Takahito Mitsui, Niklas Boers

Given the likely bistability of the Atlantic Meridional Overturning Circulation (AMOC) and its recently inferred weakening, it is important to investigate the capability of identifying robust precursor signals for a possible future AMOC collapse as well as its collapses in the past. Dansgaard-Oeschger (DO) events, manifested most clearly as abrupt Northern-Atlantic temperature jumps during glacial conditions, likely reflect past switches between strong and weak AMOC modes. In general, the variance and the autocorrelation with a small lag increase in a system approaching a co-dimension one bifurcation point. Some previous studies find such statistical precursor signals for the DO warming transitions associated with a strengthening AMOC. On the other hand, statistical precursor signals for the abrupt DO cooling transitions, presumably associated with AMOC transitions from the strong to the weak mode, have not been identified. Here we identify robust and statistically significant precursor signals for several DO cooling transitions in Greenland ice core records. The important source of the statistical precursor signals stems from so-called rebound event, humps in the temperature observed at the end of interstadial, some decades to centuries prior to the transition. We propose several dynamical mechanisms that give rise to such rebound events and statistical precursor signals.