Schneider, N., A. J. Miller and D. W. Pierce, 2002:

Anatomy of North Pacific decadal variability

Journal of Climate, 15, 586-605.

Abstract. Decadal variability in the North Pacific simulated by a coupled ocean atmosphere model is investigated. It is expressed most prominently in anomalies of oceanic stream function in the upper 500m of the water column, of sea surface temperature and of latent heat flux in the Kuroshio-Oyashio extension (KOE) region off Japan. The decadal signal off Japan is initiated by winter-time Ekman pumping over the central North Pacific over the preceding five years. The winter time forcing in the North Pacific is associated with meridional shifts of the position and changes in the strength of the Aleutian Low, that also force sea surface temperature (SST) anomalies in the central North Pacific and with opposite signs in the Gulf of Alaska, off California and off Hawaii. The polarity of SST and Ekman pumping is such that warm central and cool eastern Pacific anomalies are associated with northward anomalies of western boundary transports. In the KOE region anomalous western boundary currents lead to subsurface temperature anomalies that are of the same sign as those in central Pacific. During winter time, deep mixed-layers bring this temperature signal to the surface where it is damped by air-sea heat exchange, primarily by latent, and secondarily by sensible heat fluxes. This forcing of the atmosphere in the KOE region is associated with local changes of winter precipitation such that warm KOE SST anomalies coincide with an increase of rain. Apart from this local, winter time feedback, the projection onto annual-averaged wind-stress in the central Pacific is, at best, a weak positive feedback. The dynamics underlying the decadal evolution of the oceanic stream function are consistent with a linear, equivalent barotropic, forced Rossby wave. The decadal time scale results from the integration of stochastic Ekman pumping along Rossby wave trajectories. The Ekman pumping is primarily due to internal atmospheric variability that has significant expressions world-wide. A positive feedback between coupled model KOE stream function and North Pacific wind stress curl is consistent with the enhanced variance of the coupled model at a 20-30 year period. However, the time series are too short to confidently distinguish this positive feedback hypothesis from sampling variability or from forcing that originates outside of the mid-latitude North Pacific. Comparisons with available observations confirm the seasonality of the forcing, the like signs of SST anomalies in the central North Pacific and in the KOE region, the up to five year lag between these SST anomalies, and the associated damping in the KOE region by the latent heat. In accordance with the coupled model results, observed SST anomalies in the KOE region can be predicted from the history of the wind stress curl over the North Pacific.

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