| Summary: | The Atlantic Meridional Overturning Circulation (AMOC) is characterized as a northward upper limb that carries warm near-surface waters from southern latitudes to the subpolar North Atlantic, and a southward lower limb that transports cold deep waters back to the southern latitudes. Due to its special role in distributing heat, carbon and water masses globally, AMOC as an essential part in the climate system, has long been a strong focus within the ocean community. For decades, AMOC variability has been attributed to changes in deep water production at high latitudes in the North Atlantic through a geostrophic response in the Deep Western Boundary Current (DWBC), the assumed major export pathway of the deep waters. However, recent Lagrangian studies have revealed the importance of eddy-driven interior pathways, challenging the traditional DWBC-dominated spreading pattern, hence the linkage between deep water formation and AMOC. Under the new spreading scheme of the deep waters, this dissertation provides an extended Lagrangian analysis on the spreading pathways of two major deep waters, Labrador Sea Water (LSW) and Iceland Scotland Overflow Water (ISOW), and re-examines the relationships among deep water production, deep water export and the strength of AMOC. A Lagrangian simulation of newly-formed LSW in an ocean/sea ice model (1/4°) reveals strong recirculation of the water mass in the subpolar gyre, with a small portion exported to the subtropical gyre through an advective-diffusive pathway. Furthermore, no significant correlation between LSW production and its Lagrangian export to the subtropical gyre is found on interannual to decadal time scales, suggesting a negligible or at best modest impact of LSW production on the subtropical AMOC. In a combined Lagrangian and Eulerian frame, a first comprehensive description of ISOW spreading branches in the eastern North Atlantic is presented with observational data and output from an eddy-resolving ocean model. The major export pathway for ISOW is shown to be the ...
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