Anatomy of North Pacific Decadal Variability

Decadal variability of the North Pacific simulated by a coupled ocean atmosphere model is investigated. In the Kuroshio/Oyashio extension (KOE) region off Japan weak spectral peaks at periods of 20 to 30 years are found in sea surface temperature, surface heat flux, and oceanic stream function. The latter are trapped in the u pper 600 to 800 m and are dominated by the first baroclinic mode, while the barotropic mode, being highly correlated at a lead of 2 years, has am plitudes of 10 to 20% of the surface baroclinic signal.

Complex empirical orthogonal analysis is used to describe the basin-wide evolution of this variability. The decadal signal off Japan is initiated about five years earlier by Ekman pumping in the central North Pacific that is associated with the canonical decadal temperature anomaly pattern. During the subsequent westward propagation, the anomalies of oceanic streamfunction are further strengthened by Ekman pumping, and, upon arrival at the we stern boundary region, lead to anomalies of western boundary transports. The associated adjustment of the oceanic thermocli ne at the base of the surface mixed layer, rather than advection, lead to temperature anomalies in the western boundary reg ion. These lag by five years and are of the same sign as those in central Pacific.

The oceanic perturbation of the KOE surface heat budget is balanced by venting of heat to the atmosphere, primarily due to latent, and secondary due to sensible heat fluxes. This anomalous forcing of the atmosphere is associated with local changes of precipitation such that warm KOE SST anomalies coin cide with an increase of rain. Whether this atmospheric response closes a feedback loop to the forcing over the central North Pacific remains unclear. The projections onto the initial wind stress forcing in the central Pacific are weak, and stochastic resonance is a possible explanation for the barely significant spectral peaks.

Comparison with available observations is difficult but confirms the five year lag between SST anomalies in the central North Pacific and in the KOE region, and the associated venting of latent heat to the atmosphere.