October Arctic Precipitation Changes

Wetter, but not everywhere snowier

October is the fastest warming month of the year for the Arctic overall, due in large part to the dramatic decrease in sea ice extent. I posted about changing temperatures last autumn (here). This post looks at changes over the past 50 years in precipitation and snow during October.

For a refresher, Fig. 1 shows the 50-year change in average October temperature. Virtually the entire Arctic is warming, with the largest changes from the Kara Sea east to the Beaufort Sea. The primary driver of these extreme increases is simple: the loss of sea ice. As October progresses, more and more of the Arctic receives little to no solar heating¹, so open water effectively serves as a heating blanket, releasing heat from the ocean surface into the lower atmosphere. In contrast, air directly over even a thin layer of sea ice can cool far more than air over water.

Fig. 1 Total change in October average temperature 1974 to 2023. Data from ERA5 courtesy of ECMWF/Copernicus.

October Precipitation

With much more ocean area nowadays in October having water exposed and evaporating water vapor directly to the atmosphere, it’s no surprise that October precipitation (water equivalent of snow plus rain) has increased (Fig. 2). But unlike temperatures, there are areas showing no significant change or even modest decreases in precipitation, especially over northwest North America, but also parts of the Russian and Nordic Arctic.

Fig. 2 Percent change in October average precipitation, 1974 to 2023. Data from ERA5 courtesy of ECMWF/Copernicus.

Most of the areas with decreases are over the continents, although some oceanic areas in the Atlantic Arctic between Greenland and Norway show slight decreases. Breaking down the the change in October precipitation by latitude bands (Table 1) reveals that the disparity between the land-dominated “lower Arctic” and the ocean-dominated “higher Arctic”

Table 1 Change of October average precipitation between 1974 and 2023 as a percent of the total change relative to the 50-year average. Data from ERA5 courtesy of ECMWF/Copernicus.

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October Snow

Precipitation in the Arctic transitions from predominantly (or entirely) rain during summer (except high elevations and near the North Pole) to predominantly (or entirely) snow during the autumn. By October, historically at least, snow was the dominant precipitation type except across the parts of the Atlantic Arctic and the Bering Sea and Sea of Okhotsk and adjacent coastal areas. Figure 3 shows the percent of October precipitation that fell as snow during the 1991-2020 baseline period.

Fig. 3 Average percent of October precipitation that fell as snow during the 1991-2020 baseline period. Data from ERA5 courtesy of ECMWF/Copernicus.

So with the increasing temperatures and increasing precipitation, how is October snow² changing? Figure 4 shows the 50-year change. I’ve used the same scale and color palette as for precipitation change in Figure 2 to allow for a direct comparison. Far more areas show decreasing snowfall than decreasing precipitation and the changes in many areas are larger. Nearly all ocean areas south of 70°N show decreasing October snow, but so does much of Alaska and Canadian Arctic south of 70°N, as does the European Arctic. The largest increases (percentage-wise) on land are over northern Greenland, Baffin Island, Canada and the Russian far northeast away from the Bering Sea.

Fig. 4 Percent change in October average snowfall, 1974 to 2023. Note that no trend is shown over areas where the 50-year median October snowfall (liquid equivalent) was less than 1cm (0.4 inches). Data from ERA5 courtesy of ECMWF/Copernicus.

The latitudinal changes shown in Table 2 are different from the precipitation changes. At the highest latitudes, the increase in snow is very close to the increase in precipitation, indicating that nearly all of the additional precipitation is falling as snow. In the 70° to 80°N band, snow has increased significantly, but it’s only half of the precipitation increase, while in the 60° to 70°N band, October snow is, on average, decreasing even as precipitation is increasing, a sure sign that much more October precipitation is falling as rain.

Table 2 Change of October average snowfall between 1974 and 2023 as a percent of the total change relative to the 50-year average. Data from ERA5 courtesy of ECMWF/Copernicus.

Overall, these results are not particularly surprising, but serve to reinforce the importance of the regional variability in changes around the Arctic. So while it’s straightforward to come up with single numbers for various aspects of Arctic climate change, it’s rarely helpful except at global scales.

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Technical Details

As usual in this newsletter, Arctic means “poleward of 60°N” unless otherwise specified.

ERA5 monthly temperatures and precipitation data now available here. Note that the url for accessing ERA5 data changed as of September 26, 2024.

Code by B. Brettschneider/NWS Alaska Region allowing rapid ERA5 regional analysis is invaluable for my work.

1. By the end of October, the sun has set for the year north of 76°N and the maximum solar angle at 60°N is only 16° above the horizon (for comparison, the maximum solar angle at 40°N on winter solstice is about 27°above the horizon).

2. As a reminder, snow from ERA5 reanalysis is the water equivalent of precipitation the reanalysis has occurring as snow. This is different from “snowfall” that you might shovel off your deck or driveway, which is the solid accumulation. So wet heavy snow of 5 cm (2 inches) might melt down to 6 mm (0.25 inches), while a light fluffy snow from the same amount of water might have a solid depth of 15 cm (6 inches). From the reanalysis perspective, the “snow” is the same (6 mm, 0.25 inches) for either case.