Arctic sea-ice extent headed for the annual minimum – a retrospective on the summer
22 August 2019
While Europe languished under a sweltering heat wave in the summer of 2019, Siberia was struck by massive wildfires, and record-breaking summer temperatures were observed in Germany. But the Arctic, too, was affected by prolonged warm air temperatures. Consequently, the sea-ice extent has been consistently low since spring and, as the summer minimum draws nearer, is now bound for a new extreme low somewhere between the two lowest past minimum years: 2007 and 2012. However, it’s too soon to tell for sure whether a new record will be set.
For the entire month of July, the Arctic sea-ice extent remained below the previous low in 2012 (see Figure 1). The mean sea-ice extent for the month was 7.13 million km²: a new record, and the lowest since the beginning of satellite observation in 1979 (Figure 2). The monthly mean value was ca. 0.3 million km² below the previous low from 2012 and circa 2.3 million km² below the long-term average for 1981-2010 (Figure 3). In comparison to the long-term average, the sea ice retreated in most regions of the Arctic (see Figure 4): particularly in the Laptev Sea and northern Chukchi and Beaufort Seas, which were ice-free by the end of the month. In the minimum year 2012, too, sea ice was especially sparse in the Beaufort Sea. In contrast, ice retreat was less pronounced in the Barents Sea this year, and the extent roughly reached the usual area as observed for the past several years. The East Siberian Sea is largely free of ice, which represents a substantial decline from 2012.
On 7 August the sea-ice extent dipped below 5 million km². Currently the Northeast Passage, which connects Europe with Asia via Eastern Siberia and the Laptev Sea, is largely ice-free (Figure 5). In contrast, certain routes along the Northwest Passage are not free of ice, and therefore not traversable (for more information, click here).
Prognoses for the sea-ice minimum in September, and the question of whether or not we’ll see yet another record low, remain uncertain. To a great extent, this will depend on the future ice-loss rates, and whether storm fronts break up the ice in late August, producing major ice losses. The Sea Ice Outlook for July 2019 released by the international Sea Ice Prediction Network (SIPN), which is based on a total of 39 submitted forecasts, calls for a mean extent of 4.28 million km² for September, with minimal and maximal quartiles of 4.0 and 4.6 million km². In this regard, the dynamic method currently being used at the AWI yields a projected extent of 4.71 ± 0.15 million km², while the AWI’s statistical forecasting method predicts 3.65 ± 0.3 million km² for the September mean (see Figure 6). For descriptions of the two methods employed at the AWI (dynamic and statistical), please click here.
Climatological background conditions in the Arctic
Ice conditions in the Arctic are greatly influenced not only by the ocean temperature, but also by the atmospheric conditions. The new heat wave that dominated the weather in Europe in late July also made its way north, where it produced intensified melting of the Greenland Ice Sheet. In late July, temperatures on Greenland reached 10 °C at 925 hPa, and major regions of the Arctic reported temperatures that were 1 to 7 °C above the long-term average. At the same time, surface melting of up to 60% took place on the Greenland Ice Sheet (source: NSIDC). Despite a few fluctuations in the course of the month, generally speaking, temperatures in Greenland remained above the long-term average.
The extreme loss of sea ice in July 2019 was chiefly due to the warm conditions in the first half of the month. However, the remainder of the month was relatively cool in the East Siberian Sea and Laptev Sea, as well as the Barents Sea and Canadian Arctic Archipelago, where temperatures at 925 hPa were between 1 and 4 °C below the long-term average for 1981-2010 (Figure 7). These comparatively cool conditions were the result of a pronounced low-pressure system with atmospheric pressure values below the long-term average that formed over the East Siberian Sea, combined with a high-pressure system with values above the long-term average that spread from Greenland, across the Central Arctic, to the West Siberian Plain. This dipolar atmospheric pressure pattern pushed cold air southward, driving the ice toward the coast in the process (see Figure 8).
This situation marks a continuation of the extremely warm conditions in the Arctic since the beginning of the summer. As early as May, the circulation pattern in the Arctic greatly resembled a dipole, which is known to foster sea-ice melting in the summer (high pressure above the Arctic regions of North America, Greenland and the Beaufort Sea, and a distinct low-pressure system on the Eurasian side of the Arctic and over the Kara Sea). Otherwise the pattern only appears in winter, and is highly unusual for this time of year. Accordingly, the pattern this summer also differs considerably from those in the past three years, where low-pressure systems over the Central Arctic Ocean greatly influenced the nature of the summer. This pattern brings warm southern air to the Arctic via the Laptev Sea, where temperatures were also particularly high in May (see Figures 7 and 8). As Dr Monica Ionita-Scholz, a climatologist at the AWI, explains: “The North Atlantic has been in a negative phase of the North Atlantic Oscillation (NAO) since May 2019. In 2019 we saw the lowest NAO value for May in the last 70 years (-2.38). This broad-scale pattern of variability is conducive to accelerated sea-ice loss in general, while also intensifying the melting of the Greenland Ice Sheet.” The NAO is a circulation pattern in the North Atlantic shaped by the contrasting pressures between the Azores High in the south and the Iceland Low in the north, and especially determines the weather in the North Atlantic during the winter.
It remained relatively warm in the Arctic after July. In many areas, mean temperatures at 925 hPa were at least 3° C warmer than the long-term average for 1981-2010; further, temperatures in certain regions, like the Chukchi Sea and East Siberian Sea, climbed up to 5° C above the long-term average. Conditions were especially warm in Alaska, which saw new record highs (see Figure 7).
In the Beaufort Sea, the ice began melting nearly a month sooner than expected; this was also the case in Baffin Bay, on the western coast of Greenland. In the Central Arctic Ocean, the melting began roughly 20 days earlier than usual along the line of latitude that is home to the Laptev Sea, Lincoln Sea and Hudson Bay (source: NSIDC). The start of the melting season plays an important part in the formation of meltwater pools and ice break-up, since both processes result in more sunlight being absorbed by the topmost layer of the ocean, which in turn promotes sea-ice melting. Ocean surface temperatures along the coast of Alaska and the Chukchi Sea were up to 5 °C above the long-term average (see Figure 9).
The ‘rank maps’ of the atmospheric temperature at sea level, which are used to classify the warmest years over the Arctic, show that the summer of 2019 was an especially warm one, and that the highest atmospheric temperatures for the period 1948 – 2019 were reached in the Canadian sector / Baffin Bay in May; in the Siberian sector in June; and in major areas of the Central Arctic in May, June and July (see Figure 10).
For the next several weeks, the ongoing wildfires in broad expanses of the Arctic could also affect sea-ice melting. At the moment, more than three million hectares of forest are burning in Siberia, producing tremendous quantities of soot that will cover the otherwise highly reflective ice. Because of the soot, more sunlight will now be absorbed. Since early June, several hundred major fires have been identified, and the smoke from them has since been transported over Alaska, Greenland and Siberia, as well as regions covered by sea ice. An unusually large number of forest fires have also been observed in northern Canada and Alaska, and are far more intense than usual. An analysis of their Total Fire Radiative Power (TFRP) reveals a value that is 10 times higher than the mean value for a given day (see Figure 11; source: NSIDC). The TFRP is calculated on the basis of readings from the satellite MODIS (Moderate Resolution Imaging Spectroradiometer) and takes into account the thermal radiation (intensity of the fires) and the amount of smoke produced. According to the Copernicus Atmosphere Monitoring Service (CAMS), to date the wildfires have released as much carbon dioxide into the atmosphere as the total CO2 emissions produced by Sweden in a year (more than 50 megatonnes) – and more than all fires combined in the same month from 2003 to 2018 (source: CAMS).
In closing, we can say that the summer of 2019 has been characterised by unusual weather conditions in the Arctic, and that climate change is clearly progressing in the region. As Dr Lars Kaleschke, a sea-ice physicist at the AWI, explains: “Just how much sea ice is left after this remarkable melting season is something we won’t be able to precisely determine until autumn, with the help of satellite-based ice-thickness measurements. How far to the north the sea-ice margin has shifted will be an extremely important aspect for the launch of the MOSAiC drift experiment.”