Tuesday, December 9, 2014

Climate of the 1930s

In last Wednesday's post I made a remark about the 1930's being an interesting time for climate in Alaska, so of course this idea deserves a post of its own (and probably many more).  I'll focus here on the winter climate and perhaps look at other seasons at another time; I'll also deal mostly with Fairbanks.

Readers of this blog may have noticed, as I have, that the 1930's often show up in comparisons between current and historical weather events in Fairbanks, because many records set in the 1930's are still standing.  For example, December 1934 saw the greatest winter chinook in Fairbanks history, with high temperatures in the 50s for five consecutive days resulting in a complete snow melt-out and the only brown Christmas in the city's history.  The winters of 1935-1936 and 1936-1937 saw significant rain on five separate occasions, including 0.99" of rain in January 1937 (contributing to the extraordinary monthly precipitation total cited in the previous post).  In 1938, extraordinary warmth in late October (60 °F on the 21st) resulted in the latest arrival of a 1" snow depth on record.

Of course, there was plenty of very cold weather too in the 1930's.  If we look at the daily record low temperatures for Fairbanks (1930-present), half of the daily records for cold in December-February were set or tied in the 1930's (45 out of 91 days).  However, this is partly an artifact of the much smaller urban warming effect in the early years.

A quick re-examination of the mean temperature and total precipitation from each Fairbanks winter since 1930-1931 (see below) shows that the 1930's were not particularly warm or cold overall, and except for the record wet winter of 1936-1937, the decade was not particularly wet or dry.  Therefore the mean winter climate doesn't stand out as being especially different from the long-term mean in Fairbanks.  However, with many extreme weather events having occurred in that decade, we might hypothesize that the climate was more volatile on sub-seasonal timescales.

To explore the idea that short-term weather extremes may have been prevalent in the 1930's, I calculated the frequency with which weekly average conditions fell within the top or bottom 1% for sliding 31-day windows throughout the year.  For example, for the date window centered on January 16, I pulled out all the weeks with central dates in the range January 1-31 and then sorted the ~2600 weekly average temperature and weekly total precipitation values.  The top and bottom 1% of temperature and the top 1% of precipitation were flagged as the "extremes" for the January 16 window.  After doing this for every day of the year, the total number of extreme values were added up for each winter (November through March); the results are shown below.

The colored columns show the November-March counts of extremes for temperature in the top 1% (red), temperature in the bottom 1% (blue), and precipitation in the top 1% (green), and the lines show the corresponding 10-year trailing mean for each category.

Several features are notable: first, the dramatic increase in warm extremes and decrease in cold extremes in the late 1970's, related to the PDO phase change.  Warm extremes have continued to occur in the most recent 20 years, but cold extremes have become rare.  In terms of wet extremes, the winters of 1962-1971 stand out, but the 1930's also saw a lot of very wet weeks.  It's interesting to note that warm, cold, and wet extremes were all quite frequent in the 1930's, which supports the hypothesis of highly volatile weather in that decade.

The chart below shows the sums of the counts for all three categories and confirms that the five winters of 1931-32 and 1933-34 through 1936-37 brought a sustained high frequency of extreme weather.  The decade of winters ending in 1971 was also quite extreme, but in that case the decadal mean is skewed by the outlier winter of 1970-1971.  Another notable aspect of the chart is the relatively subdued (non-extreme) winter climate of the past two decades in Fairbanks.

I also looked at monthly extremes of temperature and precipitation to see whether the same patterns are evident for the calendar months of winter.  The columns in the chart below show the number of times each winter that one of the calendar months fell into the top 5 or bottom 5 for temperature, or the top 5 for precipitation (1930-present).  The red line shows the 10-year trailing mean.  As in the weekly chart, we see a peak in about 1971 for the preceding decade - the 1960's was an extreme decade for both weekly and monthly averages - but the 1930's do not show up as particularly extreme on a calendar month basis.

In conclusion, initial analysis suggests that Fairbanks winters in the 1930's were characterized by relatively frequent weather extremes on a weekly timescale, though not on a monthly timescale.  What might explain increased volatility of daily and weekly weather?  My only hypothesis at this point - and it is really only speculation - is that this could be related to some unusual behavior of ENSO during the 1930's.  I've long been interested in the fact that ENSO volatility was particularly low during the 1930's, as seen in the chart below.  Between March 1931 and May 1938 there was not a single month in which the ENSO index exceeded a magnitude of 1.0, and the 10-year trailing standard deviation dropped to 0.48 in April 1938 (less than half the 1981-2010 standard deviation of 1.04).  My qualitative hypothesis is that the lack of tropical forcing - perhaps counterintuitively - allowed for unusually large fluctations in the jet stream position over Alaska during the 1930's, thereby causing extreme weather in Fairbanks.  In other words, with no major ENSO forcing to lock the winter jet stream into preferred orientations, it oscillated from one extreme to another with unusual frequency and amplitude.  I hope to investigate this idea soon using the 20th century reanalysis data.

Another feature of the global climate in the 1930's that may or may not have influenced Alaska climate is that the North Atlantic sea surface temperatures were relatively warm compared to other oceans, i.e. the Atlantic Multidecadal Oscillation was strongly positive (see e.g. here).  The relationship between the AMO and Alaska climate is another topic worthy of investigation.

For the sake of curiosity, I also computed the frequency of weekly and monthly extremes for Nome and Barrow, see below.  Nome saw many wet extremes in the winters of the 1930's and 1940's, but those decades were less notable for unusual warmth and cold.  Barrow saw a remarkable concentration of winter extremes from about 1950 to 1975 (mainly cold and wet), and there has been an uptick in wet extremes in the past 4 winters in Barrow.  Remarkably, Barrow has not seen a winter week in the bottom 1% for temperature since 1994-95.


  1. Alaska isn't alone. While living in the Lower 48, I noticed that many records were from the 1930's. I find it fascinating that the combinations of warmth, cold, and wet are different over different decades each 20-30 years (which implicates ENSO). This implies different regimes are responsible for the different decades. Perhaps we could correlate warmth, cold, and wet combinations with different combinations of indices (ENSO, AMO, etc.) and see what pops out.

    1. Eric, there is certainly a lot of good work that could be done here in terms of oceanic regimes affecting probabilities of extremes. This might build on well-known overall impacts of ENSO and PDO, e.g.


    2. Good read. Thanks Richard.


    3. A free paper! Since the paper was published in 2001, I wonder how another 13 years of data would affect the author's results. They'll probably be similar since similar results were found on this blog. And I love how succinctly the abstract describes Interior weather. I also love the hand drawn figures - especially Figure 1. You can tell that the author was raised without a computer.

  2. The general consensus among PDO followers is that the 1930's were squarely within a multi-decade warm phase (http://appinsys.com/GlobalWarming/PDO_files/image001.jpg), so it is interesting to compare and contrast what occurred in that era with what occurred in our most recent warm phase (1977-2007). I know you've mentioned in the past that the most recent history of the longwave pattern is one of monthly or multi-monthly stubbornness/consistency, which seems very different compared to your data from the 1930's.

  3. Andy, that's a good point about the PDO and it's interesting to note a higher frequency of warm extremes in both warm phases (for Fairbanks). The early reanalysis data may reveal more of the nature of the longwave pattern and the degree of similarity or difference from the modern era.