| Summary: | The large-scale ocean circulation is driven both by windstress forcing and by fluxes of heat and freshwater at the ocean-atmosphere surface. At midlatitudes, the most prominent features of the mainly wind-driven surface circulation are the large coherent circulation patterns (gyres) (chapter 1: Fig. 1.1). In the North Atlantic and North Pacific a relatively strong anticyclonic (subtropical) gyre is seen and a relatively weak cyclonic (subpolar) gyre. Along the western boundaries of the ocean basins strong currents exist. These currents are referred to as western boundary currents and the poleward-flowing currents carry warm water from the tropics to midlatitudes. Hence, they play an important role in the climate system. The major western boundary currents in the North Atlantic and North Pacific are called the Gulf Stream and Kuroshio, respectively. For a long time, observations have been pointing out that the Kuroshio displays bimodal meandering behavior off the southern coast of Japan (chapter 1: Fig. 1.4). For the Gulf Stream, weakly and strongly deflected paths near the coast of South Carolina have been observed (chapter 1: Fig. 1.3). This suggests that bimodal behavior may occur in the Gulf Stream as well, although less pronounced than in the Kuroshio. Apart from this bimodality, both currents show dominant variability on intermonthly timescales. Within a hierarchy of equivalent barotropic models on a ß-plane (chapter 1), it is found that multiple mean flows are dynamically possible for the North Atlantic wind-driven circulation. Besides, the modes of intermonthly variability arising as instabilities on the mean states are closely linked within this hierarchy of models. Therefore, the two main questions addressed in this thesis (chapters 3-5) are: Can the observed (possible) bimodal behavior of the Kuroshio (Gulf Stream) be explained by transitions between multiple steady states? Can the observed variability of the Gulf Stream and the Kuroshio on intermonthly timescales be explained by instabilities of the ...
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