I am comfortable asserting that the retreat of sea ice is the dominant cause of the autumn differences in the mean (not necessarily for every given year). It is both a very real local change affecting the locality and one of several Arctic feedbacks that are generally accelerating warming in the region. There is rich detail in NOAA's 2017 Arctic Report Card on the interplay between sea ice and air temperature.

The CRN is a wonderful network and was designed and deployed to help provide a baseline for situations just like these. However, the nearby CRN station did not exist in the "before" years in this comparison; it would not provide the before-and-after context needed to compare.

Thanks for your questions. I'm going to close out this thread now and devote some attention to other NCEI customers. Thanks!

Deke