Wednesday, July 11, 2018

Still Frozen on the North Slope

It has been a relatively chilly summer so far in the northernmost parts of Alaska, with Utqiaġvik (Barrow) recording the coolest June since 1994; despite a high temperature of 57°F on the 25th, the monthly mean temperature was only 33.5°F.  Only one June since 1980 has been cooler (1994); but prior to 1980 this was a typical mean temperature for June.

The cool conditions have been quite persistent for nearly two months now.


As a result of the chilly weather, some of the fresh water lakes on the North Slope are still frozen, as seen in the land-cover imagery from the Suomi polar orbiting satellite.  The image below was taken at 4pm today; ice is indicated by light blue colors in the Arctic Ocean and the larger lakes of the North Slope.


Today's webcam images from the observatory at Teshekpuk Lake (the large lake in the image above) confirm the presence of ice cover:




The same images from yesterday showed more ice cover near the shore; meltout appears to be getting into full swing now.  Compare the situation to what was observed on June 17 of last year: https://ak-wx.blogspot.com/2017/06/north-slope-thaw.html




The blame for the persistent chill lies with a trough that has transported Arctic air south into northern and interior Alaska; Fairbanks felt the effects of this about a month ago.  Here's a map of the 500mb height anomaly for the month of June.



However, the 925mb temperature map for June puts Alaska's cool anomaly in perspective - see below.  In contrast to the localized and rather mundane region of chill over northern Alaska, the central part of Arctic Siberia saw a very large and pronounced warm anomaly, and indeed the Siberian warmth was very extreme by historical standards.  At the town of Saskylakh at 72°N (nearly the same as Utqiaġvik), the June mean temperature of 60.0°F was more than 5°F above any other June, with data back to 1936, and the month was a remarkable 17.5°F above the 1981-2010 normal.


Wednesday, July 4, 2018

Possible El Niño Modoki

There has been a lot of talk in climate circles about the possibility of a new El Niño episode developing in the Pacific Ocean over the coming months, and this could become an influence on Alaska's weather patterns during the autumn and winter.  Of course it was only two years ago that the very strong El Niño of 2015-2016 ended, so I think it would be surprising to see another major episode so soon; but there is a strong consensus among the seasonal forecast models that a significant El Niño is on the way.  Here's the sea surface temperature (SST) forecast for early winter from the NMME models:


The models expect the warmest SSTs, relative to normal, in the central equatorial Pacific, and this has also given rise to a bit of speculation that the coming El Niño (if it happens) may be more of a "Modoki" variety than a classic episode.  El Niño Modoki is sometimes considered to be a distinct climate phenomenon in which warming occurs in the central portions of the equatorial Pacific, as opposed to the classic El Niño pattern that involves the most pronounced warming in the east.  However, others emphasize that the Modoki versus classic distinction is really a continuum and is mostly related to the strength of the warming episode.

Regardless of the semantics, it's interesting to compare the winter climate patterns between central-Pacific and eastern-Pacific El Niño's.  The Aleutian Low is usually stronger than normal during strong classic El Niño winters, and there is a pronounced low pressure anomaly centered to the south of the Alaska Peninsula, as shown in the map below.  The years listed here were obtained by taking the top El Niño winters (based on the Multivariate ENSO Index) that were not also top-10 Modoki episodes (based on the El Niño Modoki Index).


Enhanced southerly flow associated with the strong Aleutian trough tends to bring warmer than normal conditions to much of Alaska.


El Niño Modoki winters look very different; rather than showing a strong Aleutian Low, there's a notable tendency for high pressure from the Bering Sea to southern Alaska.  It also tends to be warmer than normal over the Bering Sea and surrounding areas (see maps below).  Note that these years are the top Modoki events that did not also have top-10 Niño3 (eastern equatorial Pacific) SST anomalies; so by definition they are also not strong El Niño episodes.  We might think of them as weak-to-moderate warming episodes that were clearly focused on the central equatorial Pacific.




As we consider which "flavor" of El Niño might be more likely this year, we can ask how the current global SST pattern compares to the typical precursor patterns for the two varieties.  First, the map below shows the SST pattern in June and July prior to east-Pacific El Niño's.  The central and eastern tropical Pacific tend to be warmer than normal already by this time of year, and there's also a strong warm signal in the Indian Ocean.



Does this resemble the current setup?  Not at all; June SSTs were near-normal in the Indian Ocean and near or even slightly below normal in the eastern Pacific.
The map below shows the pattern for summers preceding Modoki episodes; there tends to be warmth in the central Pacific and also - interestingly - in the northern North Pacific, and especially the Gulf of Alaska.  Overall this is a lot more consistent with the current pattern, and so this fits with the idea that El Niño, if it does emerge and persist into winter, is more likely to be a central-Pacific episode.  It will be interesting to see how it plays out.


Friday, June 29, 2018

Warming By Season

A few weeks ago I alluded to the fact that the temperature increase of recent decades has been smaller in summer than at other times of year in northern Alaska; but when we consider that temperature variability is lowest during summer, the warming is still very significant at that season.  It's worth illustrating this in a bit more detail.

First, the chart below shows the simple temperature difference between the 1971-2000 and 2001-2017 periods for 12 overlapping 3-month seasons throughout the year, for Utqiaġvik (Barrow) as well as Kotzebue, Nome, and Fairbanks.  The temperature difference peaks in late autumn (October-December) for all 4 sites, and the amount of warming that has occurred in summer is ostensibly quite small in comparison.


However, when we normalize the temperature difference by the interannual standard deviation of the seasonal temperatures (during 1971-2000), the picture changes - see below.  Autumn is still the time of peak warming, but the summer warming no longer appears insignificant.  Interestingly, the summer temperature change at Kotzebue has been larger in standardized terms than the winter and spring warming; but at Utqiaġvik the summer trend remains less pronounced than at other times of year.

None of this is a big surprise, but I think it provides a bit of useful perspective on the significance of relatively "small" summer warming trends in the Arctic.

Sunday, June 24, 2018

Are Fairbanks Summers Getting Wetter?

Hi,  Rick T. here with a post about Fairbanks summer rainfall.

I was at a meeting a few weeks ago and one of the folks there, who works in community planning, was wondering if Fairbanks summers are becoming wetter. After all, the past five years (especially 2014 and 2016) have featured two of the wettest summers of record. If summers are getting wetter, that is something that planners need to take into account when considering things like downtown storm drainage capacity, rural road culvert sizing or generally higher river levels. Happily, here in Fairbanks we have enough historical data to take a stab at answering that question.

Since summer precipitation often comes in the form of showers and thunderstorms (especially the first half of summer) and these are much less frequent (climatologically speaking) over the flats as compared to areas closer to higher terrain, it's not a good idea to use the usual Fairbanks "threaded" climate record, since we know that the Fairbanks Airport, being out on the flats, receives significantly less rain than areas not far to the north and east. Luckily, we have precipitation observations taken in almost the exact same place, from the UAF Ag Farm, since July 1911. While there are some data quality problems with the Ag Farm observations over the decades, summer precipitation looks reasonable. Amazingly, there appears to be only one month that is actually missing, August 1969. For this one month I used the College Observatory data (taken on West Ridge near what is now called Jack Townshend Point, about 3/4 of a mile northeast of the Ag Farm). Here is scatter plot of the accumulated June through August precipitation at the Ag Farm for the past 106 years (1912 through 2017):

Total summer precipitation June through August each year 1912 to 2017.
Just glancing at the graphic there is nothing obvious to my eye. The ten wettest and driest years are not all bunched-up on one or the other side of the plot. Naturally, there is some clustering: the 1920s saw a number of very dry summers and the 1940s several wet summers. Of course, we need not rely on our eyeballs; statistical analysis is our friend.

Ordinary linear regression and quantile regression for the median
The simplest analysis is just linear regression. In the chart above I've plotted the observations along with ordinary linear regression (green line), which in effect is modeling the average (mean), and a technique called linear quantile regression, which here I use to model the median. A reason to check both is that ordinary linear regression is sensitive to individual values that are far from other values (i.e. "outliers"), while quantile regression of the median is not sensitive to extremes. In this case, we see that both flavors of linear regression show an increase of about an inch precipitation over the past century. For the ordinary linear regression this is significant at the 90% confidence level but not at the 95% level. The trend of the median summer precipitation is not significant at even the 90% level.

Of course, it's entirely possible (even likely) that changes in summer precipitation are not best described by a simple linear fit. Often in Alaska, a piecewise linear regression (i.e. "hockey stick") provides a better estimate of trend – but not in this case. There is no evidence of any significant changes in the linear trend.

Another change we can check for are any abrupt "step" increases. When we do this, the results are also mixed:
Summer precipitation 1912-2017 at the UAF Ag Farm analyzed for abrupt step changes requiring  20 and 25 year minimum length.
Here, the results are mostly dependent on the required length of any segment. Since we are looking a long term changes that might be important for community planning, I restricted the analysis to changes that persist at least couple of decades. Requiring a minimum length of 20 years shows one step increase, at 1998 (note: the analysis was not restricted to a single step change. With 106 years of data, as many as five step changes are possible). Interestingly, increasing the minimum length to 25 years results in no step changes of statistical significance.

The last analysis I'll look at here is a simple smoothing of the observed precipitation:
Summer precipitation 1912-2017 at the UAF Ag Farm with a cubic spline fit, including the 95% confidence interval.
Here the brown line is a cubic spline fit to the data. Cubic splines are very commonly used to detect patterns (not just linear) in noisy data. In this case, looking only at the brown line, we see, much like the linear regression analysis, a general upward trend, especially since the 1990s, with a total increase of nearly two inches between 1912 and 2017.  However, this is only part of the analysis. Since we don't know what the hypothetical "true" fit is, in this case primarily due to the spread in the summer to summer rainfall, we can construct confidence intervals that give us a better idea of where the actual (but unknown) fit lies. I've done that here, shown as the gray shading. Notice that the confidence interval at least partially overlaps itself for the full 106 years of records, e.g. look along the total precipitation 6" grid line. This suggests that we don't (quite) have high confidence that there really is a significant change in the spline fit.

So where does this leave us as to the original question? Are summers becoming wetter? It looks to me like the answer is an unequivocal "maybe".

On the yes side, the ordinary linear regression trend is significant at the 90% level, as is the 20-year minimum length step change.

On the no side, the trend of the median summer precipitation is not significant, and there is no significant step change when requiring a 25-year (or longer) length.

In the maybe camp, the cubic spline analysis is certainly suggestive of a significant change, just barely falling into the no trend camp (using a 95% confidence interval).

While this analysis is perhaps not satisfying from a community planning perspective, since "maybe"  seems like it's not an "actionable" answer, from a climate perspective it is interesting that we are close to being able to detect an increase, which for precipitation in Alaska is not (yet) usually the case. However, increasing precipitation during the 21st century is exactly what the the climate model consensus have for nearly all of Alaska, and we may be starting to see that reflected in Fairbanks. The next several years will help to clarify the trend for the early 21st century.

Thursday, June 21, 2018

Update on Lightning and Fire

Prompted by some comments on my post about Alaska fire acreage a couple of weeks ago, I acquired the most recent data from the Alaska Lightning Detection Network and pulled up a comparison to recent years - see below.  Earlier this month the cumulative number of lightning strikes recorded by the network was the highest for the time of year in the modern data set, but ever since last week's cold blast there has been almost no activity (at least until today).  (Note that the lightning sensors were changed in 2012, so it's not possible to do a direct comparison with earlier years.)


Fire acreage statewide currently stands at about 210,000 acres, which is also above most recent years, although 2013 and 2015 really took off in the latter part of June.  Today's cumulative acreage is about 10 days ahead of the long-term median in terms of the rate of burning statewide.

The year-to-year variability in fire acreage is obviously much higher than that of lightning, as there are other critical factors that control fire growth.  For example, both lightning and acreage were very high in 2015, but in 2013 acreage was high while lightning was relatively sparse; of course 2013 was very hot and dry, so fuel conditions were very conducive to the spread of fire.

Unsurprisingly, the ratio of acreage to lightning strikes is wildly variable - see below (calculated here whenever the cumulative number of strikes exceeds 1000).  In 2015, over 30 acres burned for every lightning strike detected, on average.  It would be interesting to compare this number to fire behavior in the lower 48.



While fiddling with the lightning data, I also determined the days on which the most lightning strikes were detected within 100 miles of a few different sites.  This allows us to look at the typical weather patterns associated with particularly intense lightning activity in different parts of the state.  For example, here's the average 500mb height pattern (the departure from normal) for strong lightning activity near Fairbanks: unusually high pressure aloft is centered to the northeast, and Fairbanks lies just to the south of the anomalous ridge axis.

When lightning activity is intense within 100 miles of McGrath, the ridge axis tends to be located much farther west, and a trough is evident over the Gulf of Alaska.


Below are the maps for strong lightning activity within 100 miles of Ambler and Eagle, respectively.



Wednesday, June 13, 2018

Chilling in Summer

Northern and interior Alaska has seen some very chilly weather for the time of year in the past few days, as a strong upper-level trough and an unseasonably cold air mass plunged south out of the Arctic at the beginning of the week.  Despite the fact that the summer solstice is now less than 10 days away, and daylight is continuous, sub-freezing temperatures have occurred in many of the usual cold spots in the interior.

In the Fairbanks area, three consecutive days have seen temperatures falling into the 30s, with upper 20s at the colder spots like North Pole and the Goldstream valley.  The Smith Lake site on UAF's North Campus recorded 26°F yesterday morning; but the chart below (note the Celsius scale) shows that only a few days since late May have NOT dipped below freezing at this spot - even when daily high temperatures were well into the 70s.


The airport has seen 37°F, 36°F, and 37°F in the early mornings of the past three days, which is a remarkably cold series of daily minimum temperatures for this time of year.  In fact, this is the closest to the solstice that Fairbanks has ever observed 3 straight days with low temperatures of 37°F or lower at the official climate site (1930-present).

It's also interesting to note that with a high temperature of only 53°F, Monday's daily mean temperature was a mere 45°F.  It's been almost 70 years (1949) since Fairbanks saw such a chilly day this late in June (or in July).

The mid-level atmospheric pattern that created the midsummer chill is evident in the sequence of maps below.  The charts show the 500mb analysis at 24-hour intervals from 4am on Saturday through 4am today, and for ease of reference the red dot shows Fairbanks' location.  Notice the very tight pressure gradient and associated strong northerly flow that rushed down from the high Arctic into Alaska at the beginning of the week - this was a remarkable cold blast for the time of year.

Saturday:

Sunday:

Monday:

Tuesday:

Wednesday:

Finally, here's a nice view from Monday of the fresh snow that fell at Toolik Lake (2400' elevation) on the north side of the Brooks Range.  The lake is still mostly frozen despite the fact that the air temperature reached 60°F earlier this month.


Friday, June 8, 2018

Fire Season Begins

Lightning has been widespread over Alaska in the past several days, and wildfires have sprung up as an inevitable consequence.  According to the latest information on akfireinfo.com, fires have burned about 25,000 acres statewide so far this season, which is about normal for the time of year.  Fire activity typically ramps up quickly in June, with burn acreage often exceeding 200,000 acres by the end of the month.

Year-to-year variability of fire acreage in Alaska is a very interesting topic and a fascinating and challenging prediction problem.  I'd like to do an in-depth study of it one day, but today I'll just make a couple of points.  First, consider the map below, showing a 23-year correlation between sea surface temperatures in May and the subsequent fire acreage rank.  I've used the rank of the fire acreage (with higher rank for higher acreage) rather than actual acreage numbers because the distribution is strongly non-Gaussian.

The color scheme on the map rather exaggerates the statistical significance of the correlations, as the highest values are only 0.4-0.5, but nevertheless it's interesting to see that fire activity is favored by warmer ocean conditions in both the northern North Pacific and the central tropical Pacific.  The horseshoe-shaped pattern looks quite reminiscent of the PDO pattern, but actually it's a bit different; the typical PDO horseshoe hugs the coast of North America more closely and has a strong inverse correlation with SSTs extending east of Japan to south of Alaska.  Alaska fire acreage is actually nearly uncorrelated with the PDO index in May.



There is a better correlation (+0.44) between Alaska fire acreage and the North Pacific Mode (NPM) index.  The NPM pattern is focused between 40 and 50°N across the North Pacific, and according to the first map above, this is an area that shows some connection with Alaska fire activity.


So what do current conditions look like?  The map below shows the May analysis; the NPM was slightly positive, as it has been for the last 4 months, but it's not a dramatic anomaly (excepting the Bering Sea warmth).  This suggests that ocean temperature patterns are only slightly favorable for enhanced fire activity this year.  As an aside, there seems to be no sign of the strongly positive NPM phase that the long-range models were predicting earlier in the year (and are still predicting).



We can also search for fire-acreage-related precursor patterns in the atmosphere.  According to the map below, there is a statistically significant - but not highly robust - correlation between 500mb heights over Alaska in May and subsequent fire acreage.  This makes sense; if the weather pattern sets up with a ridge over Alaska during May, then dry and sunny conditions will reduce fuel moisture, and the next month or two are also more likely than not to be warm and dry.



How about May 2018?  Rather than having a ridge over the state, there was a trough over the southwest, and most of the interior was wetter than normal.  So this points to reduced fire activity, albeit with low confidence.



And now perhaps the most interesting result that I've stumbled upon in this brief analysis.  The map below shows the average SST anomaly in winters following the 6 most active fire seasons since 1995.  Most readers will recognize the pattern immediately: the warm band along the equator in the central and eastern Pacific is a classic El Niño pattern.  This suggests that very active fire seasons in Alaska have a strong tendency to be followed by significant El Niño episodes.

The chart below confirms the rather remarkable statistical connection; the 4 strongest El Niño's since 1995 were preceded by Alaska fire acreage in the top quartile (6 of 23) since 1995.  Naively this suggests we can use Alaska fire acreage as a predictor for El Niño - but why would this be?  My take is that the atmospheric and oceanic patterns that evolve into major El Niño events are already unfolding in the summer months prior to the classical winter peak of El Niño, and those patterns happen to be very favorable for Alaska wildfire.


The last point to make is that the latest data from the long-range computer models have recently shifted quite decisively in favor of El Niño for the coming winter (2018-19); so it will be most interesting indeed to see how the rest of the fire season evolves in Alaska.