Atmospheric rivers led to ice shelf collapse

Climate scientists have long debated what mechanisms trigger the collapse of ice shelves. Most recently, it was discovered that melting processes from below by warmer ocean water play a significant role. Now, a team of European researchers found that so-called atmospheric rivers – narrow bands of currents with high humidity – triggered the collapse of the Larsen-A and Larsen-B ice shelves on the Antarctic Peninsula. The study, which was published in the journal Nature Communications Earth & Environmentrevealed that this could also threaten the remaining Larsen C Ice Shelf, the largest on the Antarctic Peninsula.

Atmospheric rivers originate in the mid-subtropical latitudes of the Pacific Ocean, where temperatures are high and the air contains a corresponding amount of moisture. They form when a large stationary area of high pressure meets a cyclone. From the confluence of the two systems, a narrow stream of moist air flows toward Antarctica.

If such atmospheric rivers meet the mountains of the Antarctic Peninsula, precipitation occurs on their windward side and warm foehn winds flow down the mountain flanks on the leeward side, which – depending on the moisture content of the air flow – can melt the ice shelf on the surface above ground. The meltwater flows into cracks in the ice, refreezes, and causes the cracks to widen. If this process (called “hydrofracturing”) repeats itself several times, the ice shelf can break up.
In addition, the winds can drive the sea ice away from the ice shelf edge, which in turn makes the ice shelf more exposed to wave action that further destabilizes it.

The team of scientists, led by Jonathan Wille, a climatologist and meteorologist from the University of Grenoble in Saint-Martin-d’Hères, France, believes that these extreme weather events most likely led to the collapse of the Larsen-A and Larsen-B ice shelves in 1995 and 2002, respectively. Analyzing weather and climate data as well as satellite imagery from 2000 to 2020, the researchers found that 13 of 21 major calving events were triggered by atmospheric rivers that were particularly high in moisture and each flowed over the Antarctic Peninsula no more than five days before large ice blocks broke off.

According to the study, this scenario could also threaten Larsen-C, which is still relatively intact despite breaking off from the giant A68 iceberg in 2017. “The only reason the melting hasn’t been as strong so far is because [das Schelfeis] is farther south compared to the others, so it’s colder,” Dr. Will told The New York Times. However, as global warming continues, researchers also expect more intense atmospheric currents. “The Larsen-C will now be at risk from the same processes.”

If the Larsen C Ice Shelf were to collapse due to a large calving event, this would also have an impact on global sea level. While the ice shelf itself would not contribute to a rise because it is already floating on the ocean. The glacier ice, however, which is currently still supported and held back by the ice shelf, would then be able to flow unchecked into the Weddell Sea. All of West Antarctica, broadly speaking the region around the peninsula, harbors so much ice that sea level would rise six meters if it melted.

Just recently, an ice shelf collapsed for the first time in East Antarctica – just days after such a warm atmospheric river flowed over the region. It is not yet known if there is a connection between the two events.

Julia Hager, PolarJournal

Link to the study: Wille, J.D., Favier, V., Jourdain, N.C. et al. Intense atmospheric rivers can weaken ice shelf stability at the Antarctic Peninsula. Commun Earth Environ 3, 90 (2022). https://doi.org/10.1038/s43247-022-00422-9

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