The US AMOC Program, now in its ninth year, was developed as a US interagency program to increase understanding of the Atlantic Meridional Overturning Circulation in response to the fourth near-term priority of the SOST Ocean Research Priorities Plan. The purpose of the program is to bring together researchers studying the AMOC and to build partnerships among modeling and observational groups to address problems related to AMOC variability, predictability, and climate impacts. During 2016 the program was constituted by over 50 funded projects. Program meetings, held either independently or jointly with the UK RAPID program, have been very successful in bringing together the program PIs to share research results, develop collaborative projects, and identify near-term research priorities.
Program Objectives and Near-Term Research Priorities
Objective 1 Focus: AMOC Observing System Implementation and Evaluation
- Improving understanding of the meridional coherence (and/or lack thereof) of the AMOC and the mechanisms that control AMOC changes continues to be a high near-term priority. The newly deployed OSNAP array near 55°N, and the augmented elements of the SAMBA array at 34.5°S, will play key roles in this. Furthermore, development of dynamically consistent model-data synthesis methods to combine the heterogeneous observational pieces will also play an important role.
- Expansion of the existing observing system to better capture the deep ocean and to better quantify the role of deep temperature and salinity signals in contributing to AMOC variability continues to be a priority. Enhancements such as ‘deep Argo’, ‘bio-Argo’, full-depth gliders, and enhanced moored observations should be evaluated in the context of a full-depth observing system.
- Ensuring that AMOC estimates (and the key underlying measurements collected as part of the AMOC estimates) are made available in widely recognized locations such as the World Ocean Database, OceanSITES, the National Ocean Data Center, etc., is a new near-term priority. Improvement to communications between different observing system groups is also a recommended activity, particularly between more established observing system groups and newer groups becoming involved at the national and international levels.
- Another new near-term priority is making sure that error estimates are produced and provided alongside AMOC estimates (and the constituent components). These error estimates should be made available on applicable time scales (days, weeks, months, and years) to provide the necessary precision information for analyses, inter-array comparisons, and for numerical model studies (where data are used for validation and/or for assimilation).
- Finding and/or developing new technologies and methods for studying the AMOC and its key components will be necessary moving forward in order to address the overall observing goals for AMOC in a world of finite resources.
- Development of plans to observe and study the shallow and deep pathways of the AMOC through the basin at locations away from the places of the few trans-basin arrays will be important in the long-term. This may involve future Lagrangian studies in the South Atlantic and/or tropical Atlantic regions similar to the ongoing work in the high-latitude North Atlantic, or it may involve the development of new technologies and/or techniques.
- Rigorous testing of data assimilation schemes is needed in order to better understand how the systems are using the data collected. Better communication is needed between the US AMOC community and the data assimilation community to test and potentially expand the set of collected observations that are assimilated into models.
Objective 2 Focus: AMOC State, Variability, and Change
- Use new and existing observations in combination with modeling experiments to refine our understanding of the present and historical circulation (and related transports of heat and freshwater) in the North and South Atlantic. An emerging priority is to provide a more detailed characterization of AMOC flow pathways and their impact on variability.
- Continue development and investigation of AMOC “fingerprints.” Modeling and observational studies that seek to refine our current understanding of the connection of AMOC to large-scale, historically well-observed properties of the climate system should be encouraged.
- Investigate connections between surface forcing (e.g., freshwater, heat, and momentum fluxes, NAO-related forcing) and historical AMOC variability.
- Develop a more comprehensive understanding of the strengths and weaknesses of existing global ocean reanalysis products and hindcasts using forward models as tools for investigating the circulation and transports in the Atlantic.
- Synthesize modeling and observational evidence to build scientific consensus on the variability and change of the AMOC over the last 50 years. Efforts within the data assimilation community should focus on reaching an accurate consensus (consistent with other lines of observational evidence) on the evolution of the AMOC over the last 50 years.
- Efforts within the data assimilation community should focus on reaching an accurate consensus (consistent with other lines of observational evidence) on the evolution of the AMOC over the last 50 years.
- Observational studies should focus on mechanisms and pathways that identify and explain coherent and incoherent signals between different study sites, thereby reaching consensus on which signals represent the large-scale AMOC versus more localized circulation patterns.
Objective 3 Focus: AMOC Mechanisms and Predictability
- Investigate how surface exchanges of buoyancy and momentum between the ocean and the atmosphere/cryosphere drive the AMOC circulation across a broad range of timescales from monthly to millennial (i.e., quasi steady-state).
- Clarify the apparent disagreement between models of different complexity regarding: i) the role of Southern Ocean winds and ii) the role of Nordic Seas overflows in maintaining and modulating the AMOC.
- Quantify the magnitude, location, and physical mechanisms associated with interior diapycnal mixing in the ocean, which contribute to the diabatic AMOC, and evaluate the realism of current ocean GCMs in this regard.
- Investigate the role of freshwater forcing and South Atlantic freshwater transports in determining the variability and stability of AMOC.
- Expand the use of eddy-resolving models, particularly in regional/process studies designed to: i) test the robustness of AMOC variability mechanisms identified in coarser GCMs or idealized models; ii) address the origins of persistent model bias in the North Atlantic region (e.g., Gulf Stream separation and the North Atlantic Current path); and iii) assess the role of ocean turbulence in AMOC variability.
- Quantify the predictability properties of AMOC in idealized and comprehensive models and identify mechanisms that affect these properties.
- Explore the mechanisms associated with AMOC variability on centennial-to-millennial timescales, and evaluate the realism of GCMs on these timescales relative to available paleo proxy data, perhaps using proxy-enabled coupled climate models.
- Translate the knowledge developed about AMOC variability and predictability mechanisms into reliable decadal climate forecasts.
- Incorporate mesoscale eddy-resolving ocean models more fully into the toolkit used for AMOC mechanisms/prediction work, including long coupled GCM simulations, in order to address questions about the role of turbulence in controlling AMOC.
- Synthesize results from theoretical, idealized models, and complex GCM investigations into a common conceptual framework regarding key AMOC variability mechanisms and identify the resulting predictability of the AMOC.
Objective 4 Focus: Climate Sensitivity to AMOC: Climate/Ecosystem Impacts
- Identify the mechanisms by which AMOC variability, imprinted on SST and/or the cryosphere, affects local and remote atmospheric patterns and phenomena.
- Assess AMOC impacts on the cryosphere, particularly Arctic sea ice and the Greenland Ice Sheet.
- Assess AMOC impacts on global and regional sea level.
- Improve understanding of how AMOC variability affects ocean-atmosphere exchanges of carbon, biogeochemical cycles, and marine ecosystems.
The long-term goal of Task Team 4 is to understand how AMOC variability affects other components of the Earth system – its climate, hydrologic cycle, atmospheric circulation, coupled phenomena (e.g., ENSO, monsoons), cryosphere, sea level, marine and terrestrial ecosystems, biogeochemical cycles, and carbon budgets – both locally and remotely.