南大洋における海洋フロントの南北シフト

Abstract

The Southern Ocean plays an important role in the global climate system both at present and in the geologic past. To resolve the causes and processes of atmospheric CO2 change, it is important to understand the mechanisms and processes of sub-systems in the Antarctic Cryosphere such as change of biological productivity, sea-surface temperature, surface water frontal system, sea ice distribution, and the Antarctic Ice Sheet during the glacial-interglacial climate cycle. A large number of float observations made recently suggest that mid-depth Southern Ocean temperatures rose 0.17°C between the 1950s and 1980s. The Southern Ocean is warming faster than the global oceans, and this is concentrated within the Antarctic Circumpolar Current (ACC). Warming is consistent with a poleward shift of the ACC, probably driven by long-term poleward shifts in the winds of the region, as represented by the southern annular mode. Changes to the extent of Antarctic sea ice are difficult to quantify for the pre-satellite observation era. However, a substantially larger set of proxy records based on whaling positions indicates that a larger southward shift of the summer sea ice edge occurred between the mid-1950s and early 1970s. In the glacial to interglacial cycle, ice-rafted debris (IRD) is an important proxy for reconstructing past iceberg discharges and sea ice expansions. However, it is necessary to specify the origin of IRD in the Southern Ocean, because IRD deposition on the pelagic seafloor is controlled not only by the dynamics of the Antarctic ice sheet but also by surface water conditions such as sea-surface temperature and oceanic front migrations. For example, several layers rich in volcanic tephra were deposited in the eastern Atlantic sector of the Southern Ocean. Deposition of the tephra-rich IRD layers was controlled by changes in sea-surface temperature and sea ice conditions in the Polar Frontal Zone of the South Atlantic, rather than Antarctic ice sheet dynamics. Thus, IRD deposition is a signal of the expansion of sea ice in the South Atlantic. According to IRD records, it seems that sea ice expansion events occurred suddenly in the Atlantic sector of the Southern Ocean during the last glacial period.