PP039-10
Mechanisms for an AMOC recovery: insights from millennial-length simulations in complex coupled climate models
Monday, 14 December 2020: 12:06
Virtual
David B Bonan1, Andrew F Thompson2, Emily Rose Newsom3, Shantong Sun2 and Maria A.A. Rugenstein4, (1)California Institute of Technology, Pasadena, CA, United States, (2)California Institute of Technology, Environmental Science and Engineering, Pasadena, CA, United States, (3)University of Oxford, Department of Physics, Oxford, United Kingdom, (4)Max Planck Institute for Meteorology, Hamburg, Germany
Abstract:
The strength of the Atlantic meridional overturning circulation (AMOC) is projected to decline in response to anthropogenic climate change throughout the 21st century. Beyond the 21st century, however, the evolution of the AMOC is less clear as conceptual models and fully-coupled global climate models (GCMs) predict either an AMOC recovery or prolonged collapse. Here, we study the transient and equilibrium responses of ocean heat transport and the AMOC to global warming using an unprecedented suite of simulations from state-of-the-art fully-coupled GCMs that are evolved for thousands of years after an increase in CO2. In response to an abrupt increase in atmospheric CO2, we find that all GCMs exhibit a reduced Atlantic heat transport and an AMOC weakening on centennial timescales. In contrast, the evolution of the AMOC and its associated heat transport diverge on millennial timescales, despite the same greenhouse-gas forcing. In some GCMs, the AMOC recovers and Atlantic heat transport increases, while in others the AMOC collapses and Atlantic heat transport decreases.
We explore two mechanisms responsible for the AMOC recovery (or lack thereof) in the models: (i) a resumption of convection in the North Atlantic due to local surface forcing and (ii) a resumption of heat transport into the south Atlantic due to heat uptake in the Pacific and its redistribution to other basins. We find that the meridional density gradient between the northern sinking region and lower latitudes in the Atlantic basin provides a useful framework for distinguishing between these mechanisms and explains the inter-model spread of the AMOC strength. The decline and initial recovery are linked to northern density changes, but the final state of the AMOC appears to be influenced by density changes at the lower latitudes. These results highlight the importance of considering the evolution of the AMOC and ocean heat transport beyond the 21st century.
