[M-IS30_29AM1] 古気候・古海洋変動

[MIS30-29] 氷期の急激な気候変動における大西洋深層循環の熱的閾値の役割

*岡 顕¹、阿部 彩子¹、横山 祐典¹、川村 賢二²、羽角 博康¹ (1.東京大学大気海洋研究所、2.国立極地研究所)

キーワード：大西洋深層循環, 氷期, DOイベント

Abrupt climate changes known as Dansgaard-Oeschger events (DO events) took place frequently during glacial climate. Geological evidences support the idea that changes of the Atlantic meridional overturning circulation (AMOC) are related to these events, but question on what triggers the AMOC changes remains unsolved. Although most studies have regarded freshwater flux from melting ice sheet as a cause of the AMOC changes, we recently identified the existence of the thermal threshold of the AMOC during glacial climate. Here, from the results of numerical simulations about the glacial AMOC, we report that the thermal threshold of the AMOC serves as a triggering mechanism of DO events. We investigated the structure of the thermal threshold in glacial climate by conducting ocean general circulation model simulations under various thermal conditions in which degrees of sea surface cooling are systematically changed separately or simultaneously in northern and southern hemispheres. The results suggest that the threshold is located near the condition under which the climate is slightly warmer than the coldest glacial conditions. We also found that the amplitude of AMOC changes in crossing the threshold depends on thermal conditions in northern and southern hemispheres. The most prominent threshold is identified where the southern hemisphere is somewhat warmer than the coldest glacial conditions. It is also demonstrated that gradual warming in the southern hemisphere from the colder glacial climate leads to crossing this threshold and can cause significant strengthening of AMOC. Our results indicate that the thermal threshold could be a triggering mechanism of DO events, especially for those accompanying the gradual warming of southern hemisphere before their abrupt warming in northern hemisphere.