Springtime in the Arctic
Warmer than it used to be but more variable
The high latitudes are warming faster overall than areas further south, a feature known as Arctic amplification. The reasons for this at large scales are well understood: thinning sea ice, decreasing extent of sea ice, shorter snow cover season on land and increasing water vapor in the atmosphere. However, the factors vary in importance during the course of the year. In the autumn (September through November), the dramatic loss of sea ice in the past 20 years and resulting exposure of open ocean water is driving the (massive) warming even as solar heating wanes. In the Spring (March through May), the changes in sea ice extent are more modest. On the Pacific side of the Arctic sea ice extent changes are mostly confined to the the seas south of 65°N until the second half of May. But at the same time, solar heating is increasing rapidly, though snow covered tundra and sea ice reflect much of that energy back to space, at least when skies are mostly clear.
The 50-year change in Spring average temperature is shown in Fig. 1. The largest increases in temperature are mainly on land and exceed 5°C in parts of Russia and northwest Alaska. This is very likely tied, in part, to earlier loss of the winter snow pack and resulting exposure of the ground to the warming sunshine, though many land areas north of 68°N typically maintain some snow cover through much of May. The most dramatic warming over the ocean is the eastern Barents Sea near Novaya Zemlya (Russia), where there has been significant Spring sea ice extent decrease in recent years. The only areas north of 60°N with any appreciable cooling are parts of Canada west of Hudson Bay (where Spring snow pack meltout does not show a trend toward occurring earlier) and a small area in eastern Alaska.
With April 2023 featuring near record cold weather in western and central Alaska and northeastern-most Russia (Chukotka), a question naturally arises (often asked in many contexts): are there greater year-to-year swings in temperatures nowadays than in the past? That is, are “warm” springs getting warmer while “cold” springs staying about the same or even getting colder? We can use the same 50 years of data used to construct the average temperature change shown in Fig 1. to start to answer that question. We’ll do this by splitting the 50 years into two equal sized chunks, 25 years 1973-1997 and 1998-2022. Then we simply compare the typically variability in earlier period to that in the more recent period (for specifics of how this was done see “Technical Details” below). Figure 2 shows how the variability differs between the two periods. For most land areas the 1998-2022 period had greater variability than 1973-1997, though this was not the case for western Alaska and Chukotka, northeast Canadian Arctic, Greenland and Iceland and parts of the Nordic Arctic. By far the largest decreases in variability were over the Arctic Ocean north of Greenland and the Barents Sea west of Novaya Zemlya.
This certainly lends credence to the idea that in many (not all) areas that Spring temperatures are less predictable, i.e. more variable, than was the case in the later 20th century, at least in terms of this one way of describing Spring temperature changes.
Technical details: This analysis is based on the ERA5 reanalysis data from ECMWF/Copernicus and used the most current version of the reanalysis available as of this writing (April 2023). Figure 1 change from the trend via Theil-Sen regression. Figure 2 “variability” shown is the difference in the standard deviation of the detrended temperatures 1998-2022 minus the standard deviation of the detrended temperatures 1973-1997. Areas where the recent epoch standard is more (less) than the earlier epoch are shown as more (less) variable. Because of the short sub-interval (25 years), with the possible exception of north of Greenland, the differences in variance are not statistically significant (e.g. via Fligner-Killeen test).
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