Sea Ice and Freshwater Ice Assessments
2019 Sea Ice
Aaron Letterly, Jeff Key, Rick Thoman
6 January 2019; updated 18 January, 7 March, 10 July 2019, 15 May 2020
Ice-Free Conditions in the Bering Sea During 2019
Even during a period in which record-breaking sea ice minima are the new normal, it’s rare that ice freezes or melts the same way in back-to-back years. The surface energy budget of sea ice, which ultimately determines whether ice in an area is growing or thinning, is affected by surface temperature, snow cover on top of the ice, cloud conditions overhead, the temperature of the ocean below it, wind speeds, and many other factors. This complex system, combined with land/ocean geography and the vagaries of the weather, impose unique conditions on ice areas that contribute to ice growth or loss.
In January 2019, sea ice extent in the Bering Sea was near the 1981-2010 median, yet by March 1st, the ice extent decreased to the lowest in the satellite record. The thin ice of the region, which normally increases in area until April, experienced dramatic decreases which were far beyond the normal fluctuations due to wind- and wave-caused changes. A large high pressure system over Canada advected warm air into the region that was 10°C higher than normal for February, which resulted in the rapid drop in sea ice area. The anomalous conditions persisted through early March and resulted in ice-free conditions as far north as the Seward Peninsula.
According to the National Snow and Ice Data Center’s interactive Arctic sea ice chart, 2019 and 2020 had very similar hemispheric total ice extent (Figure 1), but the narrative of sea ice near the Bering Strait was quite different. Alaskan sea ice in early 2020 was unremarkable, remaining near average extent through the end of February. The day-to-day differences in sea ice concentration can be seen in the sea ice concentration product derived from Advanced Microwave Scanning Radiometer 2 (AMSR2) imagery. The data in Figure 2 show Alaskan sea ice concentration from 1 January to 5 May during 2019 and 2020. The ice loss in the Bering Sea during 2019 was so abrupt that the ice edge can be seen retreating hundreds of miles in a matter of a few days in early March. In contrast, the sea ice in 2020 remained close to its January spatial extent until early April.
The resulting changes in sea ice from season to season affect the local ecosystem as well as economic activity. As reported in the 2018 Sea Ice Assessment, western Alaskan communities without the coastal sea ice buffer often experience severe flooding and coastal erosion by increased wave action. The briny waters that are a result of sea ice formation were greatly reduced, with disastrous results for native north Bering species.
Fig. 1. Arctic sea ice extent as of 5 May 2020, along with daily ice extent data for 2019 and +/- 2 standard deviations of the 1981-2010 ice extent mean in gray. 2019 and 2020 both tracked similar hemispheric extents from January through April, but ice conditions in Arctic seas were dramatically different between the years.
Fig. 2. The animation shows sea ice concentration centered over Alaska for 2019 (left) and 2020 (right) between January and early May. Ice extent and concentration were very similar in January and February, but warm temperatures caused substantial melting and ice edge retreat in the beginning of March 2019. Ice concentration and extent in 2020, however, did not begin to deteriorate until mid-April.
Looking back, the winter of 2017-18 produced unprecedented low sea ice coverage in the Bering Sea. A number of western Alaskan communities battled the all-too-familiar coastal flooding during the autumn and into the winter, culminating with unprecedented flooding at Little Diomede during late February. Maritime subsistence harvest activities were modified with increased financial costs and public safety risks - or altogether abandoned. Beginning in late spring, seabirds and subadult seals were emaciated or in poor body condition. The late winter 2018 ice extent was significantly lower than any reconstructed ice extent records since 1850. Summer came and revealed profound changes in the northern Bering Sea marine ecosystem due to the lack of the thermal barrier formed by the briny waters that are created by sea ice formation. This dense salty water, which can remain liquid to -2°C, historically separated two distinct marine ecosystems (northern and southern Bering Sea). With limited sea ice formation in the northern Bering Sea, this barrier was almost entirely removed, allowing the larger predatory commercially, viable southern fishes to surge north.
Autumn 2018 started with very warm ocean temperatures in the southern Chukchi and Bering Seas and, as a result, sea ice was again slow to form. However, seasonably cold temperatures and frequent north winds during December 2018 and early January 2019 allowed for rapid ice growth and optimism for more normal spring sea ice conditions. But it was not to be: starting in late January the weather changed to persistent southerly storm conditions, with 15 separate storm systems affecting the northern Bering Sea region through February. This “parade of storms” decimated the existing sea ice by melting or pushing the ice north through the Bering Strait and into the in the southern Chukchi Sea (Figure 3). The impacts to coastal communities are immediate, and people are left to change their food-gathering strategies to a novel open water winter situation in order to provide for the nutritional, cultural, and economic needs of their families.
Overall, the February 2018 sea ice extent in the Bering Sea was the lowest record in the satellite era (since 1979), just 42% of the 1981-2010 average. Sea ice extent during February 2019 is the second lowest documented, at 56% of average. Sea ice extent in Alaska during March 2019 is not off to a good start, as there is already much less sea ice than at the same time in 2018.
Fig. 3. Sea ice concentration on 1 March 2018 (left) and 1 March 2019 (right). (Courtesy of University of Bremen and @AlaskaWx)
2019 Arctic Ice Concentration
Arctic-wide, the 2019-2020 sea ice year had low annual extents, with near-record low totals for November and December (NSIDC). The exceptionally warm 2019 summer led to open water north of much of the Alaskan, Canadian, and Siberian Arctic. Vast areas of open water persisted into September and October, when sea ice extent reached its lowest total since 2015. The ring of open water around Arctic ice is visible in the average September 2019 sea ice concentration from APP-x (Figure 4, left). Such a feature lasting into September and October is very uncommon, as shown in the ice concentration anomaly plot (Figure 4, right). Ice concentration east of Greenland and north of Svalbard, however, was higher than normal. These local increases in sea ice do little to buck the trend of decreasing minimum ice extent in the fall months.
Fig. 4. The figure (left) shows September 2019 sea ice concentration from the APP-x data record. The ice concentration anomaly compared to the 1982-2013 average (right) shows drastically reduced sea ice concentration in the Western Arctic in 2019, even though sea ice concentration near Svalbard and the Central Arctic was slightly above average.
Record Minimum Antarctic Sea Ice Extent for Early January and June
On 1 January 2019, the sea ice area around the Antarctic continent was the smallest January area since the satellite record began in 1978. Figure 5 shows time series of ice extent from two sources. The sea ice area was 30,044 sq km (11,600 sq mi) below the previous record low for 1 January, which was set in 2017. Figure 6 shows the sea ice extent from AMSR2 on 1 January 2018 and 1 January 2019. The largest differences are in the Davis Sea (~75°E longitude) and the Ross Sea (~175°W). The melt season will continue into February. By February the minimum Antarctic sea ice extent was no longer a record. However, in June it was the lowest mean monthly extent for that month (Figure 5, middle and right), surpassing 2002 and 2017.
Fig. 5. Antarctic sea ice extent through early January (left) and through July 2019 (middle and right). Left and right: NSIDC; middle: JAXA/NiPR/ADS.
Fig. 6. Antarctic sea ice extent on 1 January 2018 (left) and 1 January 2019 (right). (NOAA)
Antarctic sea ice cover is highly variable. For example, it reached a record high in 2014 and a record low in 2016 (Figure 7). Why is the Antarctic sea ice extent so low? There is some discussion that suggests it's a combination of natural variability in the atmosphere and ocean, with a possible role of human-induced changes in the climate system. For the 2016 record minimum, it appears that a strongly negative Indian Ocean Dipole (IOD) event promoted a wind pattern such that surface winds from the north pushed the sea ice back towards the Antarctic continent and also brought additional heat to the region, enhancing ice melt. That 2016 pattern may have a longer-term impact that has led to the current low sea ice extent event.
Fig. 7. Annual cycle of Antarctic sea ice extent in 2014 and 2016 with the 1981-2010 median. (NSIDC's Charctic tool)
2019 Antarctic Ice Concentration
Antarctic sea ice extent in 2019 climbed out of a record low in June to their tenth-lowest in July (NSIDC). Sea ice growth is less constrained in the open water surrounding the Antarctic continent, and this high variability (compared to the Arctic) is apparent over the satellite record. The transition from austral winter to summer led to very low 2019 sea ice extents which tracked the 2017 record minimum. The similarities of the December 2019 average sea ice concentration around Antarctica to December 2016 (one month from the record low) can be seen in Figure 8. The decrease in sea ice happened relatively quickly, as the October 2019 was very close to median ice extent. Longer-term trends in Antarctic sea ice are much less clear than in the Arctic, often due to the high seasonal variability of ice extent as seen in the 2019 Antarctic.
Fig. 8. Similarities in sea ice concentration between the December 2016 record minimum (left) and December 2019 (right) in the Southern Hemisphere from APP-x. Total sea ice extents during these time periods were within 1% of one another throughout December.