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Xiaobiao Xu, E. C. (2019).
Subpolar-Subtropical Connectivity of the North Atlantic Circulation.
Abstract: The ocean, through its large capacity to store heat, plays a critical role in Earth's climate and climate variability. Warming of the world's oceans since 1955 accounts for approximately 93% of the warming of the Earth system. However, this warming is neither spatially uniform nor temporally constant. Superimposed on the global long-term trend is climate variability on inter-annual to inter-decadal time scales and regional to basin scales. Satellite altimeters and hydrographic observations show that the North Atlantic, including the sub-polar region, has rapidly become warmer and saltier since the early 1990s. An emerging picture is that the most recent 20 years or so of warming in the North Atlantic represents, in part, a transition of the Atlantic multi-decadal variability pattern from a cold to a warm phase. These decadal climate transitions involve changes both laterally in the sub-tropical and sub-polar gyres of the North Atlantic and vertically in the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global heat and freshwater circulation system. This study of the North Atlantic circulation concentrates on a transition region around the Grand Banks of Newfoundland, where the effects of boundary currents and jets, recirculations, and mesoscale eddies (length scales typically less than 100 km) are dominant. Strong interactions occur in this transition region, laterally between the subpolar and subtropical gyres and vertically between the cold and warm limbs of the Atlantic Meridional Circulation (AMOC). There is evidence that this relatively compact region plays a key role in altering and even modulating the AMOC over a much larger scale and thus is important for the long-term, decadal variability of the Atlantic Ocean. Yet, despite many observational field programs, the dynamics and impacts of this region are not well understood. The project will contribute to understanding the variability of the AMOC by addressing the connectivity of the sub-polar and the sub-tropical gyres. The results of this model-data synthesis will be of particular significance to coupled climate models, which are central to understanding and predicting global climate change. The educational/outreach components of this project will be focused on cultivating scientific literacy with regards to ocean climate research in K-12 schools, at the university level, and in the local community through a variety of online resources/interactive tools for educators, the Florida State University Young Scholars program for high school students, and the “Scientists in the Schools” program. Finally, the requested funding will support a junior faculty member and a graduate student who will be trained in ocean modeling, data analysis and interpretation.
Through ongoing major observation programs in the sub-polar and sub-tropical North Atlantic Ocean, oceanographers are making great strides toward a better understanding of the structure and variability of the AMOC within these sub-basins. The work proposed here complements these observations by focusing on key questions pertaining to what controls the circulation in between and how much the sub-polar to sub-tropical connectivity modulates the larger scale AMOC. This project aims to elucidate the physical dynamics that controls circulation in the transition region, especially the relative importance of the eddies and the deep western boundary current, and document the role and impact of the transition region on the larger scale circulation, especially the variability of the AMOC and water properties in the sub-polar and sub-tropical North Atlantic from inter-annual to decadal and longer time scales. The interaction of eddies and time mean circulations represents one of the greatest challenges to prediction of global climate variability, and it can be studied with the fine-grid resolution model included in this project. These objectives will be met by performing a detailed model-data synthesis study, combining numerical results from a suite of high-resolution Atlantic simulations using the HYbrid Coordinate Ocean Model (HYCOM) and existing observations (satellite altimetry, drifters/floats, hydrography, tracers, and mooring arrays). The three-dimensional Atlantic circulation will be quantified by performing analysis of water mass transport and transformation, passive tracers, and potential vorticity and momentum fluxes. It has been demonstrated that the eddy-resolving HYCOM represents the basic circulation features in the transition region and larger scale North Atlantic Ocean, including both time mean structure and temporal variability.