"Objective #1.
We conducted joint analysis of the IPCL-CM6A-LR and HadGEM3-GC3.1 climate models and determined that there were significant differences in their representation of North Atlantic Ocean variability, including the AMOC, despite these models using the same underlying ocean model (NEMO). This analysis was published in Menary et al. (2018).
Objective #2.
We designed and performed sensitivity experiments, modifying the parameters of the IPSL-CM6A-LR climate models so it resembled the HadGEM3-GC3.1 model. Several different experiments were run, grouping the parameter changes into arbitrary groups based on the region/processes that we thought they would be most likely to impact. In general, we found that the parameter changes could not bring the IPSL-CM6A-LR ocean mean/variability closer to that of tHadGEM3-GC3.1. These experiments were presented at the EGU 2019 conference. Separately, new observations (OSNAP) were released during the project lifetime that called into question whether climate models could adequately represent some important processes in the North Atlantic, that were thought to drive the AMOC. We responded to this work by conducting a coordinated analysis of the models and observations, including the expertise of the observationalists. This work was published in Menary et al. (2020a) GRL.
Objective #3.
Given that the ocean parameter choices did not appear to drive these different ocean circulation behaviours, we moved our focus to the atmosphere. We investigated the role of anthropogenic aerosols in driving large scale ocean circulation changes. We showed that the multi-model mean AMOC strengthened by approximately 10% from 1850-1985 in new simulations from the 6th Coupled Model Inter-comparison Project (CMIP6), a larger change than was seen in CMIP5. Across the models, the strength of the AMOC trend up to 1985 is related to a proxy for the strength of the aerosol forcing. Therefore, the multi-model difference is a result of stronger anthropogenic aerosol forcing on average in CMIP6 than CMIP5, which is primarily due to more models including aerosol-cloud interactions. However, observational constraints - including a historical sea surface temperature fingerprint and shortwave radiative forcing in recent decades - suggest that anthropogenic forcing and/or the AMOC response may be overestimated. This work was published in Menary et al. (2020b) GRL."
