news articles on this winter's lack of success:

Here's a view from the webcam in Dawson a few days ago, looking out across the river; as in past winters, a narrow strip of open water is visible.

The lack of an ice bridge has been deemed sufficiently important that Canada's National Research Council looked into the issue last year, and a report was issued in October; it makes for very interesting reading.  The report discusses a variety of hypotheses about what may have caused the change, but there are no definitive conclusions; there may be multiple factors at play, some of which we've speculated about before on this blog (including good comments from readers).

As we've noted before, the change in ice conditions can't simply be pinned on warmer winter weather, because temperature data from Dawson doesn't show substantially warmer conditions in the winters when freeze-up failed.  The accumulated total of freezing degree days has been slightly lower this winter than the two-decade normal, but 2016-17 and 2017-18 were both near normal through this point in the season (see below).  Of course it is possible that ground and/or river water temperatures have risen, as noted in the NRC report.