Magnitude of Changes in the Atlantic Overturning Circulation Associated with Dansgaard-Oeschger Cycles During Marine Isotope Stage 3

Abstract:

Pronounced millennial-scale climate variability during Marine Isotope Stage 3 (MIS3) is considered to be linked to changes in the state of the Atlantic meridional overturning circulation (AMOC). The stability of the AMOC to North Atlantic freshwater perturbations is studied using a comprehensive coupled climate model (CCSM3) under MIS3 boundary conditions. An AMOC stability diagram constructed from a series of equilibrium freshwater hosing/extraction experiments reveals a highly nonlinear dependence of AMOC strength on freshwater forcing with a MIS3 baseline state that is remarkably unstable with respect to minor negative or positive freshwater perturbations. The global climate signal associated with a change in AMOC strength is consistent with a transition from an interstadial (strong AMOC) to a stadial (weak AMOC) state including an annual mean surface air temperature drop of >8 K in central Greenland. We further construct a global spatial fingerprint of oceanic temperature anomalies in response to AMOC changes under MIS3 boundary conditions. Highest sensitivity to AMOC changes, especially in summer, is found in northeastern North Atlantic sea surface temperature (SST), but a characteristic temperature fingerprint is also found at subsurface levels. After testing significance of the linear SST-AMOC regressions, the model results are combined with paleo-SST records from the North Atlantic in order to estimate the magnitude of millennial-scale Dansgaard-Oeschger AMOC variations during MIS3. The results suggest a difference in AMOC strength between interstadial and (non-Heinrich) stadial states of 10.2 ± 1.3 Sv (1σ).