Autumn Warming - Part 2

Last week I examined the timing of the remarkable autumn warming that has affected Barrow in the past 12 years, and I showed that an October peak in unusual warmth has been observed not only at Barrow but throughout the northern part of Alaska.  Brian showed that the warming also extended down to the Bristol Bay region.  My original hypothesis that this is mainly an effect of the reduced autumn sea ice coverage in the Arctic proved a bit controversial, as readers suggested that changing wind patterns could also be responsible; so I've been thinking about further analysis that could shed light on the subject.

To highlight the issue again, below are charts showing the October mean temperature anomaly at Barrow and Fairbanks, relative to the 1981-2010 normal.  Barrow has seen a most extraordinary change since 2002, with October temperatures well above normal every year and with very little interannual variability compared to earlier years.  The warming has been less pronounced at Fairbanks, but is still quite noticeable, with 6 of the 12 most recent years being more than 5 °F above normal.

To investigate the cause of the warming at both locations, I first looked at changes in the distribution of 850 mb wind direction in October (see below).  At Barrow the changes between 1981-2001 and 2002-2013 were fairly small, with the more recent period seeing winds from the east more often.  There was also a small increase in the frequency of southwesterly and westerly winds, and a small decrease in the frequency of northwesterly flow.  Note that the area under the curves is not the same, because winds of 10 knots or stronger increased in frequency from 68% to 78% of the time.

The October changes at Fairbanks are more noticeable, as winds from compass headings between 120° and 240° have increased at the expense of winds from the west and from the east.  In and of itself this would be consistent with overall warming, as warmer air from the south is transported to Fairbanks a greater percentage of the time.

However, this is not the end of the story.  The scatter plots below show the relationship between wind direction and temperature anomaly at Fairbanks, for the 1981-2001 period (blue dots) and the 2002-2013 period (red dots).  The blue and red lines show the median temperature anomaly for each period, based on wind direction windows of +/- 10 degrees (and with a minimum count of 10 events to calculate a median).  The top chart shows the relationship for surface temperature, and the bottom chart shows the relationship for 850 mb temperature.  Remarkably, the results indicate that surface and 850 mb temperatures have warmed similarly regardless of the direction of the 850 mb wind; there are some subtle differences in the amount of warming depending on wind direction, but overall the warming is quite uniform.

These results are NOT what I would expect to see if the October warming in Fairbanks was directly caused by the reduction in sea ice.  If the sea ice reduction was causing the warming, then there should be relatively more warming in more northerly flow, and relatively less warming in more southerly flow, but if anything the opposite has occurred: air coming from the south has become considerably warmer in Fairbanks!  I interpret this to mean that the recent October warmth in Fairbanks does NOT have its origin in warmer Arctic air masses; instead, there has been general warming regardless of wind direction, AND southerly winds have become more frequent.

Looking now at Barrow (see below), we see a result that is more consistent with October warming caused at least partly by an open ocean.  The surface temperatures have warmed dramatically for wind directions between 300° and 60°; for example, the median temperature for winds from 0-20° or 320-340° has increased by more than 15 °F between 1981-2001 and 2002-2013.  Northerly winds in October just aren't very cold in Barrow any more.  Interestingly, however, the 850 mb temperatures show by far the most warming from the southwesterly direction (210-250°), which again suggests a much larger-scale warming effect rather than local heating from the open ocean.

One more point of interest in the top Barrow scatter plot (surface temperatures) is that in the past 12 years, the median temperature anomaly is remarkably independent of wind direction in October.  This fits perfectly with the collapse of interannual variance in October temperatures: no matter which direction the wind blows from, the low-level temperature outcome is about the same.  Remarkably, it appears there is no way for the October temperature anomaly to end up significantly different from where it has been in the past 12 years.

Finally - I promise - here is the change in the temperature difference between the surface and 850 mb at Fairbanks and Barrow, for each day of the year (but heavily smoothed).  Values above zero indicate that the surface has warmed relative to the 850 mb level in the past 12 years.  The surface warming has been very dramatic at Barrow, but quite insignificant at Fairbanks; and I would suggest that this represents a better indication of the true thermal effect of the reduced sea ice.  Also, we see that the surface warming peaks in early November, which distinguishes it from the mid-late October peak that we saw in the simple temperature anomalies.