Intrinsic low-frequency variability and predictability of the Kuroshio current and of its extension

Investigations of the intrinsic low-frequency variability and predictability of the Kuroshio Current and of its extension jet (the Kuroshio Extension, KE) are reviewed. The Kuroshio and KE in the North Pacific constitute a western boundary current system of great relevance from climatological and ecological viewpoints. Both the Kuroshio south of Japan and the KE display remarkable changes of bimodal character on interannual time scales that are believed to be intrinsic, i.e., basically generated by nonlinear oceanic mechanisms rather than by direct atmospheric forcing. Model studies of the Kuroshio and KE with climatological forcing are thus reviewed. Moreover, as these changes are chaotic, their predictability requires peculiar mathematical approaches: theoretical results concerning this important issue are therefore reviewed as well. Model studies aimed at determining the optimal precursors and optimally growing initial errors for the Kuroshio are described. Techniques based on Lyapunov exponents (including their Lagrangian extension) and on data assimilation techniques (namely, sequential importance sampling using a particle-filtering approach) are reviewed for the KE. The key problem of how to identify the areas where targeted observations can improve the forecast is also addressed. The role of wind forcing in triggering the KE oscillations is finally considered.