Friday, September 2, 2011

End of Summer

With the foliage now rapidly turning to autumnal reds and yellows, thoughts turn to the end of summer in Interior Alaska, and the question arises, are summers ending later than in years gone by? Of course, I have a chart for this question.

Since a couple of cool days does not autumn make, I've defined the end of summer as the last day in the season when the 7-day centered running mean is 55F or higher. From the plot at the left, it is clear that at least until very recently, there was no trend at all. However, the gray line is the 4 year running mean, and it has, just in the past few years, popped up to the highest value in the century of instrumental records. Through September 1, this summer is already beyond the mean date for the past 40 years, and if the forecast for the next few days works out, this will push this year's "end of summer" later still.

It is very tempting to pin this recent and dramatic change on the loss of sea ice near Alaska, though we need some modeling of this idea.


  1. So you are thinking that the loss of sea ice north of AK could impact locations as far inland as Fairbanks? I agree it would be interesting to see what models say about this. Is it common for the flow to have a northerly component this time of year?

  2. Hi Matthew,

    My thinking is: sea ice cools the overlaying atmosphere more efficiently than water, so it would be a matter of producing synoptic scale cooling that then could be advected (either from the north, or first south through Chukotka/Bering Straits region and then eastward into the Interior.

    On the Arctic coast of course, and arguably on the North Slope in general, both direct and indirect airmass cooling would be in play.

    Just an idea; I was stuck by the recent surge that seems to correspond with the 2007 regime shift in late summer sea ice distribution. Could be purely coincidental. I need to gin up these numbers for other places to confirm it it is not just local, though my sense is that it is not and I'm also not sure how robust a measure the 55F "7-day centered running mean" is. It seems to correspond in the spring pretty well to the burst in biotic activity and people's perceptions, but I'm not so sure about autumn.


  3. Hmmm that is an interesting idea. It would be interesting to have been keeping a record of the leaves turning, sort of like the green up dates in the spring. One concern I have with the chart is that you could still just be on an upswing, for example like the one in the early 80's, and then if the next 3 to 4 years are cooler it would drag the average back down the and this peak would look just like every other peak in the history of it. I would be interesting to throw a couple average or early years in there to see how it reacts.

    It does seem like there has never been a period in history where there have been 4 years in a row where it was so late. Intriguing.