Monday, May 1, 2023

Fairbanks April Temperatures

April is in the books, with a monthly mean temperature of 21.5°F in Fairbanks, and that's the 4th coldest since 1930.  The last few days of the month were positively springlike with the temperature reaching 54°F only 6 days after the record cold high temperature of 17°F on the 23rd.  A big turn-around is not at all unusual when there's an amplified pattern that lends itself to temperature extremes.  (But even with the big warm-up, daily mean temperatures didn't rise above normal - only the 2nd and the 4th of the month were warmer than normal.)

The long-term chart of April mean temperatures in Fairbanks is interesting:

The thing that jumps out at me here is the skewed distribution after about 1985, with most years seeing April temperatures above 30°F, but a minority being much colder.  Prior to about 1985, the distribution looks much more symmetric.

Another striking aspect is that 8 of the 10 warmest Aprils occurred from 1983 to the present, but the 10 coldest Aprils are distributed quite evenly across the history.  Even though the climate has warmed a lot, the chance of a significantly cold April (say below 23°F, -5°C) doesn't seem to have decreased.

We can make simple estimates of the change in the mean versus the median by using linear regression in the first case and quantile regression in the second.  Here's the result:

Both trend lines have increased, but the median trend has risen nearly twice as fast as the mean trend.  It would be interesting to rearrange the data many times and see how often this difference occurs by random chance - my guess is not very often, suggesting that there's been a "real" change of climate behavior, and this isn't just a random feature of (say) normally distributed data.

What might account for this?  The only possible explanation I can think of is that it might be related to increasingly pronounced and persistent disruptions of the high-latitude circulation in connection with stratospheric polar vortex disruptions in mid/late winter; and perhaps this is being forced by pronounced warming trends in the western tropical Pacific.  We had a stratospheric vortex disruption ("sudden stratospheric warming") back in February, and I think it bears some responsibility for the April cold in Alaska; and there was also one in 2013 that seemingly had a connection.  These events don't happen every year, they are all different, and there's scientific debate over the long-term trends; but that's the only speculation I have at this point.

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