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.

Saturday, February 2, 2019

Radiation Normals

In the past week I've been digging deeper into the CERES radiation data for Alaska (see the previous post) and making some interesting discoveries along the way.  It really is quite fascinating - at least to me - to explore a new data set that deals with aspects of the climate that most of us don't often think about.

There's too much to discuss in one post, so here I'll focus on a few monthly averages over the period of record for just a couple of locations.  The chart below, for the Fairbanks area, shows the monthly average downward shortwave radiation at the surface, or simply the incoming "solar radiation" at the surface.  The CERES data set provides one set of numbers for a grid point near Fairbanks, and we can compare these to data from the high-quality Climate Reference Network (CRN) site just to the northeast of the city.

Interestingly the CERES numbers are consistently a bit higher, and especially so in March and April.  Some of the difference could arise from shading of the CRN site by nearby hills, but more investigation would be needed to find out how much solar energy is lost this way.  The larger difference in March and April looks more like a systematic bias that could be related to reflection and scattering of solar radiation from the snow-covered ground; perhaps there is locally more reflection and then downward scattering of the sun's rays at the CRN site than the CERES model estimates for the broad area contained in the (1x1 degree) grid box.

Here's the same chart for Utqiaġvik, formerly Barrow.  As expected the summer peak is narrower, but it's also higher than in Fairbanks; despite more abundant cloud cover on the Arctic coast, longer daylight hours allow Utqiaġvik to see more direct solar energy than Fairbanks in May and June (and July according to CERES).  The same positive bias is observed in the CERES data relative to the CRN site just a few miles outside Utqiaġvik.

As discussed in the previous post, longwave (infrared) radiation is just as important for the climate as direct solar radiation.  I'll illustrate this in more detail in the next post, but for now the chart below shows the overall surface radiation budget (longwave and shortwave combined) for the same two locations.

The most striking difference between Fairbanks and Utqiaġvik is seen in spring, as Fairbanks quickly sees a building surplus of energy in April and May, while Utqiaġvik really struggles to gain any ground in terms of radiative transfer until high summer - at least according to the CERES data.  The main reason is quite obvious: the universal snow and ice cover around Utqiaġvik create a high albedo in April and May, so the spring sunshine is largely reflected in the north, whereas the generally wooded environment and earlier snowmelt near Fairbanks allow for much more efficient absorption at this time of year.

The time series of CERES and CRN data also reveal some very interesting results over the ~15 year history, but I'll document these in another post.