Tuesday, August 26, 2014

Daily Temperature Departures: 2004-2013

AK Temperature Departure This is a follows-up on Richard's Autumn Warming post from a few days ago. Richard's main focus was the relationship between warming in October and how far from the Arctic Ocean its effects are felt due to a decline in sea ice coverage. However, seeing the autumn charts made me wonder what the rest of the year looks like. Therefore, let's see how the departures look for all 365 days of the year.

I replicated Richard's technique of averaging the departure from normal for a 10-year period (I used 2004-2013) for each station in Alaska. Only stations with at least 80% coverage during the 10 years were used. Stations also had to have 1981-2010 normal daily temperature values published by NCDC. In addition, I smoothed the daily data for each station using a 15-day moving average just as Richard did. Without smoothing, the data really jump around a lot. Figure 1 shows the smoothed (red) versus unsmoothed (blue) 10-year daily temperature departure for Fairbanks.

Figure 1. Average daily temperature departure from the 1981-2010 normal at the Fairbanks International Airport during the 2004-2013 time period. The blue lines are the raw daily averages and the blue line represents a 15-day smoothing of the raw data.

Statewide Departure Lines

Using the example from Figure 1, we can repeat the procedure for all other stations in Alaska with sufficient data. Figure 2 shows the result of all 112 stations in Alaska that had at least 80% data coverage and published normal values from NCDC.

Figure 2. Average daily temperature departure from the 1981-2010 normal for 112 stations in Alaska during the 2004-2013 time period. 

There are several interesting patterns that are clearly shown in Figure 2. First, there is a very high degree of variability during this 10-year period. Surprisingly high in my opinion. Over a course of 10 years I would expect the highs and lows to converge; but that is not the case at all. Second, there is a remarkable consistency to the patterns. Alaska has endured an unusual string of consecutive warm Februarys and Octobers and an unusual string of cold Novembers and Januarys. Anecdotally, I have noticed that during the cold half of the year, major patterns seem to last 30 days and they have quite remarkably coincided with calendar months. In the case of February, it seems to warm up dramatically around the 1st or February every year. Of course it is just a coincidence but it sharpens the peaks and troughs on the charts.

Even though Figure 2 clearly shows that the magnitude and direction of the anomaly vectors across the state are remarkably consistent, I thought it would be interesting to see how the trends vary across different regions. Figure 3 shows the average daily departure for all thirteen climate divisions (Bieniek et al.). Figure 4 shows the boundary of the regions. As with Figure 2, there is a remarkable consistency between climate divisions. The only notable exception is the North Slope region during November. Every other station showed strong negative anomalies during the 10-year period but the North Slope region was above normal. I would argue that if the statewide October anomaly is a result of the relativly ice-free Arctic Ocean, which is a reasonable assumption, that effect only lingers across the North Slope into November and the rest of the state no longer feels the influence of the uncovered water.

Figure 3. Average daily temperature departure from the 1981-2010 normal for 13 climate divisions in Alaska during the 2004-2013 time period.

Figure 4. Boundaries of the climate divisions of Alaska (Bieniek et al.). The yellow dots indicate the location of the 112 stations used in the analysis.

*** Update Section ***


  1. Brian, This is an excellent follow-up, thanks. It's very interesting to see that the October anomalies are nothing special compared to the magnitude of anomalies at other times of year. Also interesting is the fact that October warming has been observed down in the Bristol Bay division. I think this may point to synoptic-scale circulation changes as being at least as significant as the warming from reduced ice, except for the North Slope (presumably)... this supports Gary's and Eric's contention that it's not all about the ice. I'm working on more analysis to examine this further.

    The monthly anomaly flips are an interesting curiosity. One wonders if there are decadal-scale regimes of calendar date dependency that could be used to enhance subseasonal forecasting. In other words, does the recent Feb 1 warming trend make it more likely that the same thing will happen next year, or is it truly random?

  2. The consistency of the regions tells me that it is a state-wide advection of warmer southerly air that creates the warmer temps. I would also agree that the North Slope has the higher anomalies in October and November because of the lack of sea ice in addition to the warmer advection. I would estimate that about 1/2 of the warmth is from the reduced ice.

    I remember a couple of years ago that there was a newspaper piece about that winter's weather having 30 day periods alternating between way warm and too cold. It was very apparent. The reported meteorologist said it was due to the jet stream being blocked by some blocking highs in Greenland or something. This brings up the question: is there any Rossby or other planetary waves that have a 30 day period that could be filtered in by current global conditions? Actually, wasn't there some discussion about this on this blog a few months ago?

    And lastly, how much of the anomalies is influenced by individual years? Could last October have overwhelmed the signal since it was statewide?

    1. Eric, here is a quick time series chart of all Alaska temperature anomalies from 2002 to present. It is a 7-day running average. I stitched them together as a long strip so it is a big image (1.6 Mb). http://www.borealisscientific.com/data/TempDepart_2002_2014.png

  3. Very interesting. Maybe the old "January thaw" is now the February thaw.

    You said " In the case of February, it seems to warm up dramatically around the 1st or February every year. Of course it is just a coincidence but it sharpens the peaks and troughs on the charts." What do you mean by that? Just a coincidence that this warming has tended for the last 10 years to come close to the same time on many of the years, or that that time coincides with the first of the month?

    1. Thanks for the question Jim. The coincidence that I refer to is unusual number of extreme periods that seem to start and end at the same time that a new month begins and ends. Take January 2012 in Anchorage for example. The extreme cold of January 2012 began on December 29, 2011 and ended on January 30, 2012. The month of January was 14 degrees below normal and the second coldest on record. If that cold snap started and ended 7 days later, January 2012 would have been 9 degrees below normal and might fall outside the Top 10 coldest Januarys. Four different months in 2013 had a similar situation as January 2012 and three months in 2011. Again, it is more of a statistical curiosity in my opinion but it magnifies monthly departures from normal even if the annual departures are pretty benign.

      By the way, Rick suggested that we touch base regarding possible mapping collaboration. If you have a chance please send me an e-mail at: bbrettschneider@outlook.com

    2. I added a section at the end of the post with a zoomable statewide departure from normal chart using the Highcharts Javascript API. It works on my Windows-based PC and my Android-based phone. I cannot vouch for Apple-based products though.