Impact of ocean resolution and mean state on the rate of AMOC weakening

• Published in: Climate Dynamics
• Main Authors: Jackson, Laura C., Roberts, Mark J., Hewit, Helene T., Iovino, Doroteaciro, Koenigk, Torben, Meccia, Virna L., Roberts, Chris D., Ruprich Robert, Yohan, Wood, Richard A.
• Other Authors: Barcelona Supercomputing Center
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Link 2020
• Subjects: Àrees temàtiques de la UPC::Desenvolupament humà i sostenible; Databases; Atlantic Ocean; Ocean temperature; Climate; Atlantic Meridional Overturning Circulation (AMOC); Weakening ocean heat; Climate models; Bases de dades; Greenland; Iceland; north atlantic current; North Atlantic
• Online Access: http://hdl.handle.net/2117/192990; https://doi.org/10.1007/s00382-020-05345-9

We examine the weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing CO2 at different horizontal resolutions in a state-of-the-art climate model and in a small ensemble of models with differing resolutions. There is a strong influence of the ocean mean state on the AMOC weakening: models with a more saline western subpolar gyre have a greater formation of deep water there. This makes the AMOC more susceptible to weakening from an increase in CO2 since weakening ocean heat transports weaken the contrast between ocean and atmospheric temperatures and hence weaken the buoyancy loss. In models with a greater proportion of deep water formation further north (in the Greenland-Iceland-Norwegian basin), deep-water formation can be maintained by shifting further north to where there is a greater ocean-atmosphere temperature contrast. We show that ocean horizontal resolution can have an impact on the mean state, and hence AMOC weakening. In the models examined, those with higher resolutions tend to have a more westerly location of the North Atlantic Current and stronger subpolar gyre. This likely leads to a greater impact of the warm, saline subtropical Atlantic waters on the western subpolar gyre resulting in greater dense water formation there. Although there is some improvement of the higher resolution models over the lower resolution models in terms of the mean state, both still have biases and it is not clear which biases are the most important for influencing the AMOC strength and response to increasing CO2. LJ, MR, DI, TK, VM, CR, YR-R were funded by the PRIMAVERA project, funded by the European Union’s Horizon 2020 programme under grant agreement 641727. LJ, HH, MR, RW were supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra (GA01101). We wish to thank two anonymous reviewers for their comments which improved this manuscript. Peer Reviewed Postprint (published version)