Arctic sea-ice extent rapidly approaching its summertime minimum
22 August 2018
After the sea-ice extent in the Arctic showed a moderate loss, hovering in the lower portion of the double standard deviation range from the long-term average with ca. 92,000 km² / day in the first half of July, the retreat significantly accelerated (110,000 km² / day) in the second half of the month, meaning it lost an area roughly the size of Bulgaria every day (Fig. 1). The average extent for the month was 7.89 million km²: the seventh-lowest value ever recorded in July and substantially below the long-term average, but slightly above the trend line for July (Fig. 2). Whereas the melting during the first half of the month chiefly took place in Eurasian coastal waters (the Barents, Kara and Laptev Seas), later in the month the sea-ice cover opened from the Beaufort Sea into the Canadian Basin, forming a large ice-free region. By the end of July, Hudson Bay and Baffin Bay were almost completely devoid of ice (Fig. 3).
In terms of the monthly average, July 2018 was characterised by a higher sea-ice extent than last year in the Pacific sector (Chukchi Sea and East Siberian Sea), but a significantly lower extent in the Atlantic sector (Fig. 4 left). Here the melting had begun quite early in the year. Compared to the long-term average for 1981 – 2010, the ice margin in the Pacific sector was in nearly the same position, while the regions in the Atlantic sector with less ice are readily apparent (Fig. 4 right).
Meteorological conditions in July
In July there continued to be extremely low atmospheric pressure at sea level over the central Arctic Ocean (Fig. 5): a weather pattern that normally slows the loss of Arctic sea ice in the summer, and which determined the weather in both of the past two years. There was also low atmospheric pressure at sea level over Greenland, but high atmospheric pressure over Northern Europe and northern Siberia, Alaska and Canada. This was combined with air temperatures at 925 hPa (ca. 750 m) that measured 0.5 °C to 3.0 °C below the long-term average over the Kara and Laptev Seas, and 0.5° C to 1.5 °C below average over the Beaufort Sea and the Canadian Basin. Over the North Pole, temperatures largely matched the long-term average or were slightly above it (up to +1.0 °C). Over Greenland, temperatures were ca. 2.0 °C below the long-term average (Fig. 6).
July 2018 also brought a number of record high temperatures for Scandinavia. In Finland (Turku) the mercury hit a whopping 33.3 °C on 17 July, the highest temperature observed in Finland since 1914. At Trondheim Airport in central Norway, a temperature of 32.4 °C was recorded on 16 July. In Bardufoss, south of Tromsø in the northern polar circle, there was a record high of 33.5 °C on 18 July. During the unprecedented heat wave in mid-July, more than 40 forest fires broke out across Sweden; there were also fires in Lapland and Latvia. But Scandinavia wasn’t the only region hit by the heat wave and lack of precipitation: the same extreme conditions were to be found throughout Western Europe. Widespread forest fires in Greece claimed nearly 90 lives. Japan was also struck by a heat wave, in the wake of which 65 people died and 22,000 required medical attention. (Source: NSIDC). In Germany, the “blocking” situation created by the stationary high-pressure cell over Northern Europe produced a virtually unprecedented period of heat and drought, which especially posed major challenges for the agricultural sector.
Smoke from forest fires in Siberia over the Arctic Ocean
During the past few weeks, fires in the western United States have often been mentioned in the news, yet the massive fires in Siberia received much less attention. Satellite images from 3 to 6 July, available on the NSIDC website, show smoke from these fires as it spreads across the Arctic Ocean. For several days, the combination of a low-pressure cell and winds pushed the smoke clouds out over the Arctic Ocean, where they spread across the East Siberian, Chukchi and Beaufort Seas, and most likely ultimately made their way to northern Canada via Alaska (see Figure 7).
The smoke can have two potential effects on sea ice: initially, smoke particles scatter sunlight, reducing the amount of light that reaches the Earth’s surface in the process. This cooling effect can reduce the melting of sea ice. However, carbon particles that accumulate on the ice (referred to as “black carbon”) also darken its surface, reducing its reflectivity (albedo). This increases the amount of sunlight absorbed by the ice, accelerating the melting process. The atmospheric scattering effect is short-lived, and only lasts until the smoke dissipates; in contrast, the surface-level albedo effect is more lasting, and can accelerate summertime melting. However, the intensity of this effect depends on how many carbon particles accumulate on the surface, on the surface’s original background albedo, and on the quantity of clouds that reduce the incoming sunlight. The effect is most pronounced on bright, snow-covered ice and less intense on darker ice that has already begun melting, as well as meltwater pools. In open water, there is no effect whatsoever. (Source: NSIDC)
We asked Dr Christoph Ritter, a member of the ‘Physics of the Atmosphere’ Working Group at the AWI’s Potsdam facilities, to assess this phenomenon: “When it comes to the radiative forcing of smoke particles (aerosols), the key factor is always the difference between the ground albedo and the aerosol albedo. On light surfaces, like fresh snow, aerosols have a warming effect, since without their influence the surface would reflect the sunlight back into space. But part of the sunlight is absorbed by the aerosol layer, warming the atmosphere. Over darker surfaces, aerosols have a cooling effect, because they generally absorb less sunlight than the dark ground. Due to their scattering effect, aerosols ensure that far less sunlight makes it to the ground. In other words, aerosols over light surfaces produce a mild warming of the atmospheric layer where the aerosols are; over darker surfaces, they produce significant cooling,” he concludes, before adding, “In the Arctic, I generally consider the albedo effect to be negligible. The concentration of sunlight-absorbing aerosols is most likely too low to meaningfully reduce the ground albedo; other characteristics, like the surface roughness of the ground, are much more important.”
Sea-ice forecast for the September minimum
With the forecast at the beginning of August, this year’s Sea Ice Outlook 2018 (Sea Ice Prediction Network, SIPN) drew to a close. As in the past, this year the AWI contributed two methods to the Outlook: a statistical variant and a model-based approach (click here for more information). For both methods, the balance for this year looks quite similar:
Accordingly, we can expect to see a mean sea-ice extent of 5.15 ± 0.58 million km² in September 2018, which would represent a continuation of the low values throughout the observation period since 1979. With regard to this year’s forecasting period, Dr Monica Ionita-Scholz explains, “In addition to its mean overall extent, the spatial distribution of the sea ice is of particular interest, though it’s an aspect we won’t be able to fully assess until the end of the month, after we’ve reached the absolute minimum.”
Dr Christoph Ritter (AWI)
Dr Monica Ionita-Scholz (AWI)
Dr Frank Kauker (AWI)
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