Two main modes of SST and atmospheric variation are found in the Pacific domain – the El Nino-Southern Oscillation (ENSO), whose signature is most pronounced in the tropics, and the lower-frequency Pacific Decadal Oscillation (PDO), whose signature is most pronounced in the mid-latitudes. The complex interplay between these modes has been the subject of considerable debate in recent years. Is it the tropics – ENSO, or the mid-latitudes – PDO, that govern the relationship?

Using the 1948-2001 data set of NCEP wind reanalysis, we investigate the interaction between the tropics and the mid-latitudes on ENSO and PDO time scales over the Pacific domain. We address this issue by focusing on the kinetic energy interchange between triplets of frequencies encompassing the tropical oscillations on 24-60 month time scales associated with ENSO and the extra-tropical oscillations on 90+ month time scales associated with the PDO. A co-spectrum formulation is used to calculate the transfer of kinetic energy into/out of the PDO or into/out of the ENSO frequencies resulting from the interaction with two other ENSO or PDO frequencies.