Without the current positive phase of the Arctic dipole anomaly, sea ice loss in the Arctic since 2007 would have been much greater, says a new study published in Science. Now, apparently, the transition to the negative phase is imminent, with greater “atlantification” of the Arctic Ocean and accelerated sea ice retreat.
Reliable continuous satellite data on Arctic sea ice extent have been available since the late 1970s, and a downward trend in ice cover has been observed ever since: The minimum summer sea ice area has shrunk from about 7 million square kilometers then to about 4 million square kilometers today. But since the then record minimum of 4.17 million square kilometers observed in 2007, this trend has virtually stopped, except for another record minimum in 2012 of 3.41 million square kilometers.
In their new study, an international research team led by the University of Alaska Fairbanks explains why the trend of sea ice loss in the Arctic Ocean has not continued in recent years. Based on their results, the researchers predict that there will be another accelerated decline in sea ice. The cause is the so-called Arctic dipole, a phenomenon in the atmosphere that reverses in a roughly 15-year cycle.
The Arctic environment responds to the Arctic Dipole in a variety of ways, and the study helps to understand “atlantification”, the influence of North Atlantic water on Arctic Ocean climate.
Currently, the Arctic Dipole has been in its positive phase since 2007, according to the research team. This means that over the Canadian Arctic there is a high pressure area with clockwise winds and over the Siberian Arctic there is a low pressure area with counterclockwise winds.
This wind pattern drives ocean currents in the upper water layers, affecting regional air temperatures, heat exchange between the atmosphere, ice, and ocean, sea ice drift and sea ice export, and ecology throughout the year.
Depending on which phase the Arctic dipole is in, the flow conditions also change. The researchers therefore emphasize in their study that “Water exchanges between the Nordic seas and the Arctic Ocean are critically important for the state of the Arctic climate system”.
The team analyzed ocean responses to the wind pattern that has prevailed since 2007 and found that the flow from the warmer North Atlantic into the Arctic Ocean through the Fram Strait east of Greenland is decreasing, while the flow from the Atlantic into the Barents Sea north of Norway and western Russia is increasing. This resulted in more warming of the Barents Sea between 2007 and 2021. The authors refer to these alternating changes in Fram Strait and the Barents Sea as a “switchgear mechanism” caused by Arctic dipole regimes.
The researchers believe the cause of the slowed sea ice loss in the years between 2007 and 2021 is the Siberian low pressure area, with its winds driving freshwater from Siberian rivers into the Canadian sector of the Arctic Ocean during the current positive phase of the Arctic Dipole. This increased the thickness of the surface freshwater layer, which became too thick and stable to mix with the heavy salt water below. The warmer salt water did not come into contact with the sea ice thanks to the freshwater layer, which would otherwise have caused the ice to melt from below. Freshwater flowing westward thus helped slow the overall loss of Arctic sea ice compared to the 1992-2006 period.
In addition to the effects on sea ice extent, there are also multiple ecological effects of the physical changes. For example, the authors describe that Arctic vegetation responds markedly differently to the two phases of the Arctic Dipole. During the negative phase between 1992 and 2006, photosynthetic productivity increased and vegetation increased in biomass. By contrast, it lost biomass in the current positive phase.
Moreover, the switchgear mechanism has “profound” implications for ocean life. For example, conditions for subarctic boreal species may be better in the eastern Eurasian Basin than in its western part, which recent observations confirm, according to the study.
The authors see indicators of a reversal of the Arctic dipole, which may be abrupt, as it was in 2007. “We are beyond the peak of the currently positive Arctic dipole regime, and at any moment it could switch back again,” Igor Polyakov, a professor in the College of Natural Science and Mathematics at the University of Alaska Fairbanks, said in a news release. “This could have significant climatological repercussions, including a potentially faster pace of sea-ice loss across the entire Arctic and sub-Arctic climate systems.”
Julia Hager, PolarJournal
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