17:15 〜 19:15
[MIS15-P03] The Climatology of the Cosmonaut Polynya and Its Formation Mechanism Inferred from a Mixed Layer Model
キーワード：ポリニヤ、海氷、南大洋、混合層モデル
A polynya is an area of thin ice or open water that exists within a sea-ice-covered region. There are two types of polynyas: latent heat polynyas, which mainly form in coastal areas when sea ice is blown offshore by wind, and sensible heat polynyas, which primarily occur in the open ocean due to the upwelling of warm water that suppresses sea ice formation and melts existing ice. Among the sensible heat polynyas, the Weddell Polynya of 1974–1976 is the largest and most prominent one, with a reoccurrence in 2016–2017 (Campbell et al., 2019). For this polynya, it has been suggested that deep convection occurred, with heat supplied from the intermediate/deep layers to prevent the ice formation. In turn, this deep convection has altered the properties of intermediate/deep waters, potentially influencing the thermohaline circulation.
The Cosmonaut Polynya is also considered a large-scale sensible heat polynya, although open water was not maintained throughout the year (Comiso and Gordon, 1996). The occurrence area and timing of the Cosmonaut Polynya vary from year to year, and its formation mechanism remains poorly understood. Furthermore, it is still uncertain whether deep convection occurs in association with the Cosmonaut Polynya. The purpose of this study is to provide a climatological analysis of the Cosmonaut Polynya and elucidate its formation mechanism using the mixed layer model of Kraus and Turner (1967), which assumes that entrainment and mixed-layer deepening are driven by wind-induced turbulence and cooling-induced potential energy. We created a climatology of the Cosmonaut Polynya's occurrence area and period based on 40 years of SSM/I ice concentration data. We found that the high-occurrence area is confined to 41°–47°E and 64°–65.5°S. The occurrence period ranges from June to September, with the highest frequency observed in July. According to the climatological dataset of water temperature and salinity compiled by Mensah and Ohshima (2023), the region where the Cosmonaut Polynya frequently occurs is characterized by a thin mixed layer.
First, we conducted climatological experiments using the mixed layer model. The initial conditions were set using the climatology of temperature and salinity in March, when surface cooling begins, while the climatology of wind speed and heat flux (heat loss) from ERA5 data were used as forcing variables. The calculations continued until the mixed layer temperature reached the freezing point. The model results indicate that freezing is significantly delayed in the area where the polynya frequently occurs. Next, we used individual temperature and salinity observations from March as initial profiles and applied the wind speed and heat loss from the corresponding observed years. The model results suggest a tendency for delayed freezing in the areas where the polynya frequently occurs. Interestingly, in approximately 20% of the cases, deep convection occurred, with heat supplied from the intermediate/deep layers, preventing freezing throughout the year. A particular focus was placed on the case of 2019, when a prominent polynya occurred and a larger number of hydrographic observations were conducted. The results of the mixed layer model indicate that the area of non-freezing cases exactly coincides with the polynya occurrence area in 2019. In the area of non-freezing cases, anomalously high temperature and salinity existed just beneath the mixed layer.
This study suggests that the occurrence of the Cosmonaut Polynya is primarily attributed to the thin mixed layer and the presence of high-temperature, high-salinity water beneath it. Additionally, it is likely that deep convection occasionally occurs in conjunction with polynya formation.
The Cosmonaut Polynya is also considered a large-scale sensible heat polynya, although open water was not maintained throughout the year (Comiso and Gordon, 1996). The occurrence area and timing of the Cosmonaut Polynya vary from year to year, and its formation mechanism remains poorly understood. Furthermore, it is still uncertain whether deep convection occurs in association with the Cosmonaut Polynya. The purpose of this study is to provide a climatological analysis of the Cosmonaut Polynya and elucidate its formation mechanism using the mixed layer model of Kraus and Turner (1967), which assumes that entrainment and mixed-layer deepening are driven by wind-induced turbulence and cooling-induced potential energy. We created a climatology of the Cosmonaut Polynya's occurrence area and period based on 40 years of SSM/I ice concentration data. We found that the high-occurrence area is confined to 41°–47°E and 64°–65.5°S. The occurrence period ranges from June to September, with the highest frequency observed in July. According to the climatological dataset of water temperature and salinity compiled by Mensah and Ohshima (2023), the region where the Cosmonaut Polynya frequently occurs is characterized by a thin mixed layer.
First, we conducted climatological experiments using the mixed layer model. The initial conditions were set using the climatology of temperature and salinity in March, when surface cooling begins, while the climatology of wind speed and heat flux (heat loss) from ERA5 data were used as forcing variables. The calculations continued until the mixed layer temperature reached the freezing point. The model results indicate that freezing is significantly delayed in the area where the polynya frequently occurs. Next, we used individual temperature and salinity observations from March as initial profiles and applied the wind speed and heat loss from the corresponding observed years. The model results suggest a tendency for delayed freezing in the areas where the polynya frequently occurs. Interestingly, in approximately 20% of the cases, deep convection occurred, with heat supplied from the intermediate/deep layers, preventing freezing throughout the year. A particular focus was placed on the case of 2019, when a prominent polynya occurred and a larger number of hydrographic observations were conducted. The results of the mixed layer model indicate that the area of non-freezing cases exactly coincides with the polynya occurrence area in 2019. In the area of non-freezing cases, anomalously high temperature and salinity existed just beneath the mixed layer.
This study suggests that the occurrence of the Cosmonaut Polynya is primarily attributed to the thin mixed layer and the presence of high-temperature, high-salinity water beneath it. Additionally, it is likely that deep convection occasionally occurs in conjunction with polynya formation.