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      Contrasting internally and externally generated Atlantic Multidecadal Variability and the role for AMOC in CMIP6 historical simulations

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          Abstract

          Atlantic multidecadal variability (AMV) has long been thought to be an expression of low-frequency variability in the Atlantic Meridional Overturning Circulation (AMOC). However, alternative hypotheses have been forwarded, including that AMV is primarily externally forced. Here, we review the current state of play by assessing historical simulations made for the sixth coupled model intercomparison project (CMIP6). Overall, the importance of external forcing is sensitive to the type of AMV index used, due to the importance of globally coherent externally forced signals in the models. There are also significant contrasts between the processes that drive internally and externally forced AMV, but these processes can be isolated by exploring the multivariate expression of AMV. Specifically, internal variability in CMIP6 models is consistent with an important role of ocean circulation and AMOC and the externally forced AMV is largely a surface-flux forced mechanism with little role for the ocean. Overall, the internal multivariate fingerprint of AMV is similar to the observed, but the externally forced fingerprint appears inconsistent with observations. Therefore, climate models still suggest a key role for ocean dynamics, and specifically AMOC, in observed AMV. Nevertheless, models remain deficient in a number of areas, and a stronger role for externally forced dynamical changes cannot be ruled out.

          This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’.

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          Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization

          Karl E. Taylor, Ronald J Stouffer, Bjorn Stevens … (2016)
          By coordinating the design and distribution of global climate model simulations of the past, current, and future climate, the Coupled Model Intercomparison Project (CMIP) has become one of the foundational elements of climate science. However, the need to address an ever-expanding range of scientific questions arising from more and more research communities has made it necessary to revise the organization of CMIP. After a long and wide community consultation, a new and more federated structure has been put in place. It consists of three major elements: (1) a handful of common experiments, the DECK (Diagnostic, Evaluation and Characterization of Klima) and CMIP historical simulations (1850–near present) that will maintain continuity and help document basic characteristics of models across different phases of CMIP; (2) common standards, coordination, infrastructure, and documentation that will facilitate the distribution of model outputs and the characterization of the model ensemble; and (3) an ensemble of CMIP-Endorsed Model Intercomparison Projects (MIPs) that will be specific to a particular phase of CMIP (now CMIP6) and that will build on the DECK and CMIP historical simulations to address a large range of specific questions and fill the scientific gaps of the previous CMIP phases. The DECK and CMIP historical simulations, together with the use of CMIP data standards, will be the entry cards for models participating in CMIP. Participation in CMIP6-Endorsed MIPs by individual modelling groups will be at their own discretion and will depend on their scientific interests and priorities. With the Grand Science Challenges of the World Climate Research Programme (WCRP) as its scientific backdrop, CMIP6 will address three broad questions: – How does the Earth system respond to forcing? – What are the origins and consequences of systematic model biases? – How can we assess future climate changes given internal climate variability, predictability, and uncertainties in scenarios? This CMIP6 overview paper presents the background and rationale for the new structure of CMIP, provides a detailed description of the DECK and CMIP6 historical simulations, and includes a brief introduction to the 21 CMIP6-Endorsed MIPs.
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            A signature of persistent natural thermohaline circulation cycles in observed climate

            Jeff Knight (2005)
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              Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability.

              Ben Booth, Nick Dunstone, Paul Halloran … (2012)
              Systematic climate shifts have been linked to multidecadal variability in observed sea surface temperatures in the North Atlantic Ocean. These links are extensive, influencing a range of climate processes such as hurricane activity and African Sahel and Amazonian droughts. The variability is distinct from historical global-mean temperature changes and is commonly attributed to natural ocean oscillations. A number of studies have provided evidence that aerosols can influence long-term changes in sea surface temperatures, but climate models have so far failed to reproduce these interactions and the role of aerosols in decadal variability remains unclear. Here we use a state-of-the-art Earth system climate model to show that aerosol emissions and periods of volcanic activity explain 76 per cent of the simulated multidecadal variance in detrended 1860-2005 North Atlantic sea surface temperatures. After 1950, simulated variability is within observational estimates; our estimates for 1910-1940 capture twice the warming of previous generation models but do not explain the entire observed trend. Other processes, such as ocean circulation, may also have contributed to variability in the early twentieth century. Mechanistically, we find that inclusion of aerosol-cloud microphysical effects, which were included in few previous multimodel ensembles, dominates the magnitude (80 per cent) and the spatial pattern of the total surface aerosol forcing in the North Atlantic. Our findings suggest that anthropogenic aerosol emissions influenced a range of societally important historical climate events such as peaks in hurricane activity and Sahel drought. Decadal-scale model predictions of regional Atlantic climate will probably be improved by incorporating aerosol-cloud microphysical interactions and estimates of future concentrations of aerosols, emissions of which are directly addressable by policy actions.
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                Author and article information

                Contributors
                Jon Robson:
                ORCID: http://orcid.org/0000-0002-3467-018X
                Role: ConceptualizationRole: Data curationRole: Formal analysisRole: InvestigationRole: VisualizationRole: Writing – original draftRole: Writing – review & editing
                Rowan Sutton: Role: Writing – review & editing
                Matthew B. Menary: Role: Data curationRole: Writing – review & editing
                Michael W. K. Lai: Role: Writing – review & editing
                Journal
                Journal ID (nlm-ta): Philos Trans A Math Phys Eng Sci
                Journal ID (iso-abbrev): Philos Trans A Math Phys Eng Sci
                Journal ID (publisher-id): RSTA
                Journal ID (hwp): roypta
                Title: Philosophical transactions. Series A, Mathematical, physical, and engineering sciences
                Publisher: The Royal Society
                ISSN (Print): 1364-503X
                ISSN (Electronic): 1471-2962
                Publication date (Print, pub): December 11, 2023
                Publication date (Electronic, pub): October 23, 2023
                Publication date PMC-release: October 23, 2023
                Volume: 381
                Issue: 2262 , Discussion meeting issue ‘Atlantic overturning: new observations and challenges’ organised and edited by M. A. Srokosz, N. P. Holliday and H. L. Bryden FRS
                Electronic Location Identifier: 20220194
                Affiliations
                [ 1 ] Department of Meteorology, National Centre for Atmospheric Science, University of Reading, , Reading, UK
                [ 2 ] Met Office Hadley Centre, , Exeter, UK
                Author notes

                One contribution of 13 to a discussion meeting issue ‘ Atlantic overturning: new observations and challenges’.

                Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare.c.6824114.

                Author information
                http://orcid.org/0000-0002-3467-018X
                Article
                Publisher ID: rsta20220194
                DOI: 10.1098/rsta.2022.0194
                PMC ID: 10590668
                PubMed ID: 37866382
                SO-VID: 620a0c36-1441-4bfe-b666-6f638beed9a7
                Copyright © © 2023 The Authors.
                License:

                Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

                History
                Date received : April 17, 2023
                Date accepted : August 1, 2023
                Funding
                Funded by: Natural Environment Research Council, http://dx.doi.org/10.13039/501100000270;
                Award ID: NE/T013494/1
                Award ID: NE/T013516/1
                Award ID: NE/W004984/1
                Categories
                Compound subject: 1005
                Compound subject: 41
                Compound subject: 140
                Subject: Articles
                Subject: Research Articles
                Custom metadata
                cover-date December 11, 2023

                Keywords: atlantic ocean,multidecadal variability,amv,amoc
                Data availability:
                Keywords: atlantic ocean, multidecadal variability, amv, amoc

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