In this communication a minimal model of the Kuroshio Extension decadal bimodality is discussed. The eddy-permitting model is based on the reduced-gravity shallow water equations and is forced by a time-independent wind field. The obtained chaotic decadal variability (of intrinsic oceanic origin) is explained in terms of a relaxation oscillation, and is shown (Pierini et al., J. Phys. Oceanogr., 39, 2212, 2009) to be in substantial agreement with altimeter observations (e.g., Qiu and Chen, Deep-Sea Res., 57, 1098, 2010; Sugimoto and Hanawa, J. Oceanogr., 68, 219, 2012) as far as (i) the spatial structure of the mean jet and of the elongated and contracted jet states, (ii) the jet path length and mean latitudinal position, and (iii) the time evolution and temporal scales are concerned. By applying the methods of nonlinear dynamical systems theory, we then explain relevant dynamical features of the modeled flow, such as the origin of the relaxation oscillation as a consequence of a homoclinic bifurcation, the spatio-temporal character of the bimodal behavior, and the degree of predictability of the flow in the different stages of the oscillation (evaluated through a field of finite-time Lyapunov exponents and its Lagrangian counterpart). The predictability issue is also addressed by using sequential importance sampling to assess the impact of observations on an ensemble prediction (Kramer et al., J. Phys. Oceanogr., 42, 3, 2012). This particlefiltering approach gives access to the probability density of the state vector, which allows the predictive power—an entropy-based measure—of the ensemble prediction to be determined. Optimal observation locations are found to correspond to regions with strong potential vorticity gradients. For the elongated state the optimal location is in the first meander of the Kuroshio Extension; during the contracted state it is located south of Japan, where the Kuroshio separates from the coast.
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