Saturday, August 31, 2019

Sea Ice Update

Before I get to the main subject of this post, I would be remiss not to mention a publication released by UAF's International Arctic Research Center last week, titled "Alaska's Changing Environment".  The document compiles a wide range of observations from diverse sectors of Alaska's physical and biological environment, with an emphasis on illustrating the changes that have occurred in recent years and decades.  It's a really nice summary of the many ways that change has been observed across the state, and the graphical content is splendid.  Here's just one example, showing the sudden onset of drought in southeast Alaska in the last year or so, which is particularly dramatic when viewed in contrast to the wet regime that preceded it.

Here's the link to the report, whose principal author is Rick Thoman:

And now on to the topic of sea ice.  In my last update from a month ago, Arctic-wide sea ice extent was running very close to the record low of 2012, but in the past few weeks the rate of decline has slowed and the two curves have diverged; and so it now appears very unlikely that a new record low will be set this year.

A possible cause of the slowdown in ice meltout may be the unusual northerly component to the wind over the Arctic waters north of Alaska and eastern Siberia.  In the past two weeks the ridge of high pressure that has been affecting southern Alaska extended up from the Bering Sea to the East Siberian Sea, and a contrasting zone of low pressure has prevailed near the western edge of Canada's Arctic Archipelago.  Consequently there has been a strong northwesterly flow across the sea ice edge near or just south of 80°N (see the graphics below), and I surmise that this has greatly reduced ice melting and retreat on this side of the Arctic Ocean.  (It is more common for winds to be westerly or southwesterly at this time of year between 75 and 80°N, although of course there's a lot of year-to-year variabiltiy.)

Here are the latest ice extent charts for the Beaufort and Chukchi Seas.  Even though the ice meltout has slowed lately, there is still considerably less ice than last year, and only a few years in the satellite era have had less ice at this date.  Given the very late meltout in the Chukchi Sea last year, it's still entirely possible that significant ice loss still lies ahead in the next few weeks.

The chart below shows the annual minimum extent for both of these basins as a percent of total area; this year will probably come in around 35% for the Beaufort and 5% or less for the Chukchi.  The Chukchi Sea essentially melted out completely (less than 1% ice) every year from 2007-2012, except 2009.  We should bear in mind, though, that ice "extent" includes any sea area with at least 15% ice coverage, so the actual area fraction covered with ice could be close to zero whenever the basin-wide extent drops below 5% or so.  Sea ice area is tracked separately, but extent is the preferred metric at NSIDC for reasons that are explained here:

Saturday, August 24, 2019

Wildfire Latitude

Last weekend, as I was writing about the stark rainfall contrast across Alaska, the effects of the drought in the south-central region were unfortunately illustrated by an outbreak of several rare late summer wildfires, leading to evacuations and significant property damage.  And "drought" it most definitely is: Anchorage has still seen no measurable rain this month and remains on track for the driest summer of record.

Even the national Drought Monitor, a subjective drought assessment issued every week by experts in the field, shows widespread drought, both in south-central Alaska and in the southeast of the state.  In the vicinity of Anchorage the drought is classified as "extreme", the second highest category, and remarkably the analysis estimates that there are twice as many people in "extreme drought" in Alaska as in the rest of the USA put together.  Here's the Drought Monitor website link and the latest analysis for Alaska:

An article in the Anchorage Daily News this week discussed the broader context for the unusual late wildfire activity near Anchorage: check out the article for excellent commentary from Rick Thoman.

In light of the discussion in the article, I thought it would be interesting to look at the historical variations in the latitude of Alaska wildfire; for example, do the historical data show a notable southward shift in fire locations?  Somewhat remarkably, fire acreage and location data is available from the Alaska Fire Service/AICC all the way back to 1939; I have no idea how reliable the numbers are from the earlier decades, but the analysis produces some interesting results.

First, the total annual fire acreage from this source shows the 80-year history of overall fire activity in Alaska.  As many others have noted, the frequency of big fire years has increased since about 1990, although there have always been highly active years every now and then.

Here's a histogram comparison of fire acreage in latitude categories for three different periods in the history.  In this chart we're looking at the fraction of total acreage in each period, so the columns add up to 1.0 for each period (i.e. the columns do not correspond to absolute acreage numbers).  Note that the AFS data provides the location at which each fire started, not (for example) the centroid of the acreage burned.

The overall pattern of latitude dependence has of course remained quite similar over the years, but it is intriguing to see that the peak latitude band changed from 66-67°N in the first 40 years, to 65-66°N in 1979-2008, and then to 64-65°N in the past 10 years.  This does suggest a southward shift, although it's not easy to judge by eye whether the median of the distribution has changed; more on that later.

The next chart shows absolute acreage totals (average per year) for each period, and here the increase in the 64-65°N band stands out more distinctly because of the upward trend in total acreage.

To gain another perspective, I calculated the acreage-weighted mean latitude of fire activity (for all fires of 10 acres or greater) in overlapping 15-year periods; see below.  Here the result is interesting and quite different, because it seems that Alaska wildfire acreage occurred relatively far south - indeed farther south than in recent years - prior to about 1970.  But beginning with the big fire year of 1969, more northern acreage tended to be burned, and that trend continued until 1997, when the trend switched back; in that year, the 2 million acres of fire were focused around 63°N, which is the farthest south of any 1+ million acre fire year.

Finally, here's a scatter plot of acreage versus acreage-weighted latitude by year, with different colors highlighting the two halves of the 80-year history.  Recent decades have seen some fairly big years focused south of 64°N (1997, 2002, 2013), but if the data are to be believed, 1940 and 1957 were very active fire years with a southern focus (around 64°N).  Surprisingly, the overall acreage-weighted mean latitude is actually higher for the last 40 years than the preceding 40 years.

In conclusion, the historical trends in Alaska wildfire latitude are not simple to interpret, and the older history clearly suggests that the baseline of recent years may not be atypical on a longer time scale.  However, a couple of caveats are in order: first, my acreage-weighted analysis is obviously dominated by a relatively small number of big fire years, so the trends may not be statistically robust.  And second, it seems wise to remain cautious about the quality of the data from the early decades; for example, if acreage was systematically underestimated in the northern interior (where perhaps aerial monitoring was less intensive in the early days), then the early fire acreage reports would be biased towards the south.  Perhaps someone among the readership can chime in on the history of fire monitoring in Alaska.

Sunday, August 18, 2019

Rain: Feast or Famine

Another round of heavy rain fell across the central interior late last week, taking the month's rainfall total to over 5 inches in Fairbanks, and exacerbating flood problems in the area.  According to the NWS, the Salcha River to the southeast of Fairbanks is the highest it's been since 2014 (the wettest summer of record in Fairbanks); here's the hydrograph:

A webcam view of the Tanana River at Nenana from yesterday morning (the latest available) shows a rather swollen river, and according to the NWS there is some flooding in the town.

Only a couple of years in the past have seen so much rain in Fairbanks in the entire month of August, with one of course being the flood year of 1967.  With flood control measures now in place, a repeat of that flood will not occur in Fairbanks, and indeed the Moose Creek Dam is operating tonight to regulate the Chena River flow volume.

Readers in south-central Alaska may well wish that some of the rain would make its way south of the Alaska Range, because while Fairbanks is near-record wet, Anchorage has seen no measurable rainfall at all this month so far.  This is an extraordinary contrast: Fairbanks is second-wettest on record for the month to date (only lagging 1967), while Anchorage is record dry.  The chart below shows August 1-17 rainfall totals for the two cities over their common climate history since 1952.

How can we explain the difference in rainfall fortunes over such a relatively short distance?  The map below shows the 500mb height anomaly (departure from normal) for the first 16 days of the month; the glaring feature is the high pressure anomaly just to the southwest of Anchorage, which has deflected the jet stream and associated weather systems farther north into western and central Alaska.

Below is the actual 500mb height map, instead of the departure from normal.  The ridge near southwest Alaska extends far to the south and southwest, and on the west side of the ridge a long fetch of southerly flow can be inferred, stretching from the central western Pacific up to the Bering Sea and then east into the interior.  This is the moist flow that has created the persistent rain in the interior.

Here's what the flow looks like at 700mb, or about 10,000 feet up in the atmosphere.

Let's compare the recent pattern to what typically occurs when Fairbanks is considerably wetter than Anchorage in the first half of August; the map below shows the average 500mb height anomaly in 6 such years.  There are some distinct similarities to the height anomaly map above, but in the historical (and less extreme) events the unusual ridge was farther west along the Aleutians, rather than sitting in close proximity to Anchorage.

In the opposite scenario where Anchorage is notably wetter than Fairbanks at this time of year, it's not too surprising to see that there is a trough (lower heights) near Bristol Bay, rather than a ridge.  This pattern would allow a vigorous jet stream to affect south-central Alaska, but a downslope component to the flow over the Alaska Range would bring much drier weather to the interior.

Sunday, August 11, 2019

Rainy Season

Parts of interior and western Alaska have seen a lot of rain in recent days, which is of course good news as far as wildfires are concerned: there has been minimal growth in fire acreage this month so far.  But the copious rainfall has brought other problems to some locations, including flooding and (at least in Denali NP) mud and rock slides.

Here are maps of daily estimated precipitation since the beginning of the month:

Rain was especially remarkable across the Seward Peninsula and the northwest interior in the first few days of the month, with Nome reporting its highest 24-hour rain total of record (2.47").  Rain amounts from some other sites were even more remarkable; see below for a graphic published by the NWS on the evening of August 3rd.

This time of year does of course tend to be relatively wet in interior and western Alaska, but there's a very pronounced west-east gradient in when the wettest conditions tend to occur.  Consider the contrast in timing of peak rainfall frequency between Nome, Fairbanks, and Northway - see the charts below.  Far up the Tanana River valley in Northway, measurable rainfall is most frequent at the beginning of July, and August actually sees much less rain than July on average, so the rainy season, such as there is, ends quickly there as summer wanes.

In Fairbanks the peak in precipitation frequency is about 4 weeks later than in Northway, according to 1981-2010 normals from NOAA, and August tends to bring considerably more hours of rain, and more cloud, than July.  (However, the total August rainfall is a bit less than in July, because there are fewer heavy rain events.  For more discussion of July versus August, see posts here and here.)

In contrast, if we go all the way over to Nome, August is much wetter than July and clearly stands out as the wettest month of the year.

The map below gives a sense of how the date of peak precipitation frequency varies across the entire state.  It generally advances from early July to early August from east to west across the interior, reaches late August or early September along the west and southwest coast, and then most of the south-central region sees the height of its rainy season in late September (when interior weather is usually much improved).  Of course the epic rainy season of southeastern Alaska doesn't reach its peak until October.

Here's a zoomed-in map to reveal a bit more of the local variation in the southeast interior and south-central.

Finally, the peak climatological value of precipitation frequency is depicted in the map below.  Unlike the seasonal timing, the maximum frequency of precipitation is surprisingly uniform from west to east across central Alaska, with peak values of 40-50% in many locations.  In the Fairbanks area, the only locations with normal precipitation frequency rising above 50% are the Keystone Ridge and Two Rivers co-op sites; and North Pole apparently never exceeds 35%.

Friday, August 2, 2019

Sea Ice Update

An update on Arctic sea ice conditions has been long overdue on this blog, although Rick Thoman has been providing excellent updates all summer via his Twitter feed.  Nevertheless it's worth reiterating the main headlines, which are that Chukchi Sea ice is at record low levels for the time of year, and Arctic-wide sea ice stands a chance of breaking the record low from 2012.

Unsurprisingly, the extraordinary shortfall of ice in the Bering Sea last winter set the stage for extremely early meltout farther north, and Chukchi Sea ice extent has been almost continuously at record lows ever since meltout began in early May.

The figures below show the NWS sea ice analyses from today (top) and last year on this date (below).  It's quite a difference in both the Chukchi and Beaufort Seas, although the Beaufort Sea ice deficit is not at record levels.

For the Arctic Ocean as a whole, sea ice is running just about neck-and-neck with 2012, which saw the lowest seasonal minimum on record.  However, a new record low this year isn't necessarily likely, because the rate of meltout was so great in August 2012 that it may be tough to keep pace this year.  Much will depend on the weather pattern in the next several weeks, including the degree of storminess and the amount of warm air that is imported into the Arctic from southerly latitudes.  In 2012 an intense low pressure system in early August probably contributed to the rapid ice loss by breaking up ice and mixing it with warm water.  The forecast for the next week or two looks more suggestive of high pressure in the Arctic, but on the other hand there's no lack of warmth in waters south of the ice edge, so I think a challenge to the 2012 record must be a real possibility.  We'll know soon enough.