5:15 PM - 7:15 PM
[MIS15-P05] On the melting land-fast ice near the Showa Station in January 2025
Keywords:JARE66, Showa Station, land-fast ice, sea ice melting, current, heat flux
In the activities of the Japanese Antarctic Research Expedition (JARE), transportation of supplies by the icebreaker Shirase is critically important. For the organized and planned operations, accurate understanding and prediction of sea ice conditions near the Showa Station is required. This presentation will introduce the results of the sea ice and ocean observations conducted from January 2 to January 29 during JARE-66, in conjunction with satellite observation data as a preliminary report. During JARE-66, active sea ice melting was observed around Showa Station and in the sea ice covered Kitanoura area located north of the base. This research on the sea ice melting observed over the Kitanoura area is expected to provide insights into the melting of sea ice.
The icebreaker Shirase reached a stable land-fast ice approximately 300 meters away from Showa Station on December 31, 2024 and started the transportation afterward. Sea ice conditions in the Ongul Strait including the period from landing to departure of the Shirase, were monitored using Landsat satellite images. Ongul Strait is the area where Shirase cruised over to reach Kitanoura. In late December, a thin ice region was observed off the western coast of Ongul Island where the Showa Station located on. The thin-ice region was extending north-south. From early January, open water areas began to appear, expanding over time. By late January, open water area in the Ongul Strait had advanced from the south. By early February, stable ice south of Ongul Island began to drift, and the open water area in the strait continued to expand. Thus, the series of satellite images suggested a gradual northward expansion of open water area from the south.
On the other hand, as part of the JARE-66 summer team, field observations of sea ice and ocean conditions were conducted near the Shirase landing site in the northern bay. The observation site was chosen not to affect the supply transport and the icebreaker Shirase’s navigation. The water depth was about 37 meters. The ice thickness at the observation point was 130 cm on January 10, but after recording 130 cm on January 15, it significantly decreased, reaching 116 cm on January 20 and 99.5 cm on January 29. The snow thickness at the observation site never exceeded 10 cm, and the surface was white, suggesting high albedo. On the other hand, nearby sea ice showed the developing of pools of meltwater, indicating active melting during the field observation.
Electromagnetic flow meters were installed at depths of 3 meters and 23 meters below the surface to measure tidal currents as well as ocean currents. The associated temperature sensor also provides temporal variation of the seawater temperature.
Measurements were conducted during both spring and neap tide periods. The tidal flow amplitude at both depths was less than 10 cm/s. The upper layer measurements also showed signals likely derived from meteorological disturbances. Water temperature in the upper layers was as low as -1.4°C at 1/10 depth, but it then began to rise. Between January 18 and 19, it rapidly increased from -1.0°C to around 0.4°C, and it continued to rise until it reached 1°C. However, after a strong wind event exceeding 20 m/s on January 24, the water temperature dropped back to around -0.5°C. Since the observation point was beneath the sea ice, direct vertical mixing from wind stress is unlikely. Therefore, the rapid temperature decrease is likely due to the intrusion of vertically mixed seawater from the open water surface.
Based on these measurements, we examined whether the observed reduction in sea ice thickness was caused by thermal forcing from the atmosphere (melting from the top) or from the ocean (melting from the bottom). Atmospheric heat flux to the sea ice was estimated from ERA5 reanalysis data, while heat flux from the ocean to the sea ice was estimated using a bulk method with the observational results described above. The results suggested that oceanic melting at the bottom may have as significant an impact as atmospheric forcing. Uncertainties in the numerical models and bulk method however need to be considered to reach the conclusion, particularly for albedo estimation and the representation of the thermal boundary layer beneath the sea ice.
Furthermore, the continuous CTD observations suggested the occurrence of sudden heat influx beneath the sea ice on January 19. Electromagnetic flow meter data also indicated a sudden increase in northward flow velocity on January 25. These events may contribute to the bottom melting of the sea ice, and to clarify the mechanism, detailed numerical modeling of the flow field near Ongul Island and in the upper water layers is needed
The icebreaker Shirase reached a stable land-fast ice approximately 300 meters away from Showa Station on December 31, 2024 and started the transportation afterward. Sea ice conditions in the Ongul Strait including the period from landing to departure of the Shirase, were monitored using Landsat satellite images. Ongul Strait is the area where Shirase cruised over to reach Kitanoura. In late December, a thin ice region was observed off the western coast of Ongul Island where the Showa Station located on. The thin-ice region was extending north-south. From early January, open water areas began to appear, expanding over time. By late January, open water area in the Ongul Strait had advanced from the south. By early February, stable ice south of Ongul Island began to drift, and the open water area in the strait continued to expand. Thus, the series of satellite images suggested a gradual northward expansion of open water area from the south.
On the other hand, as part of the JARE-66 summer team, field observations of sea ice and ocean conditions were conducted near the Shirase landing site in the northern bay. The observation site was chosen not to affect the supply transport and the icebreaker Shirase’s navigation. The water depth was about 37 meters. The ice thickness at the observation point was 130 cm on January 10, but after recording 130 cm on January 15, it significantly decreased, reaching 116 cm on January 20 and 99.5 cm on January 29. The snow thickness at the observation site never exceeded 10 cm, and the surface was white, suggesting high albedo. On the other hand, nearby sea ice showed the developing of pools of meltwater, indicating active melting during the field observation.
Electromagnetic flow meters were installed at depths of 3 meters and 23 meters below the surface to measure tidal currents as well as ocean currents. The associated temperature sensor also provides temporal variation of the seawater temperature.
Measurements were conducted during both spring and neap tide periods. The tidal flow amplitude at both depths was less than 10 cm/s. The upper layer measurements also showed signals likely derived from meteorological disturbances. Water temperature in the upper layers was as low as -1.4°C at 1/10 depth, but it then began to rise. Between January 18 and 19, it rapidly increased from -1.0°C to around 0.4°C, and it continued to rise until it reached 1°C. However, after a strong wind event exceeding 20 m/s on January 24, the water temperature dropped back to around -0.5°C. Since the observation point was beneath the sea ice, direct vertical mixing from wind stress is unlikely. Therefore, the rapid temperature decrease is likely due to the intrusion of vertically mixed seawater from the open water surface.
Based on these measurements, we examined whether the observed reduction in sea ice thickness was caused by thermal forcing from the atmosphere (melting from the top) or from the ocean (melting from the bottom). Atmospheric heat flux to the sea ice was estimated from ERA5 reanalysis data, while heat flux from the ocean to the sea ice was estimated using a bulk method with the observational results described above. The results suggested that oceanic melting at the bottom may have as significant an impact as atmospheric forcing. Uncertainties in the numerical models and bulk method however need to be considered to reach the conclusion, particularly for albedo estimation and the representation of the thermal boundary layer beneath the sea ice.
Furthermore, the continuous CTD observations suggested the occurrence of sudden heat influx beneath the sea ice on January 19. Electromagnetic flow meter data also indicated a sudden increase in northward flow velocity on January 25. These events may contribute to the bottom melting of the sea ice, and to clarify the mechanism, detailed numerical modeling of the flow field near Ongul Island and in the upper water layers is needed