The US Atlantic Meridional Overturning Circulation Program

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The US AMOC Program, now in its fifth year, was developed as a US interagency program to increase understanding of the Atlantic Meridional Overturning Circulation, in response to the 4th 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 2012, the program was constituted by over 50 funded projects. Annual program meetings, held either independently or jointly with the UK RAPID annual meetings, 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

The near-term research priorities of the program reflect discussions at the 2012 US AMOC PI Science Meeting.  Each Task Team reviewed the previous research prioirities that had been developed at the 2010 US AMOC PI Science Meeting in Miami, and which have since formed the primary focus of activity of the US AMOC Task Teams. These near-term priorities are listed below and updated to reflect the present consensus among the US AMOC Science Team of the top science priorities that the program should attempt to address.


Objective 1 Focus: AMOC Observing System Implementation and Evaluation

Assessing the meridional coherence of AMOC changes is a continuing focus of prognostic models, state estimates and enhancement of the AMOC observing system. Although the RAPID array continues to provide observations in the subtropics, and Argo and altimetry have been used to estimate overturning variability at mid-latitudes in the North Atlantic, observing system enhancements are needed to provide estimates of overturning variability in the subpolar North Atlantic as well as in the South Atlantic.

The importance of deep temperature and salinity measurements (i.e., deep Argo) in monitoring AMOC variability should be assessed.  At present, Argo, the primary component of the broad scale global ocean observing system for temperature and salinity, samples only the upper 2000 meters of the oceans.In many regions, this covers only the upper half or so of the southward flowing North Atlantic Deep Water. The importance of sampling temperature and salinity changes in the deep ocean remains unclear. More simulation efforts are needed to test the importance of deep water observations for constraining ocean models. Such work will provide guidance for building the deep ocean observing system by providing estimates for how many and what type of deep observations (e.g. deep Argo floats or full depth gliders) are needed.

Critical efforts should be directed toward synthesizing observations from existing elements of the observing system. In particular, efforts should be made to compare the transport and transport variability of the flow field at Line W, RAPID-MOCHA and the MOVE array. In addition to this comparison of directly-measured flow fields, it is also desirable to place these array measurements in the broader context of satellite and Argo float observations across the North Atlantic. The US AMOC community should consider the extent to which the proposed Oceanscope program could enhance the current and planned AMOC observing system and ocean state estimates.

Objective 2 Focus: AMOC State, Variability, and Change

Work toward the development of a multivariate fingerprint of the AMOC that would combine critical descriptors of the circulation and transport of ocean properties with those variables that are (or have been) observed extensively or can be reconstructed from paleoclimate archives.Studies aiming to develop fingerprinting techniques to better characterize AMOC variability by combining model simulations with observations should be further encouraged and supported.  Developing a set of metrics for the AMOC, in both depth and density spaces, is an important priority.

Assimilation modeling efforts should focus on reaching a consensus on the variability of the AMOC over the past few decades, and on placing realistic uncertainty bounds on these estimates. It is important that we understand the uncertainties of existing estimates and the accuracies required to detect climatically important AMOC-related changes.

Explore AMOC and MHT relationships in various models (forward, assimilation, non-eddy-resolving, eddy-resolving) in comparison with observational data being generated by the program, to understand the reasons for differences, or biases, in the relationship between model AMOC intensity and MHT in available models.

Objective 3 Focus: AMOC Mechanisms and Predictability

AMOC variability mechanisms, roles of atmospheric forcing and ocean – atmosphere feedbacks in this variability, and their model dependencies, utilizing all the available tools ranging from idealized theoretical models to the full general circulation models. A focused effort is also needed to develop a synthesis of existing observations, including synthesis of proxy data, to discriminate various model-based proposed mechanisms against the observational data.

Contributions to ongoing near-term prediction and predictability efforts with a focus on the AMOC should continue. In addition to coordinated and focused analysis and inter-comparison of the CMIP5 decadal prediction simulations, notable AMOC-related climate events should be used for verification of these prediction experiments. These efforts should seek collaboration with the WCRP Decadal Climate Prediction Panel as well as the International CLIVAR Working Group on Ocean Model Development and the Global Synthesis and Observational Panel.

Objective 4 Focus: Climate Sensitivity to AMOC: Climate/Ecosystem Impacts

Further understanding is needed of the connections between AMOC/North Atlantic SST and climate variability elsewhere, the physical mechanisms of these teleconnections, and the related impacts on humans and ecosystems. Targeted studies of the impact of AMOC variability on sea ice, ocean ecosystems, sea level changes around the Atlantic Basin, and the exchange of carbon between the atmosphere and ocean are also needed.