Saturday, January 31, 2015

Will the PDO Influence Return?

Yesterday was the fifth day in a row with a minimum temperature of -40 °F or below in Fairbanks.  This is the longest such sequence since December 2012, which saw 9 straight days of -40 °F temperatures.  The Fairbanks record (1930-present) for longest such sequence is 18 days in January 1965 and January 1971 (the latter month also produced the all-time state cold record at Prospect Creek).

It is of interest to note that the PDO phase is still strongly positive as it has been throughout the winter so far (see below).

Is it therefore a surprise that colder than normal conditions have emerged over Alaska this month?  Wouldn't we expect a positive PDO episode of such magnitude to prevent deep cold from developing over Alaska?  Perhaps - but this is so far only a brief cold spell by historical standards, and this month's mean temperature will still end up above normal in Fairbanks.  Moreover, as we discussed back in December, the PDO influence on Fairbanks temperatures is smaller in January than in early or late winter.  I don't know why this is so, but it's clear in the summary statistics I showed before and in the monthly scatterplots shown below.  Note that for these plots I computed the monthly temperature anomalies with respect to the contemporary 30-year normals.

The historical data reveal that the PDO typically reasserts itself in February, with the scatterplot showing a considerably better correlation than in January.  Given the magnitude of the current PDO anomaly, we would therefore NOT expect February to remain cold overall in Fairbanks.  Of course, there are always exceptions - for example, February 1936 was the 10th coldest February in Fairbanks but also had the second most positive PDO index for the month.  Perhaps this year will be a similar outlier - indeed the first 10 days of the month certainly look cold - but if cold persists throughout the month, then it will be counter to the long-term PDO correlation.

Thursday, January 29, 2015

North Slope Wind Chill Climatology

A few days ago the hardy residents of eastern North Slope communities experienced a combination of windy and cold conditions, leading to some sustained low wind chill values.  The chart below shows recent observations from Deadhorse airport; the wind chill was sub-minus 50 °F for quite a lengthy period over the weekend, and the lowest value was -57 °F.

How unusual is this?  The answer is "not particularly" for this time of year.  The box-and-whisker plot below shows the monthly distribution of wind chill values from the hourly observations since 1982 in Deadhorse.  The central box in each column indicates the inter-quartile range, i.e. the top and bottom of each box show the upper and lower quartile respectively for the month.  The thick horizontal line in the middle of each box shows the median, and the "whiskers" above and below the boxes show the extreme values.

We see that the lower quartile of wind chill values in January is -49 °F, so the wind chill is below that value 25% of the time.  It is between -24 °F and -49 °F half of the time.  Interestingly there is little relief from low wind chill values until April, as temperatures don't improve until then (and average wind speeds change rather little from month to month at Deadhorse).

I also looked at data from Barrow and Barter Island; the chart below shows the median wind chill from all three locations by month.  Deadhorse is the chilliest of the three locations in winter, but Barrow is coolest in summer owing to the stronger marine influence.

Monday, January 26, 2015

Really Cold Snow

The other day Rick noted that snow was accumulating on Keystone Ridge with a temperature of -25°F. That got me wondering how low the temperature can go and still receive accumulating snow around Fairbanks. With the help of daily climate summaries and hourly observations, we can identify the very coldest days with measurable snow. A key part of this analysis is that we eliminated all observations with fog in the wx code. The reason for this is because I am using visibility as a proxy measure for accumulating snow. I sometimes wonder if dense ice fog creates a rime layer that is counted as new snow or if ice crystals can pile up into a minor accumulation. I expect the Fairbanksans reading this to chime in right about now.

In conversations with Rick and Richard, they both agree that a snow observation with a visibility of 3 miles or less will start to accumulate (actually they suggested 4 or 5 miles). Since ice fog nearly always forms when the temperature is colder than -35°F, for practical purposes, this means true snow events with temperatures under this value are not captured in this analysis.

Hourly observations that are coded to identify snow go back to 1960. I paired all of the hourly observations with the daily climate summary to see which days had accumulating snow. The reason we do this is because a day might have a high of -10°F and a low of -35°F with 0.3" of snow. But did the snow fall when the temperature was -10°F, -20°F, or -35°F? That is why pairing the daily and hourly observations makes a huge difference.

Table 1 shows the dates since 1960 (54 years) that observed accumulating snow (snow wx observation, visibility of 3 miles or less, and no fog or drizzle) with a temperature of -25°F or colder. In the table, the column labelled "Warm Snow Obs" is the warmest hourly temperature that met the selection criteria on that date and the column labelled "Cold Snow Obs" is the coldest hourly temperature that met the selection criteria on that date.

Table 1. Top 25 coldest "pure" snow observations (no fog) since 1960 at the Fairbanks International Airport.

Remember that the idea here is to set practical bounds on accumulating snow and not to identify extremes. In Table 2, you will see how fog plays havoc with the analysis. On December 28, 1964, one observation met the aforementioned selection criteria (temperature was -35°F) but other observations with fog probably has a snow intensity sufficient to accumulate.

Table 2. All snow observations on December 28, 1964 at the Fairbanks International Airport.

It seems that snow can occur at, and just under, -30°F every once in a while in Fairbanks. At that temperature, there isn't very much moisture in the air (mixing ratio about 0.2 g/kg). Unfortunately the hourly observations and the daily summaries leave much of the story untold. This is a great example of where local knowledge can supplement technical documentation.

Cold - Surface and Aloft

As advertised by computer forecasts from the middle of last week, clear skies and calm winds have allowed temperatures to drop off sharply over the central and eastern interior.  At the surface there is a 1030 mb high across north-central Alaska, but at upper levels the dominant feature is not high pressure but a cold low over the western Beaufort Sea.

The lowest temperature observation I've seen so far is -55F at the Granite Creek SNOTEL just east of Delta Junction, but there were many other worthy contenders including:

-53F  Huslia
-53F  Coldfoot SNOTEL
-53F  Arctic Village
-52F  Bettles airport

Here's a delightful webcam photo from Arctic Village this afternoon: -49F and some light ice fog.

In the Fairbanks area, -43F was reached at the airport, the coldest in nearly two years.  Fort Wainwright reached -48 and Eielson -47.

Yesterday afternoon's balloon sounding revealed deep cold with only a slight inversion (prior to the overnight surface cooling).  The column maximum temperature was -26.5 C, which is again the coldest in nearly two years.  It's interesting to compare this sounding with the coldest of this winter prior to last Wednesday (as measured by 1000-500 mb thickness): see below.

Saturday, January 24, 2015

Decreasing Extremes

With potentially much below normal temperatures possible in the next few days in the central and eastern interior (latest MOS number -65 °F for Fort Yukon on Tuesday), I started to wonder about historical changes in the frequency of large temperature anomalies.  We know that cold extremes are less common in winter now, even relative to the warmer modern "normal", but what about warm extremes - have these become more or less common?

The chart below is a histogram of the daily temperature anomalies in November through March in Fairbanks, for two 30-year periods: 1931-1960 and 1981-2010.  In each case I compared the daily mean temperatures to the daily normal values for the 30-year period in question, so this is a straightforward comparison of daily temperatures to the contemporary climate normal.  The results show that large anomalies were noticeably less common in the 1981-2010 period than in 1931-1960, and that the decrease in extremes occurred on both the warm and cold sides.

In the extreme tails of the distribution, there was a large proportional decrease in frequency of daily anomalies greater than +/- 40 °F.  On the cold side, anomalies more than 40 °F below normal decreased from 0.95% of days to 0.22 % of days, and on the warm side the frequency of +40 °F anomalies decreased from 0.48% to 0.13% of days.  The chart below shows the numbers of days by decade that saw +/- 40 °F anomalies on either side.  As noted in earlier posts, the 1930s was an extreme time for Fairbanks climate.

The decrease in temperature variance over time in Fairbanks was previously noted in Brian's analysis here.

As an aside, a corresponding analysis for the warm months of the year does not show the same decrease in extremes (see below); in fact the 1990's was the decade with the highest frequency of daily anomalies greater than +/- 15 °F.

Friday, January 23, 2015

Getting Wintry

The coldest temperatures of the winter so far in Alaska were observed this morning, with the -50 °F level broken for the first time to my knowledge: -51 °F was reported at Huslia in the lower Koyukuk River valley.  The infrared satellite image below, taken at 3:53 am AKST this morning, shows the areas that cooled off under clear skies in the western interior: dark shades are colder, and Huslia is located close to the prominent dark patch in the upper center of the image.

In contrast, heavy cloud cover has so far kept temperatures relatively milder in the eastern interior, although Arctic air has been working its way in at low levels despite the blanket of clouds.  The frontal zone aloft over Fairbanks created a band of snow today that became quite narrow and fairly intense as it persisted just east of the city (see below); a storm total of 7.7" was reported from North Pole as of 4:42pm.   This kind of "mesoscale" feature is nearly impossible to predict more than a few hours ahead of time but makes a great difference for total snowfall amounts.

As clouds and moisture migrate eastward in the next couple of days, and high pressure builds across the north, the cold conditions already observed in the west will envelop the central and eastern interior.  This morning's GFS MOS forecast numbers were remarkably cold, including -49 °F at Fairbanks airport and -50's at various locations along the Tanana River valley on Monday or Tuesday.  Readers can track the latest MOS (statistical) forecast temperatures at the following page:

Wednesday, January 21, 2015

Turning Colder

This afternoon's balloon sounding from Fairbanks measured a column maximum temperature of +9 °F, which is - remarkably - the coldest such measurement of the season so far.  Much colder conditions are on the way; western Alaska is feeling a chill already, with Kotzebue reporting a stiff northwesterly breeze and temperatures hovering around -6 °F all day.

In what will be a nice change of scene both for residents and this blogger, we should have some material to discuss on the cold side in the next week or so.  Latest GFS MOS numbers for next Tuesday include -36 °F in Fairbanks, -48 °F in Eagle, and -59 °F in Chicken.  This is a long-range forecast, but gives a sense of what is easily possible given the cold airmass and the date on the calendar.

Here's this afternoon's sounding: at last, just a smidge cooler than normal at 850 mb.

Tuesday, January 20, 2015

More on Warmth Aloft

Recently there's been much attention given to the fact that - according to the National Climatic Data Center - 2014 was the warmest year on record in Alaska.  This verdict is based on surface station measurements, but is also consistent with record warmth aloft as observed by balloon soundings.  For instance, we can look at the 1000-500 mb thickness, which is an excellent measure of the average temperature of the lower half of the atmosphere; we find that the 2014 mean 1000-500 mb thickness was higher than in any other year (1948-present) in Fairbanks, Barrow, and Kotzebue.  The thickness was second only to 1957 in Nome, McGrath, Bethel, and Anchorage.

One question that arose in my mind when pondering these new records was whether there were any 365-day periods in the past that were warmer than the calendar year 2014; after all, calendar year boundaries are artificial, like week or month boundaries throughout the year.  So I calculated the running 365-day mean 1000-500 mb thickness in Fairbanks and found that - remarkably - the highest values on record occurred in the past week.  Here's a list of the top several 365-day means with data complete through today (January 20); the top 61 overlapping periods ended in 2014 or 2015, and next in line was the year ending February 7, 1994.

Jan 17, 2014 - Jan 16, 2015   5344.20 m
Jan 18, 2014 - Jan 17, 2015   5344.15 m
Jan 16, 2014 - Jan 15, 2015   5343.83 m
Jan 19, 2014 - Jan 18, 2015   5343.63 m
Jan 20, 2014 - Jan 19, 2015   5343.62 m
Jan 21, 2014 - Jan 20, 2015   5343.44 m
Jan 15, 2014 - Jan 14, 2015   5343.23 m
May 21, 2013 - May 20, 2014   5343.03 m
Feb 8, 1993 - Feb 7, 1994   5338.53 m

Below is a chart of the running means since 1949.  The new record obviously reflects the remarkable combination of persistent unusual warmth and an absence of unusual cold; the last really notable cold spell (compared to normal) in Fairbanks was the exceptionally cold spring of 2013.

Sunday, January 18, 2015

Warmest Winter To Date

The mean temperature in Fairbanks since November 1 has now moved into first place in the historical rankings since 1930; in other words, if we define winter as November-March, then this is now the warmest winter on record through January 17.  The winter-to-date temperature now exceeds that of 2002-2003 by the slimmest of margins.

The chart below shows the evolution of the winter-to-date mean temperature this year in red, compared to 2002-3 in blue and the historical range in gray.  This winter has been steadily climbing through the rankings as warmth at the surface has become increasingly anomalous.  We can see that mean daily temperatures close to +10 °F would be required to maintain first position for the remainder of winter; this would be more than 15 °F above normal for late January, so it will be a challenge to hold onto the record in the near-term.  In fact, we're very likely to fall back out of first place in the coming days, as January 2003 had a very warm spell around the 20th.

[Update: Brian coincidentally made a similar graphic, and his analysis shows that the winter of 1928-29 was warmer through January 17.  This is helpful context for the current warmth.  It's a matter of personal preference as to whether to include pre-1930 data in this kind of analysis - I usually don't do so, because the Weather Bureau/NWS era began in December 1929 and I have greater confidence in the quality of the Fairbanks data since then.  Rick Thoman went so far as to say that pre-1930 data from Fairbanks are "plagued by data quality issues", although this may apply more to pre-1920 data.  My own (relatively uninformed) opinion is that there's very useful information to be gleaned from the early years, but the data should be used with caution.]

Friday, January 16, 2015

Gulkana and Fairbanks Temperatures

Reader Gary inquired last week about temperatures in Glennallen and their relationship to Fairbanks temperatures.  The two locations are separated by more than 200 miles and the Alaska Range, but the climates are quite similar; both locations are strongly continental, with very large temperature swings from summer to winter.  In the case of Glennallen this is possible because of the Chugach Mountains to the south, which block Pacific moisture despite the ocean being less than 100 miles away.  In fact Glennallen is surrounded on all sides by high terrain, being located in the Copper River Basin, and this allows relatively clear, calm and cold conditions to prevail rather frequently in winter.

The chart below shows the 1981-2010 daily normal temperatures for the Gulkana airport, which is just a few miles from Glennallen, and for the Fairbanks airport.  Fairbanks sees a somewhat larger swing from summer to winter, but overall the large seasonal variations are similar.  Peak summer temperatures occur a few weeks later in Gulkana, which reflects a slightly greater maritime influence in the more southerly location.

We can get a sense of the variability in temperatures by looking at histograms of daily maximum and minimum temperature for summer and winter, see below.  Starting with winter, the charts show that the Fairbanks temperature distribution is shifted towards the cold side compared to Gulkana, but variability is similar.  Above-freezing temperatures are about twice as common in Gulkana in December through February (10.8% of days vs 5.7%) for the overlapping period of record.  Conversely, temperatures of -40° or below are nearly twice as common in Fairbanks in December through February (11.8% of days vs 6.8%).

In high summer (June and July), the differences between the two locations are more significant compared to the range of temperatures observed at that time of year.  As shown in the charts below, Gulkana is quite a bit cooler, especially for the daily minimum temperatures.  Minimum temperatures are generally in the 40s in Gulkana, but are more often above 50 °F than below 50 °F in Fairbanks.  In Gulkana, daily high temperatures most often fall in the range 60-75 °F, but Fairbanks is typically in the range 65-80 °F.

The cooler temperatures overall in Gulkana can be explained partly by the higher elevation (1560' vs 430' MSL).  Cooler nights in particular are the result of lower humidity: Gulkana's average dewpoint in June and July is 41 °F, compared to 47 °F in Fairbanks.  Accordingly, Gulkana is about 10 percent drier in terms of June-July precipitation.  Longer nights may also play a role in nighttime coolness: the sun is below the horizon for at least 4 hours every night in summer in Gulkana.

Lastly, I'll touch on the correlation of temperature variations in the two locations.  The list below shows the correlation coefficients between the two locations for daily temperature departures from normal, based on the 1981-2010 normals and calculated over the common period of record (1943-2014).  Correlations are highest in winter and spring, and are relatively low in late summer when it appears that local variations in cloudiness have a large effect compared to the underlying temperature variance.

Jan  +0.77
Feb  +0.76
Mar  +0.74
Apr  +0.74
May  +0.71
Jun  +0.66
Jul  +0.53
Aug  +0.62
Sep  +0.65
Oct  +0.68
Nov  +0.76
Dec  +0.76

Tuesday, January 13, 2015

Low Snow in the Northwest

One of the many notable climate anomalies in Alaska this winter is a lack of snowfall in many locations, and even Kotzebue in the northwest is suffering in this regard.  So far this winter, only 8.1 inches has fallen in Kotzebue, and the snow depth has been stuck at a meager 2 inches since November 27, after peaking earlier at 4 inches.

The only other winter with such a low snow depth in January was 2012-2013; that winter saw an even more extreme snow drought in the early going, but the snow depth then reached 5" on January 12 and 9" on January 15.  So this winter now has the distinction of the lowest peak snow depth through this date.  In a couple of days we'll also break the record for latest date to first reach 6" snow depth (January 14, 2013); the median date is November 17.

Here's a chart of the season-to-date snowfall and peak snow depth since 1949.  (I'm not sure how the snow depth exceeded the snow total in 1994-95 and 1999-2000, but that's what the data say.)  It's interesting to note the high volatility in snow totals in recent decades and especially in the last 5 years or so.

Why the lack of snow?  The answer is that a blocking ridge over the Arctic Ocean north of Alaska has prevented cyclonic storm systems from reaching Kotzebue.  The six maps below show the sea-level pressure anomaly (left column) and 500 mb height anomaly (right column) in October, November, and December of 2014; notice the general lack of low pressure in the north.

The recent flow pattern can be contrasted with the situation in a very snowy month: below are the maps for the snowiest early winter month in Kotzebue's history, November 2003 (46 inches):

It's interesting to note that Nome is not doing too badly for snow this winter, with 27.1" season-to-date and a current snow depth of 11".  Seasonal snowfall totals are correlated at only +0.42 between Kotzebue and Nome, so this isn't too surprising; but it does appear that Nome has done fairly well out of an unfavorable upper-air pattern.

Sunday, January 11, 2015

Warmth Aloft and Wind Direction

After reading Richard's excellent post the other day, I though it would be interesting to see if cyclical patterns exist in the direction of winds coming into Alaska. Richard also looked at Barrow's upper level wind direction as it related to Autumnal warming along the North Slope. This is a complementary post to those articles.

Most of Alaska has seen substantial surface warming in the winter months during the last few decades. The warmth has been well documented in numerous booksarticles, and reports. The cause of this increase in warmth is multifaceted and is not the subject of this analysis. One of the frequent responses to observed temperature increases is to question station siting and the effects of urbanization and landcover changes. One way to see if the observed surface temperature increase is a "real" or an artifact of the measurement technique is to look at air temperatures recorded from weather balloons (RAOBs) just above the surface. This blog posts looks at the case of Anchorage, Alaska, and whether the surface temperature increase corresponds to a free-air temperature increase. I know that this blog generally focuses on interior conditions but since the data were handy for Anchorage and since the effects are fairly uniform over large areas, it seems appropriate to go with Southcentral data in this case. I will look at Fairbanks data as time permits.

Surface Temperatures

An inspection of annual temperatures in Anchorage (see Figure 1) shows an obvious increase over the course of the climate record. It is worth remembering that the temperatures since 1953 represents the Anchorage International Airport location and before 1953 it represents a combination of Merrill Field, the Park Strip, and Ship Creek locations. Note the shift in temperature regime in the mid-1970s.

Figure 1. Annual temperature in Anchorage, Alaska, from 1916 to 2014. Several years in the 1920s has too many missing observations for inclusion.

Since temperatures are much more variable in winter than in summer, annual swings in temperatures are often reflected in the temperatures during the cold months of the year. For example, December or January can be +/- 15°F compared to normal but July or August cannot be more than about  5°F from normal. Figure 2 shows the December-January temperature in Anchorage for the length of the climate record. Note the high degree of correlation between the lines in Figure 1 and Figure 2.

Figure 2. December-January temperature in Anchorage, Alaska, from 1916 to 2014. 

If we use standard 30-year periods to calculate long-term normals, as NCDC and WMO does, the difference in temperatures over time is readily apparent. Figure 3 shows all 30-year daily normals since the 1921-1950 time period. The large increase in winter temperatures are clearly evident. 

Figure 3. Daily normal temperature for Anchorage for all 30-year periods beginning with the 1921-1950 time period.

Upper Air Temperatures

At high latitudes, low level temperatures are often dominated by a cold-pocket of air at the surface. This is referred to as a temperature inversion. This cold pocket of air often masks the presence of a change in airmass. Since cold air at the surface is difficult to displace, you can have a situation where an old airmass is present at the surface and a new airmass is present a few thousand feet above the surface. Sun angle, wind speed, and proximity to mountains determine the time lag between the length of time that an airmass change is reflected at the surface. Many times, the airmass changes several times without being noticed at the surface. Therefore, any study of temperature changes must also look at upper air temperatures so as to eliminate the problem of temperature inversions. For this study, we are going to use the 850 mb level (~5,000') as a proxy for free-air temperatures. All weather balloons launched in Anchorage since 1948 have sampled the air temperature at 850 mb and reported the height, temperature, dewpoint depression, wind speed, and wind direction. Figure 4 shows the the annual 850 mb (~5,000') temperature at Anchorage since 1948 and Figure 5 shows December-January temperatures only.

Figure 4. Annual 850 mb temperature in Anchorage, Alaska, from 1948 to 2014. 

Figure 5. December-January 850 mb temperature in Anchorage, Alaska, from 1948 to 2014.

The temperature increase at 850 mb is both noticeable and statistically significant. However, the magnitude is slightly less than observed at the surface. Note that the units for Figures 1 and 2 are degrees Fahrenheit and the units for Figures 4 and 5 are degrees Celsius. As we did with surface temperatures, we show the daily normal 850 mb temperatures for all 30-year periods in Figure 6.

Figure 6. Daily normal temperature for Anchorage at 850 mb for all 30-year periods beginning with the 1951-1980 time period.

A side-by-side comparison of Figures 3 and 6 shows a similar, dramatic increase in wintertime temperatures in Anchorage. At the surface, January temperature have increased about 7°F since the 1950s and the 850 mb temp has increase by as much as 1.9°C (3.4°F). The 7°F surface increase from the 1950s is equivalent to 0.55 standard deviations above the normal value at the time. The 1.9°C increase in December-January 850 mb temps is equivalent to 0.35 standard deviations above the normal value at the time.

The Role of Wind

When the airmass south of Alaska is advected northward from the sub-tropics, warm conditions ensue. Indeed, this winter has seen a never ending stream of warm air from the central North Pacific Ocean move into Alaska bringing warm air with it. Is this the case for all warm winters? The reason we spent so long introducing the upper air conditions in the previous section is because an airmass moving from the south to the north is often not apparent in the surface wind observations – but it clearly shows up in the 850 mb wind observation. Therefore, let us take a look at how the winds at 850 mb blow during warm or cold years.

Figure 7. Frequency distribution of December-January wind direction at 850 mb by decade. Inset chart shows the average 850 mb temperature by decade.

Figure 7 shows the cardinal wind direction by decade for the Anchorage balloon station at 850 mb for only the months of December and January. Since those two months showed the most dramatic surface and 850 mb warming, it is entirely possible that southerly winds might be correlated with the increase in winter temperatures. The chart shows a notable increase in southeasterly and southerly winds at 850 mb over time. There is a corresponding drop in the amount of time with northerly and northeasterly winds during these two months. The R-square value between 850 mb temp and the percentage of 850 mb southeasterly winds by decade (Figure 7) is 0.64. Therefore, we can conclude that a fair amount of the upper air warming during the winter months is due to an increase in southerly winds which advect warmer air from lower latitudes.

Figure 8. Scatter plot showing December-January 850 mb temps with the percentage of time December-January 850 mb winds are from the southeast at Anchorage, Alaska.

Looking at individual seasons instead of decades, the relationship between wind direction and 850 mb temperatures in December and January is easier to see. Figure 8 shows a scatter plot of December-January 850 mb temps and the percentage of observations with southeasterly winds. The R-square correlation is still very high. Similarly, there is a strong negative correlation between 850 mb temperatures and the percentage of time winds are coming from the north. Figure 9 displays the strength of this correlation.

Figure 9. Scatter plot showing December-January 850 mb temps with the percentage of time December-January 850 mb winds are from the north at Anchorage, Alaska.


There is a clearly defined relationship between warm winter temperatures in Anchorage (at the surface and at 850 mb) and the prevailing wind direction. This should not come as a surprise to anyone. Air from south of Alaska is obviously warmer than air from north of Alaska.

Figure 10. Percentage of December-January 850 mb wind observations from the southeast at Anchorage, Alaska..

What is surprising is the increased frequency of 850 mb winds originating from the south and southeast. Figure 10 shows the long-term trend of the percentage of 850 mb observations with a southeast wind. The question becomes why? The answer to why is not so easy. Southeast winds at 850 mb imply a low pressure center in the Gulf of Alaska. Are Gulf lows becoming more common? Are they becoming more intense? One of the more talked about climate drivers in Alaska is the Pacific Decadal Oscillation. Using University of Washington data, we can show that years with positive PDO values indeed have a larger percentage of 850 millibar southeast wind observations.

Figure 11. Scatter plot showing December-January Pacific Decadal Oscillation (PDO) index values with the percentage of time December-January 850 mb winds are from the southeast at Anchorage, Alaska.

Figure 12. ESRL Reanalysis of sea level pressure change between the 1981-2010 period and the 1951-1980 period.

How does all of this affect air pressure patterns? Since the pressure-gradient force drives winds, the air pressure regime should tell us a lot about the wind magnitude and direction. Figure 12 clearly shows that air pressure has been lower in the Bering Sea and the Aleutian Islands during the more recent 30-year normal period. This confirms the hypothesis that lower pressures are more frequent or more intense in a geographical position that promotes southerly air flow. Still, this does not answer the why question. Does the decrease in sea ice affect the pressure patterns? What about the Pacific Decadal Oscillation? This is obvious a question of great importance to Alaskans and an active area of research.

Supplemental Figures for Fairbanks

Figure S1. Annual 850 mb temperature in Fairbanks, Alaska, from 1948 to 2014.

Figure S2. December-January 850 mb temperature in Fairbanks, Alaska, from 1948 to 2014.

Figure S3. Percentage of December-January 850 mb wind observations from the southeast at Fairbanks, Alaska.

Figure S4. Percentage of December-January 850 mb wind observations from the south at Fairbanks, Alaska.

Figure S5. Scatter plot showing December-January 850 mb temps with the percentage of time December-January 850 mb winds are from the southeast at Fairbanks, Alaska.

Figure S6. Scatter plot showing December-January Pacific Decadal Oscillation (PDO) index values with the percentage of time December-January 850 mb winds are from the southeast at Fairbanks, Alaska.