10:00 AM - 10:15 AM
[MIS10-05] Swell penetration into a land-fast ice as a precursor to the breakup event
Keywords:Lutzow Holm bay, land-fast ice breakup, waves in ice
Land-fast ice in Lützow Holm Bay, Antarctica, is known to break up in Austral autumn. Past observations and records of Ice breaker Shirase suggest a semi-decadal change in the ice thickness of the land-fast ice. While the swell propagation into the bay was conjectured as a precursor to the land-fast ice breakup, no evidence was documented. To directly measure the incoming wave signatures and to associate those to the breakup events, 15 wave buoys were deployed on the land-fast ice on a grid of 5 by 3 array during the 64th Japan Antarctic Research Expedition. The buoys covered an area of about 100 km by 50 km slanted in the northwest direction around 16 degrees.
From March 2023, the land-fast ice started to break up and by May, a large part of the land-fast ice had drifted away from the bay. Even the multi-year ice reaching an ice thickness of over 3 meters has broken up. A preliminary report on the large breakup event was given at the Japan Geoscience Union Meeting 2023 (https://sj.jst.go.jp/news/202307/n0731-01k.html).
In 2022 April, the land-fast ice broke up and a large part of it drifted away from the Lützow Holm Bay, leaving the southern portion of it attached to the coast. From the SAR image taken in December 2022, one can observe a change in the surface signature possibly marking the location of the edge of the remained land-fast ice. In January and February 2023, we landed on the buoy locations north and south of the April2022-breakup-line and confirmed that the ice thickness abruptly increased from about 1.6 m to 3.1 m to the south. We, therefore, confirmed the presence of a multi-year ice in the southern part of the land-fast ice. Interestingly, while flying over the April2022-breakup-line in January and February 2023, we observed a large crack near the April2022-breakup-line.
Despite the presence of such cracks, the land-fast ice remained nearly motionless until March. The motion was rather slow and moved about 20 m to the northeast in about a month. Nevertheless, there were horizontal motions represented by distinct meridional (e.g. 5.6 hrs.) and zonal (e.g. 4 hrs. and 7.1 hrs.) oscillations. The causes of these high-frequency horizontal movement are not evident yet and are likely related to the seiches in Lützow Holm Bay (e.g. Nagano et al. 2010). On average, land-fast ice converged, and only in the fourth quadrant of the area covered by the 15 buoys, land-fast ice diverged. Near the northern edge of the land-fast ice, the surface of the sea ice seemed rough where ridges were formed. On the other hand, in the southern part, where the sea ice tended to diverge, the sea ice seemed flatter.
In addition to the 15 buoys on the land-fast ice, 6 buoys were deployed in the drift ice zone north of the land-fast ice. Except for the northernmost buoy that started drifting right after the deployment in February, other buoys remained immobile until they started to drift away in the middle of March when the wind system veered from Northerly to Southerly wind. The wind system tended to remain southerly until all the buoys on the land-fast ice drifted away at the end of April. The initiations of the drift were rather abrupt such that the large-scale deformation represented by the nominal strain did not have any anomalous increase before the drift events.
We consider that the large-scale forcings on the land-fast ice by the current and wind fields are crucial for the land-fast ice drift. However, we could not identify anomalous events that may have triggered the breakup. We therefore look into the swells as precursors to the breakup events. We have identified a few breakup events that were preceded by the penetration of swells. In this presentation, we further investigate other wave events that are not necessarily associated with drift but may have induced the creation of cracks. We focus on the wave attenuation rate that may change over time.
Figure 1: SAR mage obtained on Feb. 20th, 2023, depicting the ice fields in the Lützow Holm Bay. Sea ice thicknesses (m) are shown next to the buoy positions.
From March 2023, the land-fast ice started to break up and by May, a large part of the land-fast ice had drifted away from the bay. Even the multi-year ice reaching an ice thickness of over 3 meters has broken up. A preliminary report on the large breakup event was given at the Japan Geoscience Union Meeting 2023 (https://sj.jst.go.jp/news/202307/n0731-01k.html).
In 2022 April, the land-fast ice broke up and a large part of it drifted away from the Lützow Holm Bay, leaving the southern portion of it attached to the coast. From the SAR image taken in December 2022, one can observe a change in the surface signature possibly marking the location of the edge of the remained land-fast ice. In January and February 2023, we landed on the buoy locations north and south of the April2022-breakup-line and confirmed that the ice thickness abruptly increased from about 1.6 m to 3.1 m to the south. We, therefore, confirmed the presence of a multi-year ice in the southern part of the land-fast ice. Interestingly, while flying over the April2022-breakup-line in January and February 2023, we observed a large crack near the April2022-breakup-line.
Despite the presence of such cracks, the land-fast ice remained nearly motionless until March. The motion was rather slow and moved about 20 m to the northeast in about a month. Nevertheless, there were horizontal motions represented by distinct meridional (e.g. 5.6 hrs.) and zonal (e.g. 4 hrs. and 7.1 hrs.) oscillations. The causes of these high-frequency horizontal movement are not evident yet and are likely related to the seiches in Lützow Holm Bay (e.g. Nagano et al. 2010). On average, land-fast ice converged, and only in the fourth quadrant of the area covered by the 15 buoys, land-fast ice diverged. Near the northern edge of the land-fast ice, the surface of the sea ice seemed rough where ridges were formed. On the other hand, in the southern part, where the sea ice tended to diverge, the sea ice seemed flatter.
In addition to the 15 buoys on the land-fast ice, 6 buoys were deployed in the drift ice zone north of the land-fast ice. Except for the northernmost buoy that started drifting right after the deployment in February, other buoys remained immobile until they started to drift away in the middle of March when the wind system veered from Northerly to Southerly wind. The wind system tended to remain southerly until all the buoys on the land-fast ice drifted away at the end of April. The initiations of the drift were rather abrupt such that the large-scale deformation represented by the nominal strain did not have any anomalous increase before the drift events.
We consider that the large-scale forcings on the land-fast ice by the current and wind fields are crucial for the land-fast ice drift. However, we could not identify anomalous events that may have triggered the breakup. We therefore look into the swells as precursors to the breakup events. We have identified a few breakup events that were preceded by the penetration of swells. In this presentation, we further investigate other wave events that are not necessarily associated with drift but may have induced the creation of cracks. We focus on the wave attenuation rate that may change over time.
Figure 1: SAR mage obtained on Feb. 20th, 2023, depicting the ice fields in the Lützow Holm Bay. Sea ice thicknesses (m) are shown next to the buoy positions.