According to analyses from the University of Bremen’s Institute of Environmental Physics, the mean sea-ice extent in the Arctic this October was only 5.44 million km², which is more than 443,000 km² below the previous minimum year, 2012 (Figure 1). Other analytical algorithms, like those used by the National Snow and Ice Data Centre (NSIDC), can vary slightly (see the article from 2018). This is an area the size of Morocco or 1.24 x the size of Germany. As Christian Haas, Head of the Sea Ice Division at the Alfred Wegener Institute, explains: “Since the beginning of continuous sea-ice observation, we’ve never seen so little sea ice in the Arctic at this time of year. It’s a troubling process, most likely caused by the increased heat input from the ocean and the atmospheric circulation in the Arctic.”
The sea-ice extent in the Arctic is an important indicator for the effects of climate change in the polar regions. Here the sea ice is manifesting a negative climatological trend that reflects the increasing warming of the Arctic. Since the beginning of continuous satellite observation, the October sea-ice extent has declined by ca. 34 percent, and reached its lowest extent this year (Figure 2). Especially in the northern Canada Basin, in the Chukchi, Siberian and into the Laptev Sea, there was considerably less ice than in the comparison year 2012, the year with the lowest summertime sea-ice extent to date (Figure 3). The observed changes in comparison to other years are discussed in more detail below. As can be seen in Figure 4, in the years 2016 and 2018, too, the sea-ice growth began late, producing low sea-ice extents in October / November. Potential causes for the weak ice growth this year include the unusually high ocean surface temperatures, which were up to 4 °C above the long-term average (1971 – 2000) in major expanses of the Arctic Ocean (Figure 5). Here it will take a prolonged cooling phase to reduce the stored heat and drop the ocean surface temperature to the freezing point. In addition, atmospheric temperatures up to 6 °C higher than the long-term average were dominant in the Beaufort Sea (see Figure 5). To the north of Greenland the air was also unusually warm, which slowed ice growth. In Figure 6, an integrated, vertical depiction of atmospheric temperatures in the Arctic across all lines of longitude, we can clearly see how the high ocean surface temperatures contributed to a warming of the air masses above. This is especially apparent in the area between 73°N and 80°N, where the temperature anomaly (deviation from the climatological mean, 1971 – 2000) was ca. 6 °C. This area largely corresponds to the position of the ice margin, especially in the Beaufort Sea and Chukchi Sea. Since the humidity was also especially high in these areas (see Figure 7), the formation of new ice was made far more difficult.
Analysis and Significance of Sea-ice Formation in October
When discussing individual years in comparison to climatological changes, the considerable influences of year-to-year changes (interannual variability) must be taken into account. Especially in high latitudes, these variations can be so much more pronounced than the actual climatic changes that climate changes essentially become invisible. This effect is especially prominent in the Arctic – due in part to the ice-albedo feedback. In this regard, climate researchers speak of a poor ‘signal-to-noise’ ratio – the climate changes (the signal) are large, but so are the interannual and decadal variations (produced by ‘internal variability’; the background noise). This often produces ostensibly contradictory conclusions when examining changes in individual years.
An analysis of the sea-ice conditions in October that takes into account the ‘signal-to-noise’ ratio has been conducted by Dr Frank Kauker, a sea-ice physicist at the AWI. Using model studies, Kauker compared e.g. the number of non-freezing days (NNFD) from 1 September to 31 October with a climatological mean, and with the number in selected individual years. In this regard, the NNFD also roughly represents the time at which freezing begins (1 September plus NNFD): however, the latter is difficult to define, since early in the winter, freezing phases often alternate with warm phases, until prolonged wintry conditions finally set in. Principally speaking, the NNFD can also be calculated using the changes in the observed ice concentration; however, the basis of the calculation is the change in sea-ice extent. In the high-resolution sea-ice model employed at the AWI, not only the sea-ice extent but also the change in ice volume – which is used here to determine the NNFD – is simulated. This parameter can’t be determined using observations, since Arctic-wide monthly ice information can only be gathered from October to April. Figure 9 shows this number for 2019 as an anomaly in comparison to the climatology for the period 1980 – 2019, and for the years 1990, 2007, 2012 and 2015 – 2018.
Contacts
- Prof. Dr. Christian Haas (AWI)
- Dr. Thomas Krumpen (AWI)
- Dr. Frank Kauker (AWI)
- Dr. Renate Treffeisen (AWI)
- Dr. Klaus Grosfeld (AWI)
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