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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