Observational and analytic analysis of the Pacific Decadal Oscillation

Decadal to interdecadal timescale variability in the Pacific region, commonly referred to as Pacific Decadal Oscillation (PDO), is studied in this research by using observational data, numerical and analytic models. Besides PDO, it is well known that the El Niño/Southern Oscillation (ENSO) shows its major signatures in the region. The striking similarity between the spatial features of ENSO and PDO suggests that these two phenomena may be generated by similar mechanisms.; The phase transition mechanism for ENSO has been suggested to be related to the recharge/discharge of the equatorial region. The recharge/discharge is realized by the collective effects of the equatorial oceanic waves. It is demonstrated in our research that the Very-Low-Frequency (VLF) eigen modes of the equatorial β-plane bounded in zonal direction are central for the understanding of the recharge/discharge process. These slow and free eigen modes, which have zonal uniform thermocline changes in the equatorial region, might also play an important role in the generation of decadal to interdecadal climate variability.; The recharge/discharge process is thus analyzed through idealized numerical experiments of the equatorial thermocline response to wind stress forcing in the off-equatorial region. The off-equatorial wind stress anomalies generate zonal uniform thermocline responses in the equatorial region. The spatial structures of the responses are very similar to the VLF modes. The eastern subtropical region is identified as the most effective region in generating equatorial thermocline response.; Furthermore, a coupled analytic model is formulated. It has the equatorial β-plane dynamics coupled with wind stress of fixed spatial patterns and an amplitude proportional to sea surface temperature (SST) anomaly in the eastern equatorial Pacific. The SST anomaly is governed by a simple thermal dynamic equation used for ENSO modeling. The analytic eigen solutions are solved. We find that when a warm SST is coupled with a cyclonic wind stress pattern in the eastern subtropical Pacific, an oscillation with timescale around 10–15 years could be generated. In contrast, when a warm SST is coupled with only a westerly wind stress in the central equatorial Pacific, an ENSO-like oscillation could be generated with timescale around 3 years. Sensitivity of these two kind coupled modes to different parameters is tested. Numerical results with a reduced-gravity shallow-water model in a similar coupled framework are consistent with the results of the analytic model. Our model results lead us to suggest that broad-scale tropical ocean-atmosphere interaction is the most important source for the generation of PDO.