Saturday, May 20, 2017

Does Wet=Cool in Summer? The Fairbanks Evidence

Hi, Rick T. here. On Friday's ACCAP monthly Alaska Climate Outlook Webinar, the question came up afterwards about whether the Climate Prediction Center's June through August 2017 outlook of both a warm and wet tilt in the odds over a large part of mainland Alaska was reasonable, on the idea that if it's a rainy (dry) summer, then would not it be more likely to be cool (warm)? So I thought it would be worth reprising that question here on Deep Cold.

Below is the basic Fairbanks evidence in the form of a scatterplot. For summer precipitation I'm using the Standardized Precipitation Index (SPI). The SPI is widely used in drought assessment  and is extremely useful for comparing precipitation anomalies for regions with quite different climates (say, Fairbanks vs. Yakutat). However, it's also a handy way of characterizing anomalies at a single locations, since the SPI  effectively takes into account the distribution of precipitation amounts, which very often do not follow a bell (Gaussian) curve and turns the anomalies into standard deviations. For temperatures, which at the monthly and seasonal scale usually do follow a bell-curve distribution (in the absence of trend), standardized anomalies are just the observed minus the long term average, and that difference divided by the long term standard deviation.When we work-up those statistics for each summer during the Weather Bureau/NWS era at Fairbanks (using the full 1930-2016 record as the baseline), here's what we get. The summer SPI is on the vertical axis and the summer temperature anomaly is on horizontal axis. The red dashed line is the linear correlation, which is significant, and does lend support to the idea that warm summers (right-hand side of the plot) do trend to be dry, while cool summers due tend to be wet.

That's all well and good, but with 87 data points it's kind of hard to make out the details. Since we're interested in the warm/dry and cool/wet idea, here's the same plot, but I've masked out all the points in the "near normal" category (using the three-category characterization, so ±0.43 standard deviations) and colored the quadrants that correspond to the four combinations of significantly wet/dry/warm/cool.
From this it is much clearer that there have been summers that have been both warm and wet overall (e.g. just last year, upper right quad) and summers that have been cool and dry (e.g. 1947, lower left quad), but clearly wet and cool or warm and dry are much more favored. It's very hard to read in the plot, but by this measure there have been no "cool and dry" summers (lower left quad) in Fairbanks since 1969. Upon reflection, this isn't  surprising, since we all know there is a very marked trend in June through August average temperatures.
We can make some account of the current climate by using a more recent baseline period for temperatures (there is no multi-decadal trend in summer rainfall). Since the Climate Prediction Center is basing the outlook on the 1981-2010 reference period, we'll use that. We can also adjust the correlation between rainfall and temperature to take into account the temperature trend (by using generalized least squares regression). When we make those changes, we get this:
So we see the overall correlation between summer temperature and precipitation is weaker (but still significant) and that the modern criteria of what a warm and wet summer wound be really are an unusual combination. Come back in September to see how the outlook worked out.

Update: A couple of folks have asked about how wildfire activity fits into this, so here's the basic temp vs. rainfall anomaly plot with the million acre plus seasons highlighted. In this view, only two seasons, 2002 and 2015, had significantly above average summer rainfall in Fairbanks and also burned a million acres or more (statewide).

Thursday, May 18, 2017

Not So Cold in Barrow

The sun is now remaining above the horizon for 24 hours a day on the Arctic coast of Alaska, but temperatures are only just getting up to about the freezing point.  Barrow (or Utqiaġvik) has not yet exceeded 32°F this spring, although it was 36°F back on January 1st!  The normal date to break the freezing mark for the first time in Barrow is May 9th; temperatures have been less variable than usual of late.

Looking back at the 2016-2017 winter, it's remarkable to note that Barrow failed to reach -40° for the 5th consecutive winter; the lowest temperature observed all winter was -36°F, which is exactly the same as the last two winters.  The two winters before that only saw -39° and -38°.  The persistent absence of deep cold is quite remarkable.  In terms of daily maximum temperatures, not a single calendar day remained at or below -20°F in the past winter, and it's the first time that has happened in the modern history of daily observations from Barrow.

On another note, here's an interesting chart showing the date of the first spring day with a temperature of 40°F or higher in Barrow.  The absence of variability in the 1950s and 1960s is rather striking, and the two outlying early occurences in 1931 and 1936 really stand out.  There seems to be no particular reason to doubt that it really did get that warm in late April in those years; it was very warm in Kotzebue too.  The scanned observer forms are below (click to enlarge; note "rain" on April 23, 1931).

Monday, May 15, 2017

More on Spring Warming Trends

Some readers may find this post a bit esoteric, but I think the end result is worth reporting.  My interest was piqued by Rick Thoman's chart that I posted last week, showing the dates on which 65°F or warmer was first observed each year in Fairbanks, dating all the way back to 1905.  The early data may be a bit dubious, but it's interesting to look at the complete series.  As I noted before, the chart shows that the earliest "first warm" days have become much earlier over time, but there is essentially no trend in the latest 10% of the distribution - there have been several instances of very delayed warmth in recent decades.  This implies that not only has the climate warmed at this time of year, but inter-annual variance has increased considerably.

The question that comes to mind here is, "Is the increase in year-to-year variance statistically significant?"  Visually it's clear from the chart that the early decades saw more consistency in the arrival date of summer-like warmth in Fairbanks, whereas recent decades have been more variable.  But given a warming climate, could random chance have produced a chart like this?  Or can we conclude that there really is something going on here in terms of the year-to-year variability?

To address this question, I created 1000 simulations of the daily temperature history in Fairbanks for the full 113-year period.  First I calculated the daily normals for overlapping 30-year periods throughout the history, i.e. the daily mean and standard deviation of daily maximum temperatures, and then I produced random Gaussian simulated data constrained by these normals and the observed lag-1 autocorrelation coefficient (which also varies through the year).  Finally, for each simulated history I pulled out the annual dates of first 65+°F and then performed the same trendline regressions as shown in Rick's chart.

Here's an example of what I'm talking about.  The first chart below shows the observed dates from the actual data - the same dates as Rick plotted, although there's a minor difference because of the way leap years are handled.  The upper and lower trendlines diverge, as Rick showed.  The next two charts show two of the simulations; in the first, the trendlines show the kind of slope we might expect in a warming climate, but in the second, the trendlines happen to converge.  Both simulations use the same climate normals as the real history, and the differences are just caused by random chance.

Now we come to the interesting part.  Based on the full sample of 1000 simulations, the charts below show the distribution of trendlines for the 10th, 50th, and 90th percentiles.  On each chart I've marked the category in which the actual observed slope falls; so for example, the observed 10th percentile (early arrival of summer) has advanced much more than most of the simulations.  This means that the slope of the lower trendline in Rick's chart is much steeper than we would expect based on the underlying climate normals.  In contrast, the middle (50th percentile) trendline is not far off where we would expect it to be; and the 90th percentile (late arrival of summer) has advanced less than we would expect.

The final step in my analysis was to look at the distribution of the difference between the 90th and 10th percentile trendlines; in the actual history of Fairbanks the lower and upper tails have diverged, but what about the simulations?  The chart below shows that nearly all of the simulations have a smaller difference in slopes; less than 1% of the simulations have a divergence as large as in the real world.  In fact, it's more common for the trendlines to converge than to diverge, as the upper trendline tends to be steeper than the lower trendline (this can be seen in the charts above).

So what do we conclude?  The bottom line is that the observed increase in year-to-year variability in timing of the "first warm day" appears to be highly statistically significant.  Simulations based on a Gaussian model of daily temperature variations show a less than 1% chance that the observed divergence in the upper and lower trendlines could have occurred by random chance.  We can also say that recent decades have seen both more unusually early arrivals of warmth and more unusually late arrivals of warmth than would be expected; if the variations were just random, we probably would have seen less change on the warm side and more change on the cool side.

Of course, all of this depends on the assumption that my Gaussian simulation model is realistic; we know the temperature distribution in Fairbanks is skewed in spring, but exploring whether that makes a significant difference here is a topic for another day.

Tuesday, May 9, 2017

River Ice Update

Break-up is essentially complete now on the Tanana and Kuskokwim Rivers and is under way on the Yukon River; the Yukon ice moved out on May 3 at Dawson, May 4 at Eagle, and today at Circle.  These dates are very close to long-term normals.  The tripod at Nenana fell on May 1, which is also about normal, but Bethel's break-up guessing game ended on May 6, a few days ahead of normal.

As we've noted before, the long-term trend towards earlier break-up is less statistically significant at Bethel than at Nenana.  There are two reasons for this: first, the (assumed linear) rate of change itself is smaller at Bethel, and also the inter-annual variance is greater at Bethel.  However, the early break-ups of recent years (2014-2017) have been much more unusual at Bethel compared to long-term norms.  The reason for this is probably the proximity of Bethel to the persistent and very unusual warmth in the northern Pacific Ocean.

While we're on the topic of variance and spring warming, here's a very interesting chart that Rick Thoman sent over, showing the long-term history of the first date of 65°F or higher in Fairbanks.  The three trendlines represent linear regression applied to the 90th percentile (blue), the median (green), and the 10th percentile (orange) of dates.  While the trend lines for median and early (10th percentile) warmth have advanced over time as expected, the 90th percentile has barely changed.  This implies that the variance has increased quite substantially.  While this could perhaps be a statistical quirk of this particular temperature threshold, it's an interesting result that deserves further investigation.

Wednesday, May 3, 2017

Howard Pass Study

This week I'm in Anchorage at the Climate Prediction Application Science Workshop, and among many other interesting presentations I noted a poster by Ken Hill and Pam Sousanes on weather at Howard Pass.  On this blog we've looked at extreme winds and wind chill at Howard Pass on several occasions.  A reduced-size photo of the poster is below, and here's a link to a full-size pdf (2.5MB):

Sunday, April 30, 2017

Spring Warming Trends (continued)

In my last post I suggested that annual dates of river break-up in Alaska may be able to provide quantitative information about long-term temperature changes at this time of year; this is possible because of the high degree of correlation between break-up dates and spring temperatures.  The annual date of "green-up" in Fairbanks can function in the same way as a climate marker.

After pondering the topic for a couple more days, it now seems clear that the method I used in the first post (read it here) was far from robust.  There's surely no significant difference between a correlation of (say) -0.86 and -0.87, so it's probably not reasonable to pick out the precisely optimal correlation and then claim to have identified a "true" temperature trend.  Accordingly, I think it's probably just coincidence that the results for Fairbanks and Nenana lined up so closely.

Not being one to give up, however, I moved on to a different approach by considering what range of temperature trends could be consistent with the observed correlation between temperature and break-up (or green-up) dates.  This is something that can be addressed with statistical simulation; so I produced 1000 simulated histories of southeast interior April-May temperatures by generating values based on the observed correlation with break-up date.  Each of the simulated histories has approximately the same correlation with break-up date as the actual reported temperature history, but the trends differ widely owing to the random component of the simulations.  Note that I'm assuming there are no other long-term changes that have systematically affected the break-up dates one way or the other.

Here are a couple of examples: the top chart shows the break-up dates and a simulated history that happens to have a very low (near zero) temperature trend, and the second chart shows an example with a very high temperature trend.

For both of these examples, the correlation of the simulated temperatures with break-up dates is very close to the correlation observed in reality, so these are outcomes that "could have" happened based on the physical connection between break-up and temperature.  However, both of these are unlikely outliers; the chart below shows the full distribution of trends from the 1000 histories, plotted as a cumulative distribution function.

It's nice to see that the 50th percentile of simulated trends lies almost exactly on the reported trend, so we can say that the changes in break-up at Nenana are entirely consistent with the temperature trend reported in the southeast interior climate division.  Bear in mind that the simulation process has no knowledge of the actual temperature trend; we have backed it out from the break-up dates.

Here's the same chart derived from green-up dates and April-May temperatures in Fairbanks.

In this case the simulated trends do not line up perfectly with the reported trend, as about 60% of the simulations have more warming than reported by the thermometers.  This is consistent with the simplistic result in my first post, but now we can quantify the probability that the trend is higher than reported.  Based on these results, and if my assumptions are correct, it is about 60% likely that Fairbanks has warmed more than the official climate record indicates (based on a 1974-2016 linear trend line).

Finally, here's an interesting result based on break-up dates of the Koyukuk River in Bettles.

Here we find that nearly all of the simulated histories have less warming than the reported April-May temperatures from the Bettles observing site; it appears to be 90% likely that Bettles has over-reported the warming trend.  In fact, more than 50% of the simulated trends are negative, and when we look at the break-up dates (see chart below) it becomes immediately obvious why this is the case: the linear trend-line for break-up dates shows a very slight increase over this period (although several years are missing near the beginning).  The lack of change in break-up date appears to be inconsistent with the reported warming at Bettles; more investigation is required.

Wednesday, April 26, 2017

Spring Warming Trends

Break-up season is about to get under way across interior Alaska, as seen in the following map from NOAA's Alaska-Pacific River Forecast Center; there is considerable open water on the upper reaches of the Tanana River, and the lower Chena River appears to be mostly open too.  This is supported by a webcam image from Fairbanks showing water pooling on or flowing over the ice.  The ice at Nenana is also deteriorating.

It is well known that break-up at Nenana has tended to occur earlier in recent decades, and of course this is directly related to the long-term warming trend.  Warmer spring weather has also caused trees to leaf out somewhat earlier in Fairbanks; we know this because the annual dates of "green-up" have been recorded since 1974 by visual observation of the east-facing slopes of Chena Ridge.  The chart below shows the green-up data along with the April-May mean temperature from Fairbanks airport.  Note that the high degree of variance prevents the trends from being statistically significant despite last year having the earliest green-up and warmest April-May period on record.

The chart below shows a corresponding analysis for break-up at Nenana, except that here I've plotted the April-May mean temperature for the Southeast Interior climate division (which shows a slightly higher correlation with break-up than temperatures in Fairbanks).  In this case the trends are marginally significant over this period of 43 years.

Here's a scatter plot of green-up and break-up dates versus April-May temperature, showing the rather high degree of correlation for both physical phenomena.  Spring temperatures aren't the only influence on timing of green-up and break-up, but they do explain most of the year-to-year variation.

In view of the high correlations, it's interesting to consider what the green-up and break-up dates can tell us about long-term temperature trends.  For example, if there was no long-term trend in green-up and break-up dates, then it would be difficult to believe that temperatures have risen, even if the thermometers have reported a warming trend; we might infer that urbanization trends or other factors were creating "artificial" warming in the temperature data.  However, the fact that green-up and break-up dates have advanced supports the warming trend observed by the thermometers.

We can take this idea a step farther by examining what temperature trend would be needed to optimize the correlation between temperature and green-up or break-up dates.  I performed this analysis by systematically applying small adjustments to the temperature trend and re-calculating the correlations each time; the results are shown below.  Note that the correlation coefficients are around -0.85 or so, with small variations depending on how the temperature trend is adjusted.

The result of the analysis is rather fascinating: for both Fairbanks green-up dates and Nenana break-up dates, the correlation with temperature is optimized when the reported warming trend is increased by 0.2°F/decade.  This reflects a change from +0.4 to +0.6 °F/decade in Fairbanks April-May temperatures and from +0.6 to +0.8 °F/decade in the Southeast Interior climate division.  This appears to suggest that the actual rate of warming, which is reflected in the dates of green-up and break-up, has been higher than reported by the thermometers at this time of year.

I'll look at break-up dates from some other rivers around Alaska in a subsequent post.  For now, I'd welcome comment from readers on the idea I'm proposing here: that physical date markers with long-term histories can be used to infer true temperature trends and thereby test whether historical weather data is adequately measuring the changes that have actually occurred.

Saturday, April 22, 2017

Spring Progress

After a cold March (14°F below normal), Fairbanks has seen warmer than normal weather overall this month so far (4°F above normal).  Recent days have been relatively cool, however; the high temperature was only 35°F on Wednesday, compared to a normal of 47°F.

Total thawing degree days (accumulation of mean daily temperatures above freezing, in Fahrenheit) are up to 48 as of yesterday, which is the lowest for the date since 2011 and far behind last year's near-record pace.  However, we're actually only a couple of days behind the normal pace for thawing.  The snowpack is diminishing steadily and river ice is starting to look a bit rotten.

The chart below shows daily statewide maximum and minimum temperatures for the last several months according to data from NOAA's ACIS tool.  I've excluded Snotel stations, and for maximum temperatures I also removed RAWS sites because of their known warm bias in sunny weather.  It's striking to see the dramatically higher variance of statewide daily minima compared to maxima; this is largely because the warmest parts of the state in the cold season have highly maritime climates and therefore low temperature variance in comparison to the much colder continental areas.

Sunday, April 16, 2017

Snowy Weeks in Fairbanks

Rick T. here with a post about snowy weeks in Fairbanks.

Interior Alaska has a long snow season, easily from late September to early May, and occasionally even longer. The average number of days between the first measurable snow in the fall and last measurable snow in the spring is a bit shy of seven months (202 days). But of course, the snowfall is not evenly distributed during that time. On the seasonal timescale, early winter is, on average, much the snowiest time of year, while March and April are the climatological dry season. On shorter time scales, depending on what's happening with the flow aloft, we go through days or weeks that are drier and then snowier. However, over the years it sure seemed to me that a significant fraction of the seasonal snowfall comes in a relatively short window. Is that so, or is it just my back complaining from too much shoveling? To address that question, below is a plot of the maximum 7-day snowfall as a percentage of the winter total, i.e how much of the winter's snowfall fell in the snowiest week (any 7-day period) for the Weather Bureau/NWS era of observations (since 1929-30).

As I expected, a fair chunk of the total snowfall does indeed fall in short stretches. The 88 winter average is that almost 22 percent of the seasonal total falls in some seven day period. Although not statistically meaningful, it is interesting that the two highest percentages have occurred since 2010 and that there have been no winters since the early 1990s with unusually low (compared to the long term average) weekly snow percentages.

A surprise to me was the lack of correlation between the total seasonal snowfall and snowiest week. In the graphic, each year's dot is sized according to the total seasonal snowfall: bigger dots indicate higher seasonal totals. Just eyeballing it you can see the size of the dots varies randomly above and below the average line (correlation only +0.03, effectively zero). I expected that low snowfall winters might tend to have higher percentages fall in a few days, but this analysis says otherwise.

———Update April 17, 2017

Inspired by Richard's question on extremes of timing of the snowiest week, here's a histogram of the date of the the snowiest week by half-months. Note that there is no well defined peak: the late November spike is likely no more than random variability.
And for completeness, here's a histogram of the amount of snow that fell in the snowiest week each winter. Not surprisingly, there is a concentration of values around a foot but with a long tail of higher amounts.


Friday, April 14, 2017

Arctic Update

It's been more than two months since my last update on basin-wide Arctic warmth, but not much has changed; even the remarkable cold spell in Arctic Canada in early March didn't put a dent in the overall high-latitude temperature anomaly.  According to the mean temperature from my set of 19 long-term surface observing sites, both February and March were more than 4°C warmer than the 1981-2010 normal.  The coolest of the last 6 months relative to normal was December, at "only" 3.4°C above normal.

This winter's November-March average temperature was the highest since at least 1971 for the 19 stations, as shown in the chart below.  Remarkably, Vize Island (on the northern side of the Kara Sea) was more than 10°C above normal for the 5-month average.  To get a sense of how enormous this anomaly is, consider that 10°C is greater than the difference between the record coldest and record warmest November-March periods in Fairbanks.

If we look at the temperature in terms of standard deviations away from normal, the warmth really hasn't taken a break since the remarkable events of January 2016.  Monthly mean temperatures now seem to be remaining near levels that were very unusual only a few years ago.

Not surprisingly, March average Arctic sea ice extent was the lowest in the satellite era according to the NSIDC, and estimated sea ice volume is also well below previous records.

Interestingly, however, the Danish Meteorological Institute estimates that the Greenland ice sheet has gained considerably more mass than usual so far this cold season.  This was probably caused by the unusual North Atlantic circulation pattern, as low pressure and storminess have been more dominant than usual near southern Greenland.

Monday, April 10, 2017

Chena Basin Snowpack

The April 1 snowpack update from the National Resources Conservation Service (NRCS) shows the cumulative effect of a very snowy winter in the east-central interior, as the snowpack is considerably greater than normal in the vicinity of Fairbanks - see below.  Fairbanks airport has received 83.1" of snow in total, the most since 1992.  However, most of the measuring sites to the south of the Tanana River did not do nearly as well, and it appears that nowhere else in the state saw such a snowy winter in relation to normal.

The chart below shows the 1981-2010 normal snow water equivalent for the SNOTEL sites near Fairbanks, revealing that peak water content is typically reached in mid to late April, depending on elevation.  The higher sites receive more snow in total owing both to orographic forcing and to the longer period with sufficiently low temperatures; so Munson Ridge, at 3100' elevation, tends to build snowpack all the way until the end of April.  In 1982, Munson Ridge was still reporting snow on June 13!

Friday, April 7, 2017

North Slope Winter Warmth

Yesterday NOAA released the U.S. climate division monitoring data for March, and the results for Alaska show a bit of a puzzle.  I was expecting to see significantly below-normal March temperatures for most of the state and a significant warm anomaly for the North Slope district.  However, it turns out that the climate division data show the North Slope division with a -3.0°F anomaly relative to the 1981-2010 normal, and -1.3°F relative to the 1925-2000 normal (as shown in the map below - click to enlarge; note that the normal is 1925-2000, not 1901-2000 as stated in the legend).

In contrast, the graphic below (courtesy of Rick Thoman) shows that Barrow, Deadhorse, and Umiat all saw temperatures in the upper two-thirds of the 1981-2010 distribution; the month of March was significantly warmer than normal at these sites and almost certainly at many other locations north of the Brooks Range.

Here's a time series of the March mean temperature in the North Slope district according to the climate division data:

Oddly the March 2017 temperature is well below the March 2016 temperature, even though Barrow and Umiat were warmer this year than last year in March.  Looking back over the past 5 months (November-March), the same discrepancy is observed: the climate division data indicate that this winter was colder than 2015-2016, but in fact this winter was the warmest on record at Barrow and appears to have been warmer than last winter at Umiat too (although there is a fair amount of missing data).

The two charts below show a closer comparison between the two winters for Barrow, Deadhorse, and Umiat.  The January through March mean temperature was slightly cooler this year, but November and December were much warmer this winter than last winter.  Notice the +54°F daily temperature anomaly at Umiat on January 2 of this year: with a high temperature of 38°F and a low of 29°F, this is the largest departure from normal of any day in Umiat's history.  The 1981-2010 normal for Umiat on January 2 is -20°F, the lowest of any station in Alaska for that date.

What can we conclude from all of this?  First, it's plain to see that climate monitoring is a challenge in the data sparse regions of northern Alaska; different analysis methods can give very different answers even for something as "simple" as year-to-year temperature changes.  It's possible that NOAA's methodology for the climate division calculations allowed this winter's North Slope result to be influenced by colder temperatures south of the Brooks Range and outside the boundary of the district; this would be unfortunate, but these kinds of issues are not uncommon in the world of near-realtime climate monitoring.

An interesting question to consider is, was this the warmest winter on record for the North Slope as a whole?  According to the climate division data it wasn't even close, but my cursory analysis of the numbers from Barrow, Deadhorse and Umiat suggest it may well have been so.  A more thorough investigation, including a careful accounting for missing data, and additional observing sites, would be required to say anything with greater confidence.