Seasonal and interannual variability of the North Pacific Ocean: modeling results and their validation through altimeter data
Università di Napoli Parthenope,
Via A. De Gasperi, 5,
Altimetric measurements have allowed significant validation of hypotheses, based on modeling results, concerning the functioning of two relevant aspects of the variability of the North Pacific Ocean: (a) the wind-driven seasonal variability in the eastern tropical ocean and (b) the decadal variability of the Kuroshio Extension.
As for the first dynamical aspect, the wind-driven seasonal variability in the tropical North Pacific is analyzed by means of an ocean model implemented in an idealized North Pacific and forced by an analytic seasonally varying wind stress field derived from ECMWF winds. The oceanic response in the tropical region is found to be mainly in the form of annual beta-refracted baroclinic Rossby waves radiating from the eastern boundary, and generated by the passage of northward propagating coastal Kelvin waves which are, in turn, produced remotely by seasonally varying winds in the equatorial wave guide through a mechanism known to play a major role in the El Niño dynamics. These modeling results suggest, therefore, that the annual Rossby waves observed in the eastern tropical North Pacific may be mainly generated remotely in the equatorial band rather than by varying winds along the eastern coast, as considered in other studies. Validation of these results with TOPEX/Poseidon altimeter data is carried out. Zonal x-t diagrams of sea surface height anomalies and zonally integrated meridional transports computed from T/P data and from the winds through the Sverdrup relation by Stammer (1997) are compared with the corresponding quantities obtained from the model results. Such comparison shows that the model does capture the essential features of the wind-driven seasonal variability in the eastern tropical North Pacific.
As for the Kuroshio Extension dynamics, a model study in which forcing is provided by a time-independent climatological wind yields an internal decadal variability of the jet in significant agreement with the altimeter observations for the period 1992-2004 presented by Qiu and Chen (2005). A reduced-gravity primitive equation ocean model is implemented in a box spanning the whole North Pacific, including a schematic coastline at the western side, and the wind, though idealized, is chosen according to the ECMWF and COADS climatologies. The low-frequency variability (due to intrinsic nonlinear mechanisms) is found to be a chaotic bimodal oscillation between an energetic meandering state and a much weaker state with a reduced zonal penetration. These high and low energy states are found to be very similar to the “elongated” and “contracted” modes of the Kuroshio Extension detected through altimetric measurements, and also the period (of around 10 years) and transition details of a typical bimodal cycle are in good agreement with the above mentioned altimeter observations. A dynamical mechanism supporting this self-sustained oscillation of the modeled Kuroshio Extension is then proposed, and its strict connection with the bimodal behavior of the Kuroshio south of Japan is analyzed. On the basis of these modeling results and of their significant altimetric validation, it can be hypothesized that the observed bimodal decadal variability of the Kuroshio Extension is basically due to a self-sustained internal oscillation related to the barotropic instability of the Kuroshio south of Japan without any crucial intervention of wind-driven Sverdrup transport fluctuations and of topographic interactions, although such effects certainly play an important role in shaping the finer structure of the jet variability.