[M-IS14] Paleoclimatology and paleoceanography

[MIS14-02] Role of thermal component for driving the millennial-scale oscillations of the Atlantic meridional overturning circulation

★Invited Papers

*Yuta Kuniyoshi¹, Ayako Abe-Ouchi¹, Wing-Le Chan¹, Sam Sherriff-Tadano² (1.The University of Tokyo, 2.University of The Ryukyus)

Keywords:Dansgaard-Oeschger event, AMOC, AOGCM

During the mid-glacial periods, abrupt warming/cooling events (Dansgaard-Oeschger (D-O) events) occurred with a millennial time scale, and were closely related to abrupt changes in the Atlantic meridional overturning circulation (AMOC) and its heat transport. Several studies using coupled atmosphere-ocean models including our MIROC4m AOGCM have recently shown millennial-scale self-sustained oscillations with no change in external forcings (Kuniyoshi et al. 2022GRL). These studies propose several internal feedbacks, i.e,, hydrological changes in salt oscillator, deep decoupling and thermal oscillator, which involves sea ice change and subsurface temperature change in the North Atlantic, and stochastic changes in surface wind. These feedbacks, however, are proposed only from analysis of the self-sustained oscillations in their AOGCMs, and further investigation on the role of each feedback has not been carried out yet. Here we developed a method to evaluate each feedback involving hydrology, heat, and wind. Specifically, we modified the coupler in the MIROC4m AOGCM so that each of the freshwater flux, heat flux, and momentum flux passed from the atmosphere to the sea ice-ocean component every three hours is replaced with a specified climatology. In this study, using this modified coupler, we conduct three kinds of partially coupled experiments by fixing freshwater, heat, and winds separately. These experiments are branched from the simulation of self-sustained oscillation with the fully coupled MIROC4m (Kuniyoshi et al. 2022GRL). The climatology of each flux used in the partially coupled experiments is taken from the range of amplitude of the self-sustained oscillation in the fully coupled MIROC4m. This means the time-averaged values of the weak AMOC period and of the strong AMOC period, and the intermediate values between the two states are used as the climatology. As a result, while millennial-scale climate oscillations are observed when we fix freshwater flux and wind stress, no oscillation occurs when we fix heat flux. This suggests that the AMOC oscillations could be driven even without changes in hydrology and surface wind stress and that changes in sea surface heat flux could be the main driving factor of the oscillations. The influence of surface heat flux on the self-sustained oscillations of the AMOC will be further investigated and the role of the sea ice - heat flux feedback will be discussed.