A Scripps Institution of Oceanography, UC San Diego-led research team found evidence that ocean-wave energy generated thousands of miles away in the North Pacific Ocean vibrates ice shelves in Antarctica, which can expand existing fractures that in turn may play a role in their disintegration.
Scripps oceanographer Peter Bromirski and colleagues reported on the data they collected from a 34-station network of seismometers distributed across Antarctica’s Ross Ice Shelf in 2014-2015. In the National Science Foundation-funded project, the team is hoping to better understand the characteristics of seismic signals generated by ocean surface wave impacts, with the spatial variability of signal strength affected by propagation through zones of less competent ice. These vibrations could trigger the collapse of weakened ice shelves, a process that is difficult to observe since collapses usually happen over time periods too short to be adequately captured by infrequent satellite imagery, and what triggers collapse events is yet unexplained.
“The seismic survey studying the vibrations of the Ross Ice Shelf (RIS) in response to wave impacts will provide on-ice information on the structure and strength of the RIS, giving baseline “state-of-health” ice shelf measurements that can be used to identify the magnitude of changes in its integrity over time,” said Bromirski, the project’s principal investigator.
The study, “Ross ice shelf vibrations,” appeared Sept. 16 in the online version of the American Geophysical Union journal Geophysical Research Letters.
The project will be expanded this field season to include 13 geodetic GPS stations co-located with selected seismic stations, including five stations near the shelf front. These GPS stations will give continuous measurements during the austral summer, with one station operating year-round. This study investigates shorter-term changes than the research by Scripps glaciologists published earlier in 2015 that revealed ice loss as much as 18 percent in other Antarctic ice shelves over the past two decades. Combined with an upcoming analysis of atmospheric conditions over Antarctica, these studies will provide a comprehensive view of physical processes that could lead to substantial sea-level rise in all global oceans.
Ice shelves are slabs of ice that extend from land over the ocean like a half-cover on a jacuzzi. Though their disintegration does not directly cause sea level to rise, the buttressing effect they have on the grounded ice sheet is removed and the flow of land ice into the ocean accelerates. The Ross Ice Shelf is the largest ice shelf in Antarctica, measuring 487,000 square kilometers (188,000 square miles), an area roughly the size of Texas, and restrains a sector of the grounded West Antarctic Ice Sheet that would contribute as much as three meters of sea-level rise.
The seismic data showed that impacts of infragravity (very long wavelength ocean waves) and ocean-swell induced vibrations in the ice that can be felt 100 kilometers (62 miles) away from the front of the Ross Ice Shelf. Much of the ocean wave energy originated in the North Pacific Ocean, and caused higher amplitude vibrations than waves generated locally in the Southern Ocean.
Bromirski said the dominance of the long-distance wave energy is still not understood. Nor do the preliminary data explain whether the signals recorded by the seismic stations were generated at the shelf front or closer to the stations at the base of the ice shelf by wave energy penetrating the sub-shelf water cavity. The array will continue gathering data for 24 months, giving observations of two full annual cycles of change. A team of Scripps researchers will recover the first full year of data in October and November 2015.
Institutions participating in the study include Woods Hole Oceanographic Institution, Washington University in St. Louis, Colorado State University, and Penn State University.
– Robert Monroe