Warm Winds Bringing Peril to Antarctic Ice Shelf

This Nov. 10, 2016 aerial photo released by NASA, shows a rift in the Antarctic Peninsula’s Larsen C ice shelf. A vast iceberg with twice the volume of Lake Erie broke off in 2017. The iceberg named A68 weighed 1.12 trillion tons. (John Sonntag/NASA via AP)

(CN) – Warm dry air currents passing over the Antarctic Peninsula’s central mountain range are causing an unusual spike in late-season surface melting on the continent’s fourth largest ice shelf, according to a new University of Maryland research study published Thursday in Geophysical Research Letters.

The Larsen C ice shelf on Earth’s coldest continent is vulnerable to global climate change, according to researchers who are concerned about a three-year spike in foehn-induced melting late in the melt season.  Foehn winds occur when air passes over mountains before reaching land where it can pick up heat through moisture loss, changes in pressure, turbulence or warming by the sun.

Rajashree Tri Datta, a faculty assistant at UMD’s Earth System Science Interdisciplinary Center and the lead author of the research paper, said “the Larsen C ice shelf is of particular interest because it’s among the most vulnerable in Antarctica.”

Larsen C is located just south of the former Larsen B shelf that collapsed in 2002 and the Larsen A shelf that dropped in 1995. Both ice shelves dramatically broke off into the sea rather than gradually receding at a slow and steady glacial pace.

“Because it’s a floating ice shelf, a breakup of Larsen C wouldn’t directly lead to a rise in global mean sea level,” Datta said. “However, the ice shelf does brace against the flow of the glaciers that feed it. So if Larsen C goes, some of these glaciers will be free to accelerate their rate of flow and melt, which will result in a rise in global sea level.”

The researchers used two different methods to quantify patterns of foehn-induced melt from climate model outputs that correspond to real-world satellite observations and weather station data. Brilliant blue pools of melt water can be seen collecting atop the Larsen C ice shelf in images created by NASA’s Earth Observatory using Landsat data from the U.S. Geological Survey.

As foehn winds race down the colder eastern slopes of the Antarctic Peninsula’s central mountain range, they can raise air temperatures by as much as 30 degrees Fahrenheit, producing localized bursts of snowmelt. According to Datta, these winds exert their greatest effects at the bases of glacial valleys. Here, where the feet of the glaciers adjoin the Larsen C ice shelf, foehn winds could destabilize some of the most fragile and critical structures in the system.

“Three years doesn’t make a trend. But it’s definitely unusual that we are seeing enhanced foehn winds and associated melting in late summer and early autumn,” Datta said. “It’s unusual that we’re seeing increased foehn-induced melt in consecutive years – especially so late in the melt season, when the winds are stronger but the temperatures are usually cooling down. This is when we expect melting to end and the surface to be replenished with snow.”

Enhanced surface melting causes water to trickle into the underlying layers of firn – uncompacted, porous snow – in the upper layers of the ice sheet. This water then refreezes, causing the normally porous, dry firn layers to become denser. Eventually, the firn layers can become too dense for water to enter, leading to a buildup of liquid water atop the ice shelf.

The greater study, spanning 35 years from 1982 to 2017, quantifies how much of this additional melting can be ascribed to warm, dry foehn winds and how they have begun to restructure the snowpack on the Larsen C ice shelf. If this pattern continues, it could significantly alter the density and stability of Larsen C, potentially putting it at further risk to suffer the same fate as the Larsen A and B shelves.

“With enhanced densification, the ice enters the next warm season with a very different structure. Our modeling results show that, with less open space for the surface water to filter into, runoff increases year after year,” said Datta. “The dominant theory suggests that enhanced densification led to the fracture of the Larsen A and B shelves. Despite an overall decrease in peak summer melt over the last few years, episodic melting late in the melt season could have an outsized impact on the density of the Larsen C ice shelf.”

Marco Tedesco, Xavier Fettweis, Cecile Agosta, Stef Lhermitte, Jan Lenaerts and Nander Wever also contributed to the study with additional funding from the National Science Foundation.

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