October Temperature Changes

Very Scary 🎃

In a previous post we looked at how sea ice has declined during October in recent decades. In this post we’ll examine the impacts this ice loss has had on air temperatures.

The Big Picture

For the Arctic (poleward of 60°N) as a whole, the average October temperature has increased 4.6°C in the past 50 years, the most for any month of the year. However, as Fig. 1 shows, there is enormous regional variability in the observed changes, from more than 10°C over parts of the Asian Arctic seas to 2°C or less over large portions of the Atlantic sector south of 70°N.

Fig.1 Total change in October average temperature between 1973 and 2022. Data from ERA5 reanalysis.

When we parse out the total change by latitude, we find a peak in the 75° to 80°N band with a dramatic drop off south of 70°N, where sea ice changes in October are only locally significant (mostly in some smaller, shallow bays in Alaska and Canada).

Table 1: Total change in October average temperature between 1973and 2022 based on Theil-Sen regression. Data from ERA5 reanalysis.

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Regional Temperatures

Taking a closer look at the region with maximum change, that is along and north of the Russian and western Alaska coasts, basically encompassing, from west to east, the Kara, Laptev, East Siberian and Chukchi Seas, we find a really remarkable time series. The dramatic warming in this region has all taken place since 1990s. From 1950 into the 1990s there is pretty clearly no trend, or even significant decadal scale variability. In this region, all the warming is packed into the past 30 years or so, when the average temperature has increased a stunning 8.1°C. This kind of change over this large an area (more than 3 million km²) is a direct result of the loss of sea ice and the exposure of ocean water to the atmosphere. Since ocean water can’t be colder than about -1.8°C (otherwise it’s ice not liquid), this water serves as a major heat source to the Arctic atmosphere that just was not available prior to the late 20th century.

Fig. 2 October average temperature along and north of the Asian and northwest Alaska coasts to 80°N. Data from ERA5 reanalysis.

Figure 3 plots the average October temperature for pan-Arctic land areas north of 60N. The shape of the trend is similar to that in Fig. 2, although there are difference is detail, and there is somewhat more variability, as we expect to find over land. Note that the five warmest Octobers have been in the past five years.

Fig. 3 October average temperature 1950-2000 over land areas poleward of 60°N. Data from ERA5 reanalysis.

Based on very preliminary analysis, it’s quite possible that October 2023 will also finish up in the top five warmest October in the Arctic. Stay tuned.

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Technical details: Temperature and precipitation analysis spatial maps and time series from ERA5 reanalysis data from ECMWF/Copernicus. Code by B. Brettschneider/NWS Alaska Region that allows rapid ERA5 regional rankings and time series.