Soutenance de Lennard Miller

Ocean gyres are basin-wide patterns of horizontal circulation driven by surface wind stress. They feature energetic currents at the western boundaries of oceanic basins, a phenomenon known as western intensification. Western boundary currents produce chaotic flow that we here refer to as western-intensified turbulence. However, the fate of this turbulence remains unclear: How is it dissipated? Why does it sometimes self-organize into eastward jets in certain basins but not in others? This thesis aims to bridge the gap between modern modeling capabilities of western-intensified turbulence and our physical understanding of it by investigating western-intensified turbulence across a hierarchy of ocean models of increasing complexity. First, using a barotropic model, we show that the energy of western-intensified turbulence can be dissipated at a fixed rate, independent of the smallness of viscosity. The dissipation occurs in nonlinear vortices generated at the coasts but populating the entire ocean basin, while the time-averaged gyre pattern remains remarkably close to the linear response of the model to the applied wind forcing. Coherent jets are absent in the barotropic model, but we show that they emerge in a reduced-gravity model when the density stratification is sufficiently weak. In that case, both eastward and westward jets are present. In a two-layer model, we find that baroclinic instability disintegrates the westward jets into westward-drifting vortices but leaves the turbulent eastward jets intact. We identify key nondimensional parameters related to the density stratification and map out a phase diagram describing the emergence of coherent eastward jets. In the final part of this thesis, we leverage these results to study how altered stratification profiles in different climates may impact Atlantic eastward jets. Using a realistic ocean circulation model, we show that intensified stratification leads to major changes in the properties of the Gulf Stream jet and the formation of a novel eastward jet detaching from the Brazil Current.