On the stability of the Atlantic Meridional Overturning Circulation and its climate impact during Late Neogene

This thesis focused on the Atlantic Meridional Overturning Circulation (AMOC) stability and global climate responses of an altered AMOC in the late Neogene by performing freshwater hosing/extraction computer simulations, using both intermediate complexity climate models and state-of-the-art coupled climate models. Two time slices were studied: 1) 38 kya (kilo years ago), during Heinrich event 4, and 2) around 4-5 million years ago, when the Panamanian Seaway finally gradually closed. Chapter 2 demonstrated a very sensible 38 kya global climate as response to external freshwater forcing in the North Atlantic and the AMOC is in a monostable mode during cold stadials. What is more, a threshold behaviour was observed in both Greenland temperature and Northern Hemisphere ice cover with regard to the AMOC maximum in the simulations, which indicates a system transition between a mild interstadial state and a cold stadial state. The global imprint of the AMOC change was also investigated in detail. Applying the Analysis of Variances (ANOVA) statistical method, several locations with high sensitivity to the AMOC strength change were captured at the surface, e.g. the Southern Indian Ocean and the North Atlantic. It was estimated the AMOC experienced a deduction of 10.2±1.8 Sv of its circulation strength from warm interstadials to cold stadials and this decrease in vigour was stronger from interstadial states to Heinrich stadials, with a larger estimated uncertainty (Chapter 3). By model inter-comparison, chapter 4 emphasized that the closure of the Panamanian Seaway could lead to an enhancement of the AMOC and also causes a shoaling of the equatorial Pacific thermocline. The change in the thermocline depth in the Equator is then preconditioned to the present Pacific cold tongue state. Wind stress feedback was able to amplify this process.