[M-IS14] 古気候・古海洋変動

[MIS14-13] Bistability of AMOC and Millennial scale climate variability; dependence on background climate condition in the past, present and future

*阿部 彩子¹、陳 永利¹、シェリフ多田野 サム²、國吉 優太¹、三ツ井 孝仁³、小長谷 貴志⁴、岡 顕¹ (1.東京大学大気海洋研究所、2.琉球大学、3.順天堂大学、4.JAMSTEC)

Glacial periods were punctuated by abrupt millennial scale climate changes and have a strong link to the Atlantic Meridional overturning circulation (AMOC) changes. Although paleoclimate is important to understand the present and future, how the bistability and tipping points at present and future are related to the glacial millennial scale climate change is not clear. Here we present several hysteresis diagram of AMOC and climate and compare them with the cases of millennial scale climate variability with our Atmospheric Ocean coupled GCM, MIROC4m. A series of long transient experiments (> 10, 000 years) were performed systematically with different steady glacial conditions (CO2 level, obliquity, precession, meltwater, ice sheet size), to study the dependence of the sweet spot of millennial scale variability on the background climate and summarize the results as phase diagrams. We also show the hysteresis diagram of our model by adding anomalous freshwater flux in the North Atlantic in the model little by little. We chose the model version which we simulate LGM AMOC weaker and shallower than the AMOC under Pre-Industrial condition. We show that a reasonable sweet-spot of intrinsic oscillation exists when the Northern Hemisphere ice sheets exist even without freshwater perturbation. In the sweet spot, self-sustained oscillation with bipolar seesaw pattern and shift between interstadial and stadial occur, with interval between abrupt events ranging from 1000 years to more than 5000 years depending on the background condition, while an abrupt shift from stadial to interstadial mode occurs in about 100 years. We show the cases of CO2 from 155 ppm up to 840ppm and discuss the hysteresis diagram and the oscillatory mode depending on CO2 level as well as the existence of glacial ice sheets.

co-author Wing-Le Chan, Sam Sherriff-Tadano, Takahito Mitsui, Takashi Obase, Yuta Kuniyoshi, Akira Oka and Christo Buizert