A Generalized Stability Analysis of the AMOC in Earth System Models: Implications for Decadal Variability and Abrupt Climate Change

Funding Program: 

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial factor controlling the transport of heat from low to high latitudes in the Atlantic – changes in this transport affect ocean temperatures, atmospheric circulation and hence climate. Variations in the strength of the AMOC are believed to contribute to climate variability on timescales ranging from decades to millennia. Moreover, paleorecords and simulations with climate models indicate that the AMOC can undergo dramatic changes in response to external perturbations, such as freshwater pulses or temperature anomalies in high latitudes. Emerging research also suggests that AMOC variations have a significant decadal predictability. However, different climate models show a very broad range of key properties of the AMOC – its sensitivity to external forcing and variability. Some models exhibit a rapid collapse of the AMOC for very small external perturbations, resulting in abrupt climate change, while others have a very stable overturning. Some models show robust decadal or multidecadal variations, whereas others have almost no variability. The overarching objective of this project is to understand these differences by investigating what controls the stability and variations of the AMOC, and related abrupt climate changes, in earth system models. The main tool of this study will be the generalized stability analysis applied within a hierarchy of GCMs, including comprehensive earth system models (in particular, the newly released CESM).

The generalized stability analysis uses tangent linear models in conjunction with their adjoints to determine a variety of characteristics of ocean circulation and variability. We have developed an efficient method for conducting such an analysis with the use of Lagrange multipliers. Using this method, we can extract the dominant internal mode of the AMOC associated with ocean dynamics and study the properties of this mode as a function of model parameters. In addition, by calculating the AMOC optimal perturbations, we can assess the sensitivity of this circulation and the entire climatic state to initial anomalies in temperature and salinity and explore the possibility of rapid changes in the system. We can also calculate optimal steady and finite-time perturbations in surface heat and freshwater fluxes that affect the AMOC the strongest, which is essential for understanding the AMOC response to climate change. First, we will apply this method to the linear version of an ocean model (OPA-NEMO), linearized with respect to the model's basic climatological ocean state. Next, we will calculate the basic ocean state of CESM and substitute it into the original linearized model, where the key model parameters will have been adjusted to match those in CESM. We will refer to the new model as CESM-LO (i.e., Linear Ocean). We will then conduct the generalized stability analysis of the new system. The results obtained with CESM- LO, such as optimal perturbations, will be applied to the full version of CESM to study the effects of ocean-atmosphere coupling on the evolution of the perturbations, the properties of the leading decadal eigenmodes and the likelihood of abrupt climate change. Finally, we will apply this method to a subset of models from the CMIP5 dataset. Two interwoven tasks of this part of the project will be to develop our method into a universal tool one could use with any earth system model and to explain differences in the AMOC properties between different models.

Ultimately, the goal of this study is to understand the fundamental physical mechanisms that control AMOC variations and rapid changes within earth system models. This is of significant practical value because AMOC changes are likely to become a major element of the ocean response to global warming. The project will serve as a test bed for applying the generalized stability analysis to any earth system model with the goal of improving simulation and prediction of the AMOC. Thus, this project conforms to the two goals of the current solicitation – to improve the accuracy and skill of climate models and to understand the principle causes and effects of climate change, including potential abrupt changes in climate. Educational impacts of this project include further development and maintenance of ocean and climate modeling capabilities at Yale University, which benefit both graduate and undergraduate students. Funding originating from this grant is critical to support the career growth of a talented young scientist at Yale, Dr. Florian Sévellec. His thorough expertise in the generalized stability analysis is critical for the success of this project. The project will also involve a new Ph.D. student at Yale, Ivy Tan (limited funds are requested for her during summer in year 2 of the proposal).

Project Term: 
2011-2014
Project Type: 
University Funded Research

Publications:

A Subbasin-based Framework to Represent Land Surface Processes in an Earth System Model
A Technique for Generating Consistent Ice Sheet Initial Conditions for Coupled Ice Sheet/Climate Models
Appendix 3: Climate Science Supplement. Climate Change Impacts in the United States
Appendix 4: Frequently Asked Questions. Climate Change Impacts in the United States
Aquaplanet Experiments Using CAM's Variable Resolution Dynamical Core
Are channels standalone? Analysis of channel-land interactions using PAWS+CLM
Assessing the CAM5 Physics Suite in the WRF-Chem Model: Implementation, Resolution Sensitivity, and a First Evaluation for a Regional Case Study
Attribution of Extreme Weather to Anthropogenic Greenhouse Gas Emissions: Sensitivity to Spatial and Temporal Scales
Attribution of Floods in the Okavango Basin, Southern Africa
Climate and Energy-Water-Land System Interactions: Technical Report to the U.S. Department of Energy in Support of the National Climate Assessment
Climate Change and Energy Supply and Use: Technical Report for the U.S. Department of Energy in Support of the National Climate Assessment
Climate Feedbacks in CCSM3 under Changing CO2 Forcing Part II: Variation of Climate Feedbacks and Sensitivity with Forcing
Climate Simulations and Projections with a Super-Parameterized Climate Model
Could a Future “Grand Solar Minimum” Like the Maunder Minimum Stop Global Warming?
Detection and Attribution of Climate Change Impacts – Is a Universal Discipline Possible?
Increase in the Intensity of Postmonsoon Bay of Bengal Tropical Cyclones
Influence of Continental Ice Retreat on Future Global Climate
Intercomparison and Evaluation of Global Aerosol Microphysical Properties Among AeroCom Models of a Range of Complexity
Investigating soil moisture spatial scaling using Reduced Order Models and analysis of fractal temporal evolution patterns
Methods of Projecting Future Changes in Extremes
National Climate Assessment: Valuation Techniques and Metrics Workshop Report
North Atlantic Warming and the Retreat of Greenland's Outlet Glaciers
Northern Winter Climate Change: Assessment of uncertainty in CMIP5 projections related to stratosphere-troposphere coupling
On the Interpretation of Constrained Climate Model Ensembles
On the Lack of Stratospheric Dynamical Variability in Low-Top Versions of the CMIP5 Models
Optimal Initial Conditions for Coupling Ice Sheet Models to Earth System Models
Parameter and State Estimation with a Time-Dependent Adjoint Marine Ice Sheet Models
Parameterization of Basal Friction Near Grounding Lines in a One-Dimensional Ice Sheet Model
Projections of the Tropical Atlantic Vertical Wind Shear and Its Relationship with ENSO in SP-CCSM4
Prospects for Simulating Macromolecular Surfactant Chemistry at the Ocean-Atmosphere Boundary
Prospects for the Simulating Macromolecular Surfactant Chemistry at the Ocean-Atmosphere Boundary
Quantifying Components of Aerosol-Cloud-Radiation Interactions in Climate Models
Reducing the Computational Cost of the ECF using a nuFFT: A fast and objective probability density estimation method
Retrievals of Cloud Fraction and Cloud Albedo from Surface-based Shortwave Radiation Measurements: A Comparison of 16 Year Measurements
Robust Direct Effect of Increasing Atmospheric CO2 Concentration on Global Tropical Cyclone Frequency - A Multi-Model Inter-Comparison
Scalability of Grid- and Subbasin-Based Land Surface Modeling Frameworks for Hydrologic Simulations
Sea Ice Volume and Age: Sensitivity to physical parameterizations and thickness resolution in the CICE sea ice model
Sensitivity of Global Terrestrial Gross Primary Production to Hydrologic State Simulated by the Community Land Model using Two Runoff Parameterizations
Sensitivity of Tropical Cyclone Rainfall to Different Warming Scenarios at the Global Scale
Skill in forecasting extreme ozone pollution episodes with a global atmospheric chemistry model
Technical Note: Simple Formulations and Solutions of the Dual-Phase Diffusive Transport for Biogeochemical Modeling
The climate impact of ship NOx emissions: an improved estimate accounting for plume chemistry
The Pattern of Anthropogenic Signal Emergence in Greenland Ice Sheet Surface Mass Balance
The Response of the Southern Hemispheric Eddy-Driven Jet to Future Changes in Shortwave Radiation in CMIP5
The Robust Dynamical Contribution to Precipitation Extremes in Idealized Warming Simulations Across Model Resolutions
The Role of Moist Processes in the Intrinsic Predictability of Indian Ocean Cyclones
WRF-Chem Simulations of Aerosols and Anthropogenic Aerosol Radiative Forcing in East Asia

Research Highlights:

Regionalization of Subsurface Stormflow Parameters of Hydrologic Models: Derivation from Regional Analysis of Streamflow Recession Curves Highlight Presentation