Tuesday, March 26, 2019

Yet More Warmth

Winter is coming to an end with truly exceptional warmth across Alaska this year, and a record early breakup is looking increasingly likely as the pattern persists.  The map below shows temperatures well above freezing and far above normal today across nearly all of the state.  A long list of records could be drawn up, but standing out to me today is the high of 60°F at Yakutat; this is the second time this month that 60°F has been reached there, and it's a record for the month of March.

Fairbanks will probably set a record for highest monthly average temperature in March; Utqiaġvik (Barrow) will certainly do so.

As I noted about 10 days ago, the breakup situation at Nenana is interesting, because the accumulation of thaw heat units (thaw degree days) is so far ahead of normal that there's a chance breakup could occur before the Ice Classic closes to new entries.  The chart below (click to enlarge) shows evidence for this contention; the dashed ascending line shows the accumulation of TDDs through today, and then going forward as predicted by NOAA's probabilistic EKDMOS system (see here).  The expanding gray range shows a plausible range of possibilities based on the 10th and 90th percentiles of the daily probability distribution, and the horizontal lines show percentiles of the TDDs that have been required to produce breakup in the past.

The smallest number of TDDs that have ever been observed prior to breakup is 75, in 2002 and 2006, and the short-range forecast suggests that Fairbanks will probably reach that number by the end of the month; we're already above 50 TDDs as of today, which is easily a record for this early in the season.  Last year's breakup occurred after only 88 TDDs of thaw.  So it's theoretically possible that the tripod could fall this weekend, although I would rate this as highly unlikely because there's a tendency for earlier breakups to require more TDDs to overcome the weaker solar insolation this early in the spring.

A more reasonable estimate for breakup date is based on the median TDDs at breakup, i.e. 141 TDDs, and the latest forecast suggests this may be reached around April 9th; but of course by that lead time there is a lot of uncertainty in the forecast.  Nevertheless, with continued warmth looking likely, and with ice measurements suggesting below-normal ice thickness at Nenana, I think breakup is at least as likely to occur before April 9th as after, and April 5th or before (the contest closing date) is not at all impossible.  And if the general weather pattern holds, then a record early breakup (pre-April 20) will be almost inevitable.

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.