14:30 〜 14:45
[ACC26-04] The role of Southern warming on global ocean states and abrupt warming events during the last deglaciation
★Invited Papers
There were a number of abrupt climate changes associated with changes in Atlantic Meridional Overturning Circulation (AMOC) during the last glacial period (120 to 20 ka before present) and the last deglaciation (20 to 11 ka before present). Previous studies have elucidated that the AMOC has several distinct modes associated with sea ice extent and deep water formation in the North Atlantic, but the cause of triggering these AMOC mode shifts is still unclear. Recent climate modeling studies suggested the thermal condition in the Southern Ocean is important for the AMOC through the process of Antarctic bottom water formation and meridional density gradient in the North Atlantic.
In this study, we conduct sensitivity experiments of the last deglaciation to understand the role of the Southern thermal forcing. In the control experiment, the model was forced by changing orbital parameters and rising atmospheric CO2 based on ice core reconstructions (same as Obase and Abe-Ouchi 2019). The control experiment exhibits abrupt AMOC recovery around 14.7 ka BP, in good agreement with observations.
Next, we conducted sensitivity experiments in which the deglacial rise in CO2 radiative forcing was applied to a limited latitudinal band, with radiative forcing elsewhere held fixed at 18 ka levels. We find that only the experiments with rising radiative forcing in the southern high latitudes exhibit abrupt AMOC recovery. The results show that the rise in the Southern CO2, not the Northern CO2 is necessary for an abrupt AMOC increase, despite smaller SST increase in the North Atlantic. The analysis suggests that the abrupt changes in the AMOC is preconditioned with gradual density difference between the North Atlantic and the Southern Ocean, therefore Southern warming is more important than the Northern warming in the last deglaciation. We conclude that Southern hemisphere high-latitude warming is both a sufficient and a necessary condition for AMOC recovery at the Bølling onset at 14.7 ka before present.
In this study, we conduct sensitivity experiments of the last deglaciation to understand the role of the Southern thermal forcing. In the control experiment, the model was forced by changing orbital parameters and rising atmospheric CO2 based on ice core reconstructions (same as Obase and Abe-Ouchi 2019). The control experiment exhibits abrupt AMOC recovery around 14.7 ka BP, in good agreement with observations.
Next, we conducted sensitivity experiments in which the deglacial rise in CO2 radiative forcing was applied to a limited latitudinal band, with radiative forcing elsewhere held fixed at 18 ka levels. We find that only the experiments with rising radiative forcing in the southern high latitudes exhibit abrupt AMOC recovery. The results show that the rise in the Southern CO2, not the Northern CO2 is necessary for an abrupt AMOC increase, despite smaller SST increase in the North Atlantic. The analysis suggests that the abrupt changes in the AMOC is preconditioned with gradual density difference between the North Atlantic and the Southern Ocean, therefore Southern warming is more important than the Northern warming in the last deglaciation. We conclude that Southern hemisphere high-latitude warming is both a sufficient and a necessary condition for AMOC recovery at the Bølling onset at 14.7 ka before present.