| The bimodal behavior of the Kuroshio Extension (KE) near Japan in the North Pacific has fascinated physical oceanographers since indications of this phenomenon were found in the early seventies. Why would a western boundary ocean current switch between a large-meander and a small- meander state and back in a few years time? Why does this phenomenon not occur in other western boundary currents, such as the Gulf Stream in the North Atlantic? With the analysis of satellite data and those of in-situ measurements, a quite detailed description of the different states of the KE and their transition behavior is now available. There is still, however, no consensus on which processes cause the low-frequency variability in the KE. It appears that direct interpretation of the observations often has been based on mechanisms involving external causes (such as atmospheric forcing) and linear ocean dynamics while ocean modelers have tended to suggest mechanisms which involve elements of nonlinear ocean dynamics. A theory of the bimodality of the Kuroshio Extension should at least explain (i) why the KE can be in different states and the origin of the spatial patterns of these states, and (ii) the decadal time scale of the transition between the two states. In this presentation a nonlinear theory is proposed which is able to provide explanations of the issues above. |
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