Wer hat Schuld am kürzlichen Einbruch des antarktischen Meereises? ENSO, SAM und IOD!

Thema heute: Das antarktische Meereis. Mit dem ging es in den letzten Jahren kräftig bergab, nachdem es zwischen 1980-2015 kräftig bergauf ging (rote Kurve in der untenstehenden Abbildung).

Abbildung: Entwicklung des Mereises in der Arktis und Antarkstis. Stand: 8.2.2020. Quelle: Climate4You.

Was steckt hinter diesem seltsamen Verlauf? Stuecker et al. 2017 erklären, dass der Absturz des antarktischen Meereises mit dem El Nino 2015/16 sowie dem SAM-Ozeanzyklus zusammenhängt. SAM steht für Southern Annular Mode, ein wichtiger Zyklus für die südliche Hemisphäre. Hier der Abstract:

Conditions leading to the unprecedented low Antarctic sea ice extent during the 2016 austral spring season

The 2016 austral spring was characterized by the lowest Southern Hemisphere (SH) sea ice extent seen in the satellite record (1979 to present) and coincided with anomalously warm surface waters surrounding most of Antarctica. We show that two distinct processes contributed to this event: First, the extreme El Niño event peaking in December–February 2015/2016 contributed to pronounced extratropical SH sea surface temperature and sea ice extent anomalies in the eastern Ross, Amundsen, and Bellingshausen Seas that persisted in part until the following 2016 austral spring. Second, internal unforced atmospheric variability of the Southern Annular Mode promoted the exceptional low sea ice extent in November–December 2016. These results suggest that a combination of tropically forced and internal SH atmospheric variability contributed to the unprecedented sea ice decline during the 2016 austral spring, on top of a background of slow changes expected from greenhouse gas and ozone forcing.

Hier die dazugehörige Pressemitteilung der University of Washington (via Science Daily):

Record-low 2016 Antarctic sea ice due to ‘perfect storm’ of tropical, polar conditions

While winter sea ice in the Arctic is declining so dramatically that ships can now navigate those waters without any icebreaker escort, the scene in the Southern Hemisphere is very different. Sea ice area around Antarctica has actually increased slightly in winter — that is, until last year.

A dramatic drop in Antarctic sea ice almost a year ago, during the Southern Hemisphere spring, brought its maximum area down to its lowest level in 40 years of record keeping. Ocean temperatures were also unusually warm. This exceptional, sudden nosedive in Antarctica differs from the long-term decline in the Northern Hemisphere. A new University of Washington study shows that the lack of Antarctic sea ice in 2016 was in part due to a unique one-two punch from atmospheric conditions both in the tropical Pacific Ocean and around the South Pole. The study was published Aug. 24 in Geophysical Research Letters.

„This combination of factors, all these things coming together in a single year, was basically the ‚perfect storm,‘ for Antarctic sea ice,“ said corresponding author Malte Stuecker, a UW postdoctoral researcher in atmospheric sciences. „While we expect a slow decline in the future from global warming, we don’t expect such a rapid decline in a single year to happen very often.“

The area of sea ice around Antarctica at its peak in late 2016 was 2 million square kilometers (about 800,000 square miles) less than the average from the satellite record. Statistically, this is three standard deviations away from the average — an event that would be expected to occur randomly just once every 300 years. The record low was not predicted by climate scientists, so UW researchers looked at the bigger picture in ocean and atmospheric data to explain why it happened.

The previous year, 2015-16, had a very strong El Niño in the tropical Pacific Ocean. Nicknamed the „Godzilla El Nino,“ the event was similar to other monster El Niños in 1982-83 and 1997-98. Unlike the 1997-98 event, however, it was only followed by a relatively weak La Niña in 2016.

Far away from the tropics, the tropical El Niño pattern creates a series of high- and low-pressure zones that cause unusually warm ocean temperatures in Antarctica’s eastern Ross, Amundsen and Bellingshausen seas. But in 2016, when no strong La Niña materialized, researchers found that these unusually warm surface pools lingered longer than usual and affected freeze-up of seawater the following season. „I’ve spent many years working on tropical climate and El Niño, and it amazes me to see its far-reaching impacts,“ Stuecker said.

Meanwhile, observations show that the winds circling Antarctica were unusually weak in 2016, meaning they did not push sea ice away from the Antarctic coast to make room for the formation of new ice. This affected ice formation around much of the Southern Ocean. „This was a really rare combination of events, something that we have never seen before in the observations,“ Stuecker said.

The researchers analyzed 13,000 years of climate model simulations to study how these unique conditions would affect the sea ice. Taken together, the El Niño pattern and Southern Ocean winds explain about two-thirds of the 2016 decline. The rest may be due to unusually big storms, which a previous paper suggested had broken up ice floes. Scientists predict Antarctica’s ocean will be one of the last places on Earth to experience global warming. Eventually the Southern Ocean’s surface will begin to warm, however, and then sea ice there will begin its more long-term decline.

„Our best estimate of the Antarctic sea ice turnaround point is sometime in the next decade, but with high uncertainty because the climate signal is small compared to the large variations that can occur from one year to the next,“ said co-author Cecilia Bitz, a UW professor of atmospheric sciences. Stuecker noted that this type of big, rare weather event is useful to help understand the physics behind sea ice formation, and to learn how best to explain the observations.

„For understanding the climate system we must combine the atmosphere, ocean and ice, but we must focus on more than a specific region,“ Stuecker said. „If we want to understand sea ice in Antarctica, we cannot just zoom in locally — we really have to take a global perspective.“

Auch Schlosser et al. 2018 sehen hier den SAM-Zyklus am Werk, der besonders tiefe Werte annahm:

Atmospheric influences on the anomalous 2016 Antarctic sea ice decay

In contrast to the Arctic, where total sea ice extent (SIE) has been decreasing for the last three decades, Antarctic SIE has shown a small, but significant, increase during the same time period. However, in 2016, an unusually early onset of the melt season was observed; the maximum Antarctic SIE was already reached as early as August rather than the end of September, and was followed by a rapid decrease. The decay was particularly strong in November, when Antarctic SIE exhibited a negative anomaly (compared to the 1979–2015 average) of approximately 2 million km2. ECMWF Interim reanalysis data showed that the early onset of the melt and the rapid decrease in sea ice area (SIA) and SIE were associated with atmospheric flow patterns related to a positive zonal wave number three (ZW3) index, i.e., synoptic situations leading to strong meridional flow and anomalously strong southward heat advection in the regions of strongest sea ice decline. A persistently positive ZW3 index from May to August suggests that SIE decrease was preconditioned by SIA decrease. In particular, in the first third of November northerly flow conditions in the Weddell Sea and the Western Pacific triggered accelerated sea ice decay, which was continued in the following weeks due to positive feedback effects, leading to the unusually low November SIE. In 2016, the monthly mean Southern Annular Mode (SAM) index reached its second lowest November value since the beginning of the satellite observations. A better spatial and temporal coverage of reliable ice thickness data is needed to assess the change in ice mass rather than ice area.

Auch eine Studie von Wang et al. 2019 sieht natürliche Kräfte als Ursache für den abrupten Rückgang des antarktischen Meereises. Beteiligt sein soll auch der Indische Ozean Dipol (IOD), ein weiterer wichtiger Ozeanzyklus mit Klimarelevanz. Lesenswert ist ein Artikel in SBS-News zur Studie:

Why Antarctica’s shrinking sea ice could be down to natural causes

Research has found that natural events could be to blame for Antarctica’s sea ice reaching record lows.

Sea ice cover in Antarctica shrank rapidly to a record low in late 2016 and has remained well below average. But what’s behind this continuing dramatic melting and low ice cover? Research published earlier this month suggests that a combination of natural variability in the atmosphere and ocean were to blame, although human-induced climate change may also play a role.

[…]

The next clue was in records broken far away from Antarctica. In the spring of 2016, sea surface temperatures and rainfall in the tropical eastern Indian Ocean were at record highs. This was in association with a strongly negative Indian Ocean Dipole (IOD) event, which brought warmer waters to the northwest of Australia.
While IOD events influence rainfall in south-eastern Australia, we found (using both statistical analysis and climate model experiments) that it promoted a pattern in the winds over the Southern Ocean that was particularly conducive to decreasing sea ice.
These surface winds blowing from the north not only pushed the sea ice back towards the Antarctic continent, they were also warmer, helping to melt the sea ice.
These northerly winds almost perfectly matched the main regions where sea ice declined.

Weiterlesen bei SBS-News

Soweit zum aktuellen antarktischen Meereisminimum. Hieraus lassen sich natürlich auch allgemeine Klima-Regeln ableiten. Doddrige & Marshall 2017 beschreiben den Zusammenhang zwischen antarktischem Meereis und SAM-Ozeanzyklus:

Modulation of the Seasonal Cycle of Antarctic Sea Ice Extent Related to the Southern Annular Mode

Through analysis of remotely sensed sea surface temperature (SST) and sea ice concentration data, we investigate the impact of winds related to the Southern Annular Mode (SAM) on sea ice extent around Antarctica. We show that positive SAM anomalies in the austral summer are associated with anomalously cold SSTs that persist and lead to anomalous ice growth in the following autumn, while negative SAM anomalies precede warm SSTs and a reduction in sea ice extent during autumn. The largest effect occurs in April, when a unit change in the detrended summertime SAM is followed by a 1.8±0.6 ×105 km2 change in detrended sea ice extent. We find no evidence that sea ice extent anomalies related to the summertime SAM affect the wintertime sea ice extent maximum. Our analysis shows that the wind anomalies related to the negative SAM during the 2016/2017 austral summer contributed to the record minimum Antarctic sea ice extent observed in March 2017.

Auch Schemm 2018 und Singh et al. 2019 beleuchten die natürliche Variabilität des antarktischen Meereises. Noch kurz vor dem enormen Rückgang des Eises behaupteten Ridley & Hewitt 2014, dass ein Meereisschrumpfen in der Antarktis unumkehrbar sei. Ob die beiden heute immer noch an dieses düstere Szenario glauben, angesichts der Erkenntnisfortschritte im Zusammenhang mit ENSO, SAM und IOD.

Eine Arbeit von Eayrs et al. 2019 erinnert daran, dass Klimamodelle die hochvariable Entwicklung des antarktischen Meereises derzeit nicht reproduzieren können. Epic fail, sozusagen. Auszug aus dem Abstract:

Climate models fail to accurately reproduce mean Antarctic sea ice extent and overestimate its year‐to‐year variability, but they tend to capture the pattern and timing of the Antarctic seasonal cycle.

Im antarktischen Meereis gibt es Löcher, sogenannte Polynja. Der Spiegel berichtete 2017 über ihre Erforschung.

Genug antarktisches Meereis für heute. Es gibt noch viel zu forschen, soviel ist klar.