Kuroshio Extension Bimodality and the North Pacific Oscillation: A Case of Intrinsic Variability Paced by External Forcing

A previous North Pacific Ocean circulation model forced by climatological winds is extended here to include a time-dependent North Pacific Oscillation (NPO) forcing. The Kuroshio Extension (KE) decadal bimodal cycle (which is a self-sustained intrinsic relaxation oscillation in the climatologically forced case) is now excited by the NPO forcing. Both the timing of the cycles and the Rossby wave teleconnection mechanism that is found to govern the synchronization from 1993 to 2012 are in good agreement with altimeter observations. Sensitivity numerical experiments are carried out by varying the zonal location and amplitude of the NPO forcing, and the lateral eddy viscosity. The emergence of the KE bimodality with a correct timing is found to be extremely sensitive to changes in the dissipative parameterization; the implications of such sensitivity for deficiencies found in more realistic North Pacific Ocean general circulation models are discussed. The dynamical mechanism that emerges from this study is explained as a case of intrinsic variability in an excitable dynamical system triggered, and therefore paced, by an external forcing.