El Niño Winter Patterns

A couple of weeks ago a reader requested that we look at the likely effects of El Niño on the upcoming winter's weather pattern in Fairbanks.  This is a big topic that could fill a series of posts, so I'll just scratch the surface here.

First, published work from almost 15 years ago indicated that El Niño winters tend to be significantly warmer than normal in the eastern two-thirds of Alaska, but the signal is much weaker over western regions.  Unusual warmth over much of the state arises from the Aleutian low-pressure zone being more intense than usual, and the associated upper-level flow brings warm air from the southwest into Alaska.  However, the phase of the PDO is also a strong constraint on the winter temperature patterns, as El Niño's warming influence seems to disappear when the PDO is negative.

Some analysis I did here shows basically the same thing, with a rather high frequency of unusual winter warmth across south-central and southeastern Alaska during strong El Niño events.  Fairbanks lies within the zone where above-normal temperatures are favored, although near-normal temperatures are also common.  Again, however, a warm outcome is largely contingent on having a positive PDO phase - which is more likely than not during El Niño, as the two phenomena are inter-dependent and positively correlated.  As far as precipitation is concerned, there is a rather strong dry signal across most of western and interior Alaska in El Niño winters.

Let's look at some maps.  The following sequence of images shows, for each month of winter, the percentage of years that were warmer than normal when El Niño conditions were in the top 10 strongest for that month.  The data come from the NCEP/NCAR global reanalysis, with "normal" defined as the 1951-2010 mean.  It's interesting to see a particularly warm pattern over interior Alaska in December and a much cooler look for January; but February is warm again.  These month-to-month variations could be partly attributable to random chance.

November
December
January
February
March

Here's the same map sequence for precipitation, based on a global 1-degree objective analysis from observed data (GPCC analysis, not NCEP reanalysis).  The dry signal that I mentioned earlier shows up strongly in January and February across most of the state and persists in March across the southern interior.

November
December
January
February
March

Now let's look at some scatter plots of historical data from Fairbanks.  First, the charts below show the relationship between monthly mean temperature anomaly in Fairbanks and an index of El Niño - Southern Oscillation (ENSO) activity; a more positive ENSO index corresponds to a stronger El Niño, and a negative ENSO index corresponds to La Niña.

November
December
January
February
March

There is a lot of scatter, showing that El Niño and La Niña by themselves do not have a particularly strong influence on Fairbanks winter temperatures; the correlation is slightly better for the overall winter mean (see below), but still not great.  Compare the scatter plot for the PDO, also shown below - that's a much better correlation.  Nevertheless, the plots above do show a tendency for unusual warmth during the strongest El Niño events for every winter month except January.  It's interesting that the warmest January's appear to occur when ENSO is relatively close to neutral.

Below are the corresponding results for precipitation, with the long-term median for each month denoted with a dashed line.  It's pretty clear that late winter (January-March) is usually even drier than normal when a strong El Niño is in play.

November
December
January
February
March

The dry signal shows up in the overall winter mean, but again the PDO is more highly correlated than ENSO.

I'll mention one other aspect of El Niño's impacts on winter climate.  It has been shown in the literature that high-pressure blocking is less frequent over the Bering Sea during El Niño winters, which is consistent with a stronger Aleutian low.  The lack of blocking to the west of Alaska reduces the frequency of cold northerly flow over the state, thus contributing to the overall warming signal.  I attempted to reproduce this result with the blocking data I've used before; the map below shows the difference in frequency of blocking highs between El Niño and La Niña winters.  There is a slight reduction in cut-off high pressure centers near the western Aleutians, but interestingly the frequency rises over most of Alaska and especially the Chukchi Sea and nearby Arctic Ocean.

The corresponding map for cut-off low pressure centers shows a large increase in frequency over the Aleutians and a large decrease in the Gulf of Alaska.  Taken together, these maps show that El Niño brings more upper-level ridging and less troughing over the Gulf of Alaska and nearly all of Alaska except the Aleutians; but there is a persistent trough over the Aleutians, and this flow configuration creates warm southwesterly flow over the state. 

In summary, if the current El Niño episode continues in the "strong" category throughout the upcoming winter, Fairbanks can expect another warm - but perhaps not excessively warm - winter.  Much will depend on the evolution of the PDO, which is currently still positive but has seen strong fluctuations in recent months.  Snowfall is likely to be less than usual from January through March.

Forecast Difference

Updated Aug 23, see end of post

The end of this month could be shaping up to bring very unusual weather to interior and northern Alaska, judging from recent computer model forecasts; but there is extreme uncertainty as to how it might play out.  In fact I don't recall when I last saw such pronounced disagreement among the leading models, as illustrated below in the 7-10 day 500mb height forecast from the ECMWF, GFS, and Canadian models (click for a larger version):

The ECMWF - widely considered the best model on this time scale - is showing a strong trough and cold anomaly over Alaska, but the Canadian (CMC) model shows a huge high-pressure block.  These are ensemble mean forecasts, so typically when they show a large anomaly at this lead time, it is quite likely to occur - and therefore it's very rare to see such strong disagreement.  The GFS is taking the middle of the road, although recent runs have been flipping back and forth.  The very latest GFS run (more recent than shown above) shows cold air becoming entrenched over the state by the end of the month.

It will be fun to see which model wins out - or if they're all wrong.  In any case, it seems quite likely that there will be an interesting outcome with the potential to break records.  Those with outdoor plans towards the end of the month should pay attention, as the cold scenario would probably bring snow to the hills in many areas.

Update Aug 23: 48 hours later, and it looks like the ECMWF will be nearer to the mark.  No surprise there.  Here's the latest 7-10 day forecast:

Here's a time-height cross-section of temperature above Fairbanks from the latest GFS deterministic (not ensemble) forecast: pretty chilly by next weekend.  It could still be wrong, of course; 5-7 days is a long time in Alaska weather forecasting.

Pace of Temperature Records

Last summer we looked at changes in the rate at which temperature records have been set over the last 60 years or so; see here and here.  I've often pondered the same topic since then, and I thought it would be interesting to update the results for 2014 and 2015 to-date, and to look at which stations have warmed the fastest (and slowest).

First, the two charts below show a 19-station average of annual numbers of daily temperature records (scroll down to see the stations listed on another chart below).  The data are taken from 1954-present and records are defined relative to this period only.  The first chart shows occurrences of record warmth, whether for daily maximum (red) or daily minimum (blue) temperature; and the second chart shows occurrences of record cold.  As we noted before, cold records have diminished somewhat more rapidly than warm records have increased.  Among the 4 categories of records, the most rapid changes have occurred for low daily maximum records, followed by low minimum records, then high minimum records, and the least rapid change has occurred for high maximum records.

The 1976 PDO shift is clearly evident on the charts, and the extreme warmth of the past two years is also a striking feature.  2014 saw the highest number of warm records (both maximum and minimum temperatures) and also the least number of low maximum records.  Interestingly the number of low minimum records was slightly lower in 1978.  Remarkably, 2015 looks very likely to outdo 2014, as the number of high minimum records is already almost equal to 2014, and so far there have been just a handful of days with daily cold records at a few stations in 2015.

One minor concern I had with the results above is that there is a higher concentration of stations in southern and southeastern Alaska, so I repeated the analysis after excluding Kodiak, Homer, Talkeetna, Juneau, and Big Delta.  The results are little different, although 2015 to-date looks a little less extreme - see below.

It's interesting to compare the overall pace of change between the different stations.  I attempted to do this by adding the slopes of the linear trends for the warm record counts and subtracting the trend slopes of the cold record counts - see the results below.  Based on this metric, the long-term rate of warming has been greatest at Homer and Talkeetna, but interestingly the relatively nearby stations of Kodiak and Gulkana have seen some of the smallest warming rates.  The difference between Kodiak and Homer is particularly interesting as both stations are heavily influenced by North Pacific sea surface temperatures.  The second chart below shows that there is a long-term trend in the temperature difference between Kodiak and Homer, and 2014 was the first year in which Homer was the warmer of the two locations.  This looks a bit suspicious to me; there might be some station siting issues that have affected the reported temperatures from either or both of these places.

Lastly I'll show the record counts for several individual stations.  The situation at Homer has been very extreme during this year and last:

At Talkeetna the rapid warming pace comes from a very large number of warm records in 2002 and 2003:

The slowest-warming site, Gulkana, still shows a long-term warming trend in all 4 record categories:

Lastly, if we look at Fairbanks with data back to 1930 included, the picture changes slightly, as the high maximum records no longer show a long-term warming trend; however, the long-term decrease in cold records is still very much evident.

Late Summer Chill

Some cool conditions have emerged in recent days over interior Alaska, with freezing temperatures in the usual spots.  Here are some of the colder reports that I noted:

25F  Tok#2 COOP  Aug 12
26F  Tok School COOP  Aug 12
27F  Circle Hot Springs COOP  Aug 7
29F  Chicken COOP  Aug 7 and 8
30F  Tok 70SE CRN  Aug 12
31F  Eagle Airport  Aug 12

The Tok School COOP site has somewhat complete data back to 2003, and 2003 was the only other year with these temperatures so early in the autumn (25F on August 1, 2003).  The early frost is not at all unusual at the other locations.

In Fairbanks the balloon soundings reported sub-freezing temperatures at 850 mb on Tuesday (August 11), which seems early but in fact is only unusual compared to the "new normal" of the last 10 years or so.  In the historical data from 1948-2000, the median first date of a sub-freezing 850 mb temperature was August 14, but from 2004-2014 it was August 30, with only 2 of 11 years seeing this event prior to August 19.  Here's a chart of the long-term history of the first freeze date at 850 mb.

Besides the rather chilly conditions, Fairbanks has seen breezy weather with frequent light rain recently.  Measurable rain has fallen on 18 days since July 15, which is the most in this period since 2001.  At 850 mb, the mean wind speed for August so far is the highest since 2003, with a high frequency of winds from a direction between westerly and southwesterly.  This trajectory of air flow is the most common wind direction for every month from April to September in Fairbanks, but is especially characteristic of August, with a peak in relative frequency on about August 10.  East to east-southeasterly winds aloft are most common from October through March.

Pacific Temperature Patterns

On this blog there has often been discussion of the influence of Pacific Ocean temperature patterns on Alaska climate, and rightly so, as the evolving oceanic thermal patterns are known to create first-order impacts on seasonal (and decadal to multi-decadal) climate in this part of the world.  In this connection we usually talk about the El Niño - Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) as the two big players.

A few weeks ago reader Eric suggested that we could do an objective multi-variate analysis of the impacts of ENSO and the PDO to decipher which is more important at different times of the year.  The challenge here is that the two phenomena, as they are usually defined, are not at all independent, so it is impossible to obtain a clean separation of the impacts.  However, if we could re-define the different Pacific temperature patterns to be independent of each other, then we could proceed.  This is the approach followed by Dennis Hartmann of the University of Washington in a paper published this year:

http://www.atmos.washington.edu/~dennis/Hartmann-2015-Geophysical_Research_Letters.pdf

Hartmann looks at temperature patterns over a large domain from 30°S to 65°N across the Pacific and calculates the top 3 EOFs, which are the independent spatial patterns that dominate the variability over time.  The top pattern/EOF largely represents ENSO but also includes significant variability that we normally think of as the PDO (see Hartmann's figure 1, reproduced below).  The second pattern/EOF also resembles the PDO pattern, so we can think of the PDO as contributing to both EOF1 and EOF2.  Hartmann's third mode is what he calls the North Pacific Mode, and he focuses on this pattern as being implicated in the extreme circulation anomaly over North America in winter 2013-2014.

The independence of the three temperature patterns allows us to proceed with Eric's idea of a multi-variate linear regression, and so I've done this for Fairbanks monthly and seasonal temperatures since 1950.  The first chart below shows the estimated coefficients of regression between the 3 Pacific modes and Fairbanks monthly temperature.  This can be interpreted as the temperature anomaly that typically occurs in Fairbanks when each of the modes has an anomaly of 1 standard deviation; for example, when EOF1 is one standard deviation above normal in January, Fairbanks is typically 3 °F above normal.  Note that the coefficients are very small in summer for two reasons: first, because there is a less robust impact of the Pacific modes, and second because the summer temperature variability is much smaller.

It's interesting - to say the least - that Hartmann's North Pacific Mode (EOF3) has a small impact compared to ENSO and the PDO, and it is negatively correlated with temperature in most months.  I did not expect to see this, because the North Pacific Mode is closely related to "the Blob" of warm northeast Pacific SSTs that has recently gained notoriety, and I have speculated that this anomaly was partly responsible for the warmth of the past two winters in Alaska.  This analysis would suggest that we can't pin the recent warmth on "the Blob".

Here's the corresponding chart for seasonal (3-month) mean temperatures in Fairbanks.

If we look at the statistical significance of the 3 predictors in the multiple regression model, we find that the impacts of the Pacific modes are generally more robust for seasonal temperatures than for monthly temperatures, as we would expect.  The charts below show the t-values of the individual regression coefficients; the t-values directly indicate the probability that the coefficients could have been obtained by random chance.  On a seasonal basis, the ENSO/PDO impacts are highly statistically significant from autumn through late spring.  The NPM also shows up as significant at certain times of the year, but with a negative coefficient as noted above.

The overall correlation coefficients for the 3-predictor models (monthly and seasonal) are shown below - again generally higher for seasonal than monthly anomalies, and especially in the autumn.

From the perspective of seasonal forecasting it is interesting also to look at the statistical impacts on a lagged basis, because of course we don't know ahead of time how the Pacific temperature patterns will evolve in the months ahead.  The chart below provides a quick look at this; here I've repeated the procedure using seasonal Fairbanks temperatures and the Pacific modes one month ahead of the first month of the season - for example, the columns for DJF show the significance of the regression between November Pacific patterns and December-February Fairbanks temperatures.  The results indicate that the predictability associated with EOF1, the ENSO/PDO mode, is largely retained at a one-month lag, but EOF2 and EOF3 lose most of their predictive power.  This is not too surprising, as the coupled ENSO/PDO anomalies evolve slowly, but the 2nd and 3rd modes can change significantly over the course of a few months.

Last 80 Degree Day?

Yesterday may well have been the last 80+ °F day in Fairbanks for the season, and if so, the summer's total number of such days will end at 11, which is exactly average (1930-present and 1981-present).  Both June and July were slightly cooler than normal in the mean, but May was much warmer than normal and added 2 days to the count.

The chart below shows the climatological frequency of reaching 80 °F on or after the date shown on the horizontal axis.  As of today there is still nearly a 50% chance of reaching 80 °F again, but the forecast for the next week suggests little chance of it happening in the near-term, and after August 12 the frequency drops to under 30%.

Seasonal Transition

The weather in Fairbanks turned unusually wet in the last week of July, taking the month's rainfall total to 2.78 inches and into the top 20% of the historical distribution for July.  More than a quarter-inch of precipitation was observed on 4 consecutive days ending on the 30th, and remarkably this is only the 4th time this has happened in the history since 1930.  The previous occasions were in September 1942, April 2002, and August 1967, the latter being the occasion of the great Fairbanks flood.

Looking at a slightly broader timescale, Fairbanks recorded significant rainfall on 6 of the 8 days ending August 1st.  This is of course much more common, especially at this time of year.  Historical data show that 6 of 8 consecutive days have seen at least 0.05" of precipitation the following number of times since 1930:

8-day periods ending in May: 3 times
June: 17
July: 32
August: 59
September: 22

Nearly one-third of all Augusts see a wet spell of this kind, and of course the shift to wetter weather is a symptom of the seasonal transition to late summer.  Recently I've been wondering about the causes of this seasonal shift, so I created a few maps of climatological upper-level flow patterns to illustrate how the circulation changes over the course of the summer.

First we can look at the mean 500 mb height for June, July, and August: see below.  The upper-level height increases universally between June and July, as the atmosphere heats up in Northern Hemisphere summer, but the increase is especially pronounced over the North Pacific and Bering Sea as the Bering Sea trough is eroded.  A related change is that there's less of a tendency for upper-level riding over Alaska in July.

In August the upper-level height drops almost everywhere on the domain shown here, but the decline is greatest at high latitudes, and the 500 mb height actually increases over the Pacific south of about 50-55 °N.  This is a reflection of the changing distribution of heat input: in the northernmost areas, solar heating is much less in August than in July, and the atmosphere cools off accordingly, but farther to the south there is relatively little decrease in solar input and the atmosphere and oceans remain warm or even continue to heat up.

The effect of the changing height pattern is to increase the north-south gradient of height, especially over the Bering Sea, and this in turn causes the normal westerly flow to accelerate.  The following maps show the normal 500 mb wind speed in July (top) and August (bottom), and we can see that there is a widespread increase in wind speed over Russia, the Bering Sea, and Alaska.

The difference in normal 500 mb wind speed between July and August (below) clearly highlights the zone of energized flow circling the entire Northern Hemisphere, with the maximum change extending across southwest Alaska.

What does this transition mean for interior Alaska weather?  It implies a more progressive flow with a lower chance of persistent ridging and a higher frequency of upper-level disturbances that bring cloud and rainfall.  On the synoptic (large) scale the north-south temperature gradient strengthens from July into August, and it is this gradient that generates and feeds cyclonic weather systems.

To wrap up I'll show the frequency of 500 mb blocking highs and lows.  First, blocking high pressure systems (see below) become less common from July to August across most northerly regions, reflecting the strengthened jet stream; a stronger circumpolar circulation is less prone to developing "kinks" that produce blocking weather patterns.  Alaska in particular sees a much reduced frequency of cut-off high pressure systems as the summer advances.

The frequency of cut-off low pressure systems is not as easy to interpret (see below), although the frequency increases over southwestern mainland Alaska over the course of the summer.

In summary, it's the seasonal intensification of the north-south temperature gradient in the atmosphere that drives the circulation changes in late summer across Alaska and other high-latitude parts of the Northern Hemisphere.  The North Pacific region is especially prone to this seasonal change, presumably because the Pacific Ocean, being the world's largest water body, creates an especially large lag in seasonal warming at middle and lower latitudes.