A Theory of Interdecadal Climate Variability of the North Pacific Ocean–Atmosphere System

Abstract

A linear coupled model for the atmosphere?upper-ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the North Pacific. In this model, wind stress anomalies over the North Pacific are related to anomalies in the meridional temperature gradient of the upper ocean. The latter depends upon air?sea thermodynamical feedbacks and meridional heat transport by upper-ocean currents. Slow adjustment of the oceanic gyre circulation to the change in 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 noise wind stress forcing with organized spatial structure. Coupled dynamics produce a basin-scale interdecadal oscillatory mode. This mode arises from the dynamic coupling and the memory of the system, residing in the slow gyre circulation adjustment. Its stability is heavily controlled by the ocean thermal damping, and its period is about one and one-half to three times the decadal ocean dynamic adjustment time. In the relevant parameter regime, this coupled mode produces a robust and pronounced interdecadal spectral peak in the upper-ocean temperature and the Sverdrup transport of the gyre circulation. The interdecadal oscillations reproduced in the simple model provide insights into main physical mechanisms of the North Pacific decadal?interdecadal variability observed in nature and simulated in coupled general circulation models.