11:00 〜 11:15
[MIS15-08] Interannual variation of the Lützow-Holm Bay Land-fast Ice Extent
キーワード：リュツォ・ホルム湾、南極、定着氷、うねりによる海氷破壊、海面水温、10年規模変動
The multi-year land-fast ice in the Lützow-Holm Bay Antarctica is known to grow over a decade and then catastrophically break up and lose the multi-year ice. Another decade follows when the first-year ice repeatedly breaks up due to incoming swells. The cause of the breakup of the multi-year ice by swells was conjectured to be related to the increased proportion of the snow ice (Ushio et al. 2006). The material property of the snow ice is considered weaker than the sea ice (Toyota et al. 2016). Therefore, the land-fast ice in the Lützow-Holm Bay gradually weakens as its thickness increases. Another possible cause is related to the Sea Surface Temperature. The 2016 breakup event is associated with an anomalous SST, which is considered a part of climate variation and is correlated with the equatorial SST interannual variation (Aoki 2017).
An array of wave buoys observed the motion of the Lützow-Holm Bay Land-fast ice as a part of the Japan Antarctic Research Expedition (JARE). From 2022 to 2023, 23 wave buoys were deployed on the ice, of which 15 were on the land-fast ice (JARE64). The buoys registered a sequence of swell penetration, breakup, and sea ice drift events; swells gradually eroded the land-fast ice until all the fast ice was lost from the bay. The following year (JARE65), 21 wave buoys were placed on the land-fast ice, registering the incoming swells and drifts of the sea ice from the bay. In the past two years, a large part of the land-fast ice of the Lützow-Holm Bay was lost entirely during summertime.
AMSR2-derived sea ice concentration (sic) data was analyzed to quantify the area of land-fast ice. The method identifies the “multi-year ice” when the sic remains over a given threshold value throughout the season (from February to June). Each grid point is identified as an “open water area” if the SIC becomes lower than the SIC_threshold during the period. That means the grid points lost sea ice at least once in the season. Different threshold values were tried, and SIC_threshold=0.6 was selected to produce a 20-year long estimate of the “multi-year ice” extent, Figure 1. Together with the multi-year land-fast ice area (blue circles), the number of ramming operations is plotted from 1983 (bars).
Previous studies suggested that the number of ramming operations represents the land-fast ice thickness and material property changes, as the difficulty of ice breaking is strongly correlated to the thickness and the flexural strength of the sea ice. We therefore expected that the multi-year ice area, which is the reciprocal of the thin first-year ice area, would be highly correlated with the number of ramming operations. However, we have noticed that after 2016, despite the area of multi-year ice being relatively large, the number of ramming operations was small. This discrepancy may imply a possible weakening of the sea ice because of the increased SST in Antarctica. The abrupt change in 2016 may be related to a potential regime shift of the Antarctic sea ice extent (Kusahara et al. 2025).
Figure 1. The number of ramming operations from 1983 (30th JARE) to 2023 (65th JARE). The red bars denote the outbound number of ramming operations and the bule bars denote the inbound number of ramming operations. The vertical axis label on the left indicates the total number of ramming operations. The open blue circle indicates the are of multi-year ice that remined in the bay. The number of AMSR2 grid points are indicated in the right vertical axis.
An array of wave buoys observed the motion of the Lützow-Holm Bay Land-fast ice as a part of the Japan Antarctic Research Expedition (JARE). From 2022 to 2023, 23 wave buoys were deployed on the ice, of which 15 were on the land-fast ice (JARE64). The buoys registered a sequence of swell penetration, breakup, and sea ice drift events; swells gradually eroded the land-fast ice until all the fast ice was lost from the bay. The following year (JARE65), 21 wave buoys were placed on the land-fast ice, registering the incoming swells and drifts of the sea ice from the bay. In the past two years, a large part of the land-fast ice of the Lützow-Holm Bay was lost entirely during summertime.
AMSR2-derived sea ice concentration (sic) data was analyzed to quantify the area of land-fast ice. The method identifies the “multi-year ice” when the sic remains over a given threshold value throughout the season (from February to June). Each grid point is identified as an “open water area” if the SIC becomes lower than the SIC_threshold during the period. That means the grid points lost sea ice at least once in the season. Different threshold values were tried, and SIC_threshold=0.6 was selected to produce a 20-year long estimate of the “multi-year ice” extent, Figure 1. Together with the multi-year land-fast ice area (blue circles), the number of ramming operations is plotted from 1983 (bars).
Previous studies suggested that the number of ramming operations represents the land-fast ice thickness and material property changes, as the difficulty of ice breaking is strongly correlated to the thickness and the flexural strength of the sea ice. We therefore expected that the multi-year ice area, which is the reciprocal of the thin first-year ice area, would be highly correlated with the number of ramming operations. However, we have noticed that after 2016, despite the area of multi-year ice being relatively large, the number of ramming operations was small. This discrepancy may imply a possible weakening of the sea ice because of the increased SST in Antarctica. The abrupt change in 2016 may be related to a potential regime shift of the Antarctic sea ice extent (Kusahara et al. 2025).
Figure 1. The number of ramming operations from 1983 (30th JARE) to 2023 (65th JARE). The red bars denote the outbound number of ramming operations and the bule bars denote the inbound number of ramming operations. The vertical axis label on the left indicates the total number of ramming operations. The open blue circle indicates the are of multi-year ice that remined in the bay. The number of AMSR2 grid points are indicated in the right vertical axis.