9:30 AM - 9:45 AM
[MIS20-03] Estimation of sea-ice melt amount and net freshwater flux by sea-ice in the Southern Ocean
Keywords:Southern Ocean, salinity profile, sea-ice melt, freshwater flux
Sea ice redistributes salt/freshwater via freezing, melting, and its transport, affecting the water mass formation. The Southern Oceanwith the largest seasonal ice zone serves the major water mass formation associated with ice production/melt. Intermediate water of the Southern Ocean, which is affected by sea-ice melt via Ekman pumping, has shown a prominent freshening for recent several decades. Increase in northward freshwater transport by sea ice and its melt is suggested as its cause (Haumann et al. 2016). This freshening enhances the stratification and hampers the mixing of deeper, warmer and carbon-rich waters into the surface layer. Thus the sea-ice melt and its variation potentially impact on the climate change. However, estimation of ice-melt amount is very limited, because of the complexity of the melting process and large uncertainties of ice thickness.
Recently in addition to the ship-based observations, Argo floats and biologging data are increasing rapidly. Under such condition, Pellichero et al. 2017 provided the melting amount of sea ice only from the observation for the first time. They indirectly estimated the melting amount based on the annual cycle of the mixed layer salt budget. Except for Pellichero’s mixed layer study, there have been no investigation on the estimation of the sea-ice melt amount from the observation. They treated the annual cycle of the mixed layer and thus needs the winter data which are limited. They provide only climatological map of ice melt amount. So far interannual variability of ice melt amount has not been understood.
This study estimates sea-ice melt utilizing spring salinity profiles from ship-based, Argo float, and elephant seals data, total of nearly 25000 data. The estimation is made by calculating the salinity deficit of the upper layer affected by sea-ice melt. The key point in the estimation is to find the top of winter water, which is nearly at the freezing point and not affected by warming and ice melting. We have developed an algorithm to detect the top of winter water automatically by combined use of potential temperature and salinity profiles.
Our results show that large sea-ice melt (~1.5 m) occurs in the western side of the three gyres of Weddell, Ross, and east of Kerguelen plateau, where sea-ice is drifted from the coastal area to offshore. Large melt also occurs around the Cape Darnley polynya. In our result, clear contrast is identified compared to Pellichero et al. (2017)’s result. Our method based on many spring data probably provide higher resolution of ice melt amount. Then we compare our result with sea-ice thickness observations. The average of sea-ice thickness derived from the satellite (ICESat) and ship-based (ASPeCt) observations is 79 cm and 78 cm, respectively, which is consistent with the average melt amount from our study, 77 cm. The total freshwater flux by sea-ice melt is calculated to be 17200 Gt/yr. This value is six times larger than that of the total glacial melt (~2900 Gt/yr): the basal melt plus calving/ iceberg melt (Rignot et al., 2013, England et al., 2020). Since 2004, when the Argo and bio-logging projects started, relatively large amount of ice melt data have been obtained, which will enable analysis of interannual variability of sea ice melt, which may be related with sea ice drift and water mass change.
Our ice melt data set was used to clarify the net freshwater flux by sea ice, by combining the ice production data derived from thin ice thickness algorithm by Nakata et al. (2021) and heat flux calculation. The distribution of the net freshwater flux by sea-ice shows clear contrast: large negative freshwater flux in the coastal regions, especially in coastal polynyas, and positive freshwater flux in the offshore regions. In addition, negative freshwater flux (production > melt) occurs along the sea-ice divergence zone.
Recently in addition to the ship-based observations, Argo floats and biologging data are increasing rapidly. Under such condition, Pellichero et al. 2017 provided the melting amount of sea ice only from the observation for the first time. They indirectly estimated the melting amount based on the annual cycle of the mixed layer salt budget. Except for Pellichero’s mixed layer study, there have been no investigation on the estimation of the sea-ice melt amount from the observation. They treated the annual cycle of the mixed layer and thus needs the winter data which are limited. They provide only climatological map of ice melt amount. So far interannual variability of ice melt amount has not been understood.
This study estimates sea-ice melt utilizing spring salinity profiles from ship-based, Argo float, and elephant seals data, total of nearly 25000 data. The estimation is made by calculating the salinity deficit of the upper layer affected by sea-ice melt. The key point in the estimation is to find the top of winter water, which is nearly at the freezing point and not affected by warming and ice melting. We have developed an algorithm to detect the top of winter water automatically by combined use of potential temperature and salinity profiles.
Our results show that large sea-ice melt (~1.5 m) occurs in the western side of the three gyres of Weddell, Ross, and east of Kerguelen plateau, where sea-ice is drifted from the coastal area to offshore. Large melt also occurs around the Cape Darnley polynya. In our result, clear contrast is identified compared to Pellichero et al. (2017)’s result. Our method based on many spring data probably provide higher resolution of ice melt amount. Then we compare our result with sea-ice thickness observations. The average of sea-ice thickness derived from the satellite (ICESat) and ship-based (ASPeCt) observations is 79 cm and 78 cm, respectively, which is consistent with the average melt amount from our study, 77 cm. The total freshwater flux by sea-ice melt is calculated to be 17200 Gt/yr. This value is six times larger than that of the total glacial melt (~2900 Gt/yr): the basal melt plus calving/ iceberg melt (Rignot et al., 2013, England et al., 2020). Since 2004, when the Argo and bio-logging projects started, relatively large amount of ice melt data have been obtained, which will enable analysis of interannual variability of sea ice melt, which may be related with sea ice drift and water mass change.
Our ice melt data set was used to clarify the net freshwater flux by sea ice, by combining the ice production data derived from thin ice thickness algorithm by Nakata et al. (2021) and heat flux calculation. The distribution of the net freshwater flux by sea-ice shows clear contrast: large negative freshwater flux in the coastal regions, especially in coastal polynyas, and positive freshwater flux in the offshore regions. In addition, negative freshwater flux (production > melt) occurs along the sea-ice divergence zone.