The causal pathways of the Indian-Atlantic interocean exchange

Colorado State University, Fort Collins CO

Investigators

Abstract

A fundamental question in physical oceanography is how different oceans interact. Answering this question is crucial for our understanding of the global ocean circulation and the role the ocean plays in the climate system. One of these interaction points is the ocean area south of Africa where the Indian, South Atlantic and Southern Oceans meet. While it is now well established that the interocean exchange in this area is of global importance, much less is known on what the underlying processes are that determine the existence and size of this exchange. This project will advance the understanding of which processes are responsible for the Indian-Atlantic inter-ocean exchange and how and to what extend they work in concert to achieve the exchange. It brings together state-of-the-art developments in nonlinear data assimilation, ocean modeling, observational data sets and nonlinear causal discovery to advance that understanding. As such the project enhances our fundamental understanding of the ocean circulation, but also further develops new tools that can be used in many other oceanographic problems, in other geoscience areas, and beyond. The fully nonlinear data assimilation method implemented in the JEDI system and tested on this highly turbulent ocean area will be unique in the world and can be applied by scientists and operational centers alike. Systematic nonlinear causal discovery is a growing and exciting research field that deserves maturing into a standard well- calibrated tool, beyond more traditional linear causal discovery methodologies. The causal discovery framework can be applied to many outstanding oceanographic and wider geoscience problems. Furthermore, it can be applied to pure modeling studies to compare causal pathways and highlight model deficiencies. A student will be exposed to cutting edge data assimilation, ocean modeling and observations, and causal discovery, leading to an exceptionally rounded budding scientist. Results of the study will be broadly communicated to the oceanographic, climate and applied mathematical/engineering communities through conference presentations and publications Many processes have been put forward as important for the interocean exchange S of Africa, such as Natal Pulses (large cyclonic meanders of the Agulhas Current), Mozambique Eddies, dipoles shed from the southern tip of Madagascar, mixed barotropic-baroclinic instabilities in the retroflection area, perhaps related to Rossby-wave like basin modes, variations in the Indonesian Throughflow and the South Equatorial Current, large-scale variations in the thermodynamic structure, including vorticity generation due to NADW inflow, large-scale wind perturbations, and coupled climate modes, e.g. the Indian Ocean Dipole. This project will test the hypothesis that the interocean exchange is controlled by the tight recirculation gyre in the Agulhas Retroflection area, which is in turn modulated by the physical processes mentioned. The main contribution of this research will be to unravel and quantify these direct contributions and the contributions from nonlinear interactions using a new fully nonlinear causal discovery methodology. This methodology provides for a complete decomposition of these nonlinear interactions into two-process interactions, three-process interactions etc. Furthermore, it allows to quantify the magnitude of the contributions of any unidentified controlling processes, directing research to hitherto unknown important physical processes. This methodology will be applied to process time series from a 30-year reanalysis of the ocean area around South Africa, using fully nonlinear data assimilation, incorporating all available observations to answer the following overarching research questions: • What are the physical processes that are responsible for the interocean exchange? • How do these physical processes interact to cause the interocean exchange? This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.