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In the largest calving event from the Amery Ice Shelf since 1963, which occurred almost a decade earlier than expected, an iceberg 1,636 square kilometers in size with an estimated weight of 315 billion tons, broke away from its glacier in September 2019.
The iceberg continues to be monitored due to the threat that an iceberg this size could pose to shipping channels. However, attention is turning to the cause of this event as global warming and atmospheric changes could lead to more such calving events.
In a study recently published in The Cryosphere, Dr. Diana Francis, Senior Research Scientist and Head of the Environmental and Geophysical Sciences (ENGEOS) lab from Khalifa University, along with Dr. Kyle Mattingly, Post-doctoral Associate, Rutgers University, Dr. Stef Lhermitte, Assistant Professor, Delft University of Technology; Dr. Marouane Temimi, Associate Professor, Stevens Institute; and Dr. Petra Heil, Senior Research Scientist, Australian Antarctic Division, describes the cause of this calving event, identifying cyclogenesis—the formation of cyclones—as a major factor. The team reported for the first time the formation of polar twin cyclones near Antarctica immediately prior to the event.
“Ice shelf instability is one of the main sources of uncertainty in Antarctica’s contribution to future sea level rise,” explained Dr. Francis. “Calving events play a crucial role in ice shelf weakening but remain unpredictable, and their governing processes are still poorly understood.”
Ice shelves are platforms of floating ice that form where the Antarctic ice sheet meets the ocean. The Amery Ice Shelf, which is one of the largest glacier drainage basins in the world, is located on the Eastern coast of Antarctica.
In December 2006, Australian scientists investigated enormous cracks that had been forming for over a decade at a rate of three to five metres a day in the Amery Ice Shelf. These fractures were feared to cause a 900-square-kilometer piece of the ice shelf to break off. The cracking was particularly concerning since the last recorded activity in this part of eastern Antarctica occurred over 50 years ago.
Although early studies predicted that the Amery Ice Shelf would not experience a major calving event until at least 2025, on 25 September 2019, iceberg D28 was calved.
“The rapid collapse of several Antarctic ice shelves observed recently and the near-instantaneous acceleration of land-ice discharge into the ocean that followed the collapse, demonstrate the sensitivity of the Antarctic cryosphere to recent warming,” explained Dr. Francis. “Large uncertainty remains regarding the response of ice shelves to the global rising temperatures and the resulting changes in the atmospheric circulation. That this D28 calving took us by surprise highlights the need for an improved understanding of the underlying processes of calving events and the role of atmospheric forcing in ice shelf weakening.”
Around the world, rising seas threaten cities and infrastructure along the coast. Higher sea levels also mean that deadly and destructive storm surges push further inland than before, and high-tide flooding becomes more likely. Global mean sea level has risen about nine inches since 1880, mostly due to meltwater from glaciers and ice sheets as temperatures rise.
Although much attention focuses on the melting ice caps, calving is the fastest way by which ice contributes to sea level rise. While ice shelves themselves are floating ice and therefore already displacing water, they act as a brake on the flow of the ice further inland. As the ice shelves thin when pieces break off, this restrictive force decreases.
Despite the importance and the implications of ice shelf calving, this phenomenon remains unpredictable and poorly understood. The Khalifa University team focused on the impact of extreme cyclone activity during this largest calving event since 1963. They investigated the development of explosive cyclones and their impact on sea ice and land ice conditions in this area given that changes in cyclone tracks, numbers, and intensity may have significant impacts on Antarctic ice.
“Weather systems such as cyclones resulting from the larger-scale air and wind circulation are the main driver of the observed trends in sea ice variability,” explained Dr. Francis. “Furthermore, cyclones and their associated atmospheric rivers can induce sea ice melt, ice-shelf surface melt, and significant sea ice drift by virtue of their anomalous moisture and heat transport to high latitudes and the strong surface winds they carry. Severe storms can generate energetic waves in the Southern Ocean capable of penetrating hundreds of kilometers into the sea ice-covered ocean but this sea ice cover acts as a buffer, reducing the impact of storms on ice shelves.”
The number and intensity of cyclones around Antarctica over the last few decades have increased as the storm tracks shift towards the pole under enhanced greenhouse gas concentrations. As the climate continues to warm, the intensity of more frequent cyclones is projected to increase.
An extreme situation is the formation of explosive cyclones, which are deeper and longer-lasting compared to ordinary cyclones. Worse, they are found to be more intense in the Southern Hemisphere than in the Northern Hemisphere, with the Amery Basin, where the Amery Ice Shelf is located, standing out as one of the three main regions for explosive cyclogenesis around Antarctica.
As the cyclones are directed towards the south pole, they are blocked at the Antarctic coast by the ridges to the east. This results in stationary cyclones over the same region for longer, which has a pronounced impact on the sea ice and waves. As the extent to which the Antarctic sea ice extends reduces and the number and severity of atmospheric events increases, this may result in even more impact on ice shelves from extreme cyclones.
The KU research team also noted the formation of twin cyclones, where mutually-interacting cyclones have twice the impact as a single cyclone.
“To our knowledge, the formation of explosively developing twin cyclones has only been observed and studied in the tropics, the mid-latitudes, and in the Arctic,” explained Dr. Francis. “But we reported the formation of polar twin cyclones near Antarctica during two consecutive events just days apart in September 2019.”
The researchers found that an extended period of strong cyclonic activity in September 2019 resulted in an exceptional period of strong easterly/north-easterly winds over the western side of the Amery Ice Shelf. This exceptional wind stress on the ice shelf generated strong waves in the region in front of it, where warm and moist air masses at the ice shelf front may have contributed to a decrease in sea ice concentration. The team found that the winds during the first twin cyclone event were exceptionally unusual compared to the record, while the winds during the second were strong but not extreme. This suggests that the first event had an important role in preconditioning the ice shelf front for breakoff, while the offshore winds in the second event triggered the calving by pushing the section of ice out from the shelf.
“We found that the cyclones had a large impact on the ice conditions because they were stationary,” explained Dr. Francis. “They subjected the ice to sustained stress and strain, weakening and exposing the ice shelf before strong winds pushed part of the shelf out to sea.”
The team asserts that important changes in the atmospheric circulation in the Southern Hemisphere need to be further investigated, with an urgent need to assess the impact of cyclones on the area.
“If extreme polar cyclones are to form and more frequently reach ice shelves due to climate change, their destructive effect may have important consequences,” explained Dr. Francis. “This needs to be accounted for in models used for sea level projections.”
9 December 2020