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).