Ocean modelling and altimeter data reveal the possible occurrence of intrinsic low-frequency variability of the Kuroshio Extension

In this paper the combined use of model simulations and altimeter data suggests that, during the interval 1998-2006, the Kuroshio Extension (KE) low-frequency variability was mainly of intrinsic origin. The model is based on the primitive equations for a reduced-gravity ocean, is eddy-permitting and relatively idealized, but it includes essential elements of realism and is driven by a time-independent climatological wind field, so that the modelled variability is necessarily of intrinsic oceanic origin. The altimeter data are provided by AVISO and include all the data available to date. A new composite index Λ[Φ] specifically conceived for the KE is used in the analysis of both model and altimeter data (Φ gives the mean latitudinal position of the KE jet and Λ is a measure of the high-frequency variability of the KE path). A set of sensitivity numerical experiments shows that the frontal dynamics is extremely sensitive to the horizontal eddy viscosity. A self-sustained relaxation oscillation emerging from a reference simulation is represented by a loop in the (Λ, Φ)-plane and is characterized by four phases, each one being controlled by a specific dynamical mechanism. An interval ranging from January 1998 to March 2006 is identified in the altimeter data, during which the altimeter-derived index is very similar to the modelled one. A detailed analysis based also on sea surface height maps shows that the real KE evolution possesses four phases characterized by basically the same dynamical behaviour recognized in the model loop. The conclusion is that, during that interval, the KE dynamics was controlled by an oceanic intrinsic mode of low-frequency variability and that the directly driven dynamics and ocean-atmosphere coupled modes of variability did not overshadow the intrinsic evolution. The self-sustained vs excited character of the relaxation oscillation is finally discussed.