1:45 PM - 2:00 PM
[MIS08-12] Dominant frazil ice production in Antarctic coastal polynyas and its role on bottom water formation and material cycle
★Invited Papers
Keywords:frazil ice, Antarctic Bottom Water, coastal polynya, material cycle, sea ice production
Antarctic Bottom Water (AABW) occupies the abyssal layer of the global ocean and contributes to global overturning circulation. AABW originates as dense shelf water (DSW), which forms from brine rejection during sea-ice production. The fourth area of AABW formation was identified off the Cape Darnley polynya (CDP) (Ohshima et al., 2013). However, it remains unclear why and how high ice production leads to AABW formation. Under the Japanese Antarctic Research Expedition, two moorings were conducted inside the CDP in 2010-2011 throughout winter for the first time, which provides very valuable data set both for water properties and sea ice. We used a detection method of underwater frazil ice from the moored acoustic measurements developed by Ito et al. (2017) and the satellite microwave algorithm of active-frazil area detection developed by Nakata et al. (2019).
The most important finding of this study is the frequent occurrence of deep penetration of frazil ice down to 80 m or more in the CDP. As long as strong winds continue (typically >15 m s-1), underwater frazil ice formation persistently occurs and a mixture of frazil ice streaks and open water can be maintained at the surface without accumulation of heat-insulating thick ice. This creates an very efficient ice production system in the polynyas. The high ice-production in the nearshore combined with longer residence times creates high-salinity DSW. This DSW subducts into lower layer in the offshore shelf, descends down the shelf slope, induces compensating onshore-ward modified Circumpolar Deep Water with mixing to form new AABW.
Based on validation of the satellite algorithm for detecting active-frazil areas from our acoustic observations, we have developed a method in which deep-penetrating frazils are inferred from wind and satellite product information. Then we show the map of the mean occurrence rate of deep-penetrating frazil ice over the entire Southern Ocean. The map suggests that deep frazil is prominent particularly in the CDP. Sea ice production associated with deep frazil is also the largest at the CDP, which makes the CDP a source area of AABW formation among the Antarctic coastal polynyas. On the other hand, the map suggests that a deep frazil also occurs in other polynyas. Considering recent observations of frequently occurring supercooled water in Antarctic coastal waters, it is likely that deep-penetrating frazil ice occasionally occurs in other Antarctic coastal polynyas.
In addition to high ice production, deep-penetrating frazil ice potentially plays an important role in the transport of particulate matter. When it reaches the bottom or gets in contact with resuspended sediments, it can incorporate sediments or micronutrients such as iron, which are then transported by ice floes and released when the ice melts, possibly leading to high biological productivity. Alternatively, frazil ice could itself induce algal blooms as green frazil ice in major Antarctic polynyas. As such, dominant frazil ice could potentially contribute to high biological productivity in the Southern Ocean.
The most important finding of this study is the frequent occurrence of deep penetration of frazil ice down to 80 m or more in the CDP. As long as strong winds continue (typically >15 m s-1), underwater frazil ice formation persistently occurs and a mixture of frazil ice streaks and open water can be maintained at the surface without accumulation of heat-insulating thick ice. This creates an very efficient ice production system in the polynyas. The high ice-production in the nearshore combined with longer residence times creates high-salinity DSW. This DSW subducts into lower layer in the offshore shelf, descends down the shelf slope, induces compensating onshore-ward modified Circumpolar Deep Water with mixing to form new AABW.
Based on validation of the satellite algorithm for detecting active-frazil areas from our acoustic observations, we have developed a method in which deep-penetrating frazils are inferred from wind and satellite product information. Then we show the map of the mean occurrence rate of deep-penetrating frazil ice over the entire Southern Ocean. The map suggests that deep frazil is prominent particularly in the CDP. Sea ice production associated with deep frazil is also the largest at the CDP, which makes the CDP a source area of AABW formation among the Antarctic coastal polynyas. On the other hand, the map suggests that a deep frazil also occurs in other polynyas. Considering recent observations of frequently occurring supercooled water in Antarctic coastal waters, it is likely that deep-penetrating frazil ice occasionally occurs in other Antarctic coastal polynyas.
In addition to high ice production, deep-penetrating frazil ice potentially plays an important role in the transport of particulate matter. When it reaches the bottom or gets in contact with resuspended sediments, it can incorporate sediments or micronutrients such as iron, which are then transported by ice floes and released when the ice melts, possibly leading to high biological productivity. Alternatively, frazil ice could itself induce algal blooms as green frazil ice in major Antarctic polynyas. As such, dominant frazil ice could potentially contribute to high biological productivity in the Southern Ocean.