A THEORY OF INTERDECADAL CLIMATE VARIABILITY OF THE NORTH PACIFIC OCEAN-ATMOSPHERE SYSTEM

A linear coupled model for the atmosphere-upper-ocean system is propos ed to highlight the mechanisms of decadal to interdecadal climate vari ability in the North Pacific. In this model, wind stress anomalies ove r the North Pacific are related to anomalies in the meridional tempera ture gradient of the upper ocean. The latter depends upon air-sea ther modynamical feedbacks and meridional heat transport by upper-ocean cur rents. Slow adjustment of the oceanic gyre circulation to the change i n wind stress is accomplished by the forced baroclinic oceanic Rossby waves, which carry out the meridional heat transport. Uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white no ise wind stress forcing with organized spatial structure. Coupled dyna mics produce a basin-scale interdecadal oscillatory mode. This mode ar ises from the dynamic coupling and the memory of the system, residing in the slow gyre circulation adjustment. Its stability is heavily cont rolled by the ocean thermal damping, and its period is about one and o ne-half to three times the decadal ocean dynamic adjustment time. In t he relevant parameter regime, this coupled mode produces a robust and pronounced interdecadal spectral peak in the upper-ocean temperature a nd the Sverdrup transport of the gyre circulation. The interdecadal os cillations reproduced in the simple model provide insights into main p hysical mechanisms of the North Pacific decadal-interdecadal variabili ty observed in nature and simulated in coupled general circulation mod els.