On the synchronization between the Kuroshio Extension bimodality and the North Pacific Oscillation

The North Pacific Oscillation (NPO, the second dominant mode of sea level pressure variability in the North Pacific) is known to drive the North Pacific Gyre Oscillation (NPGO, Di Lorenzo et al., Geophys. Res. Lett., 35, L08607, 2008; Chhak et al., J. Climate, 22, 1255, 2009) and to excite Rossby waves that propagate the NPGO signature from the central North Pacific into the Kuroshio Extension (KE) region (Ceballos et al., J. Climate, 22, 5163, 2009). This, in turn, is suggested to be the cause of the synchronization between the NPGO and the KE decadal bimodality, as observed from satellite altimetry (Qiu and Chen, Deep-Sea Res., 57, 1098, 2010). In this communication modelling studies are presented, suggesting that such synchronization may be the result of the excitation -via the Rossby wave field- of a KE relaxation oscillation, whose spatial structure and evolution is basically driven by highly nonlinear intrinsic oceanic mechanisms. To arrive to this conclusion the concept of global bifurcation, of the resulting relaxation oscillations, and of the corresponding coherence resonance and synchronization with an external time-dependent forcing are first recalled in the context of dynamical systems theory by using a low-order QG ocean model (Pierini, J. Phys. Oceanogr., 41, 1585, 2011). These same notions are then applied to a reduced-gravity shallow water model of the KE bimodality (Pierini, J. Phys. Oceanogr., 36, 1605, 2006; 40, 238, 2010) in basic agreement with observations. Finally, the introduction of an idealized NPO time-dependent forcing is shown to produce a teleconnection mechanism and a timing of the KE relaxation oscillation in significant agreement with the above mentioned results. Conclusions concerning the possibility of reproducing phenomena of intrinsic lowfrequency variability of this nature in ocean general circulation models are finally presented.