Periodic Reporting for period 1 - EPICE (Emergent Properties to Improve Climate projections over Europe)
Reporting period: 2018-10-01 to 2020-09-30
The Atlantic Meridional Overturning Circulation (AMOC) represents the Atlantic branch of the global overturning circulation. It plays a significant role in both local and global climate, transporting heat from the South Atlantic to the North Atlantic and sequestering anthropogenic carbon in the deep ocean. Changes in the strength of the AMOC are thought to be an important driver of changes in North Atlantic sea surface temperature (SSTs). Hence, changes in the AMOC strength could have important impacts on a range of far-reaching phenomena, including the number of hurricanes, Sahel drought, and Atlantic storm tracks. In this project, we investigate the different behaviour of the AMOC amongst climate models, focussing initially on the underlying model parameter choices.
Our objectives are to:
1) Jointly investigate the specific mechanisms of North Atlantic variations in state-of-the-art models that share the same ocean and compare with historical observations, allowing us to...
2) Evaluate how these comparisons are sensitive to the model formulation, via targeted experiments, to determine the links between parameter choices and features of the variability , which will enable us to...
3) Derive emergent constraints to narrow the spread across the entire CMIP6 model suite of long term projections and near term predictions.
Our objectives are to:
1) Jointly investigate the specific mechanisms of North Atlantic variations in state-of-the-art models that share the same ocean and compare with historical observations, allowing us to...
2) Evaluate how these comparisons are sensitive to the model formulation, via targeted experiments, to determine the links between parameter choices and features of the variability , which will enable us to...
3) Derive emergent constraints to narrow the spread across the entire CMIP6 model suite of long term projections and near term predictions.
"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."
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."
A key outcome of this project has been the analysis and publication of the effect of anthropogenic aerosols on the large-scale circulation in the North Atlantic (the AMOC) in new CMIP6 climate models. This result is notably different to that obtained using the previous models (CMIP5; Menary et al., 2020b, GRL). This work has fed into the upcoming IPCC AR6 report, due for publication in 2021. It will help us to understand how the AMOC might be susceptible to anthropogenic forcings, which can provide insight into any potential future weakening of this important circulation under climate change. It has highlighted that the ocean (and climate) can show significant and important responses to anthropogenic forcings other than merely greenhouse gas emissions. As such, one impact of this work is to reinforce the need for environmental policies that address more than just greenhouse gas emissions.