Beyond the Tipping Point: Temporary Resilience of the Atlantic Meridional Overturning Circulation

• Main Author: Haskins, Rosalind, Kathleen
• Format: Thesis
• Language: English
• Published: University of Southampton 2020
• Subjects: Arctic; Bering Strait; Climate change; North Atlantic
• Online Access: https://eprints.soton.ac.uk/437855/; https://eprints.soton.ac.uk/437855/1/Haskins_Rosalind_PhD_Thesis_Jan_2020.pdf

The Atlantic meridional overturning circulation (AMOC) is projected to weaken due to anthropogenic climate change, partially due to ice melt freshening the North Atlantic Ocean. In order to successfully mitigate climate change it is important to consider the reversibility of temporary forcing and explore longerterm changes to the ocean state. We apply freshwater forcing to the North Atlantic and Arctic regions for various durations in global climate models (GCMs) to understand the impacts on North Atlantic density and AMOC strength. Firstly, the processes and timescales of the recovery phase were explored by considering the roles of salinity and temperature in AMOC recovery following a weakening. The behaviour of the AMOC was well reconstructed by applying ‘rotated geostrophy’ to meridional density gradient profiles between 50°N and 30°S. This makes it possible to determine the role of ocean and surface fluxes in the North and South Atlantic. Changes at 50°N dominate the weakening and early recovery. The magnitude of the overshoot to high AMOC transports in the recovery phase was related to density changes in the South Atlantic. This method was then applied to permanent hosing simulations in 2 GCMs in order to establish the changing mechanisms of AMOC weakening as the freshwater input increased. A change in AMOC weakening mechanism was found to coincide with the ‘tipping point’ for non-recovery. To explore this, a range of temporarily forced simulations was used to understand the role of feedbacks in the recovering and non-recovering ocean states. A positive salt advective feedback was found for the region 30°N in the Atlantic to the Bering Strait. However more locally to the region of convection, changes to the surface freshwater flux resulted in a positive freshwater feedback – which partially compensated for the removal of freshwater forcing and so supported the weak AMOC state.