【気候科学】海氷の動きが引き起こす南大洋の塩分低下

Northward transport of sea ice from Antarctica can explain most of the observed freshening of the waters in the Southern Ocean over the past few decades, finds a study published in this week’s Nature. These results - despite uncertainties in the underlying data sources - have important consequences for future global climate studies, the authors suggest.

Salinity observations in the Southern Ocean over the last few decades have revealed a substantial and widespread freshening of the waters. However, the underlying causes have remained elusive.

F. Alexander Haumann and colleagues use satellite observations combined with sea-ice reconstructions in order to estimate that northward freshwater transport by sea ice increased by 20 ± 10 per cent between 1982 and 2008. The authors ascribe this trend to increasingly strong southerly winds over the Ross Sea. They then estimate that the additional freshwater input entails a freshening rate of -0.02 ± 0.01 grams per kilogram per decade in open ocean surface and intermediate waters, similar to the observed freshening rate. The authors warn that there are large uncertainties in the data sets used to compute these changes in salinity, and supplement their analyses with independent data from atmospheric analysis and a comparison to another previous regional study.

They propose that changes in northward sea-ice transport contribute to salinity changes by allowing freshwater to be removed from the surface ocean where sea ice forms along the coast of Antarctica and freshwater to be released where sea ice melts along the sea-ice edge in the open ocean. This process in turn affects the deep-to-surface ocean exchange of heat, carbon and nutrients, thereby influencing the global climate.