Air-sea interactions during glacial Heinrich events

• Main Author: Krebs, Uta
• Format: Thesis
• Language: English
• Published: 2006
• Subjects: Indian; Pacific; North Atlantic; Sea ice
• Online Access: https://oceanrep.geomar.de/id/eprint/47181/; https://oceanrep.geomar.de/id/eprint/47181/1/krebs.pdf; https://macau.uni-kiel.de/receive/diss_mods_00001924

'Heinrich events' - massive iceberg discharges from Northern Hemisphere ice sheets during the last ice age - coincided with cold periods that were followed by abrupt warmings in the Northern Hemisphere. Climate reconstructions suggest that the associated freshwater pulses caused a temporary collapse of the Atlantic Meridional Overturning Circulation (AMOC) by stabilizing the stratification in the regions of deep water formation. In the present work a coupled atmosphere-ocean-sea ice model is employed under glacial boundary conditions to assess climate feedbacks after a simulated Heinrich event that lead to a fast recovery of the AMOC. Two main mechanisms have been identified. Initially, mixing and thermal processes weaken the stratification in the northern North Atlantic. Additionally, 300-400 years after the main collapse of the AMOC, the stratification is further destabilized by mean horizontal advection of anomalous saline waters within the subpolar gyre. In consequence the large-scale meridional overturning is reinitiated. The positive salinity anomaly originates from the tropical Atlantic and relies on air-sea coupling. Reduced poleward heat transport in the North Atlantic leads to a cooling north of the thermal equator. Due to advection of cold air and intensification of the northeasterly trade winds the Intertropical Convergence Zone is shifted southward and north equatorial precipitation is reduced. A dilution of the arising positive salinity anomaly is prevented because cross-equatorial oceanic surface flow is halted during the shut-down of the AMOC. Experiments with suppressed tropical air-sea coupling reveal that the recovery time of the AMOC is almost twice as long as in the coupled case. The impact of a shut-down of the AMOC on the Indian and Pacific Oceans can be decomposed into atmospheric and oceanic contributions. Temperature anomalies in the northern hemisphere are largely controlled by atmospheric teleconnections, whereas southern hemispheric ones mainly rely on ocean dynamical changes. ...