Decadal Modulation of ENSO:
Processes that cause decadal variability in an intermediate coupled
ocean-atmosphere model of the Pacific basin, both at northern midlatitudes
and in the tropics, are studied. The model's ocean component is a
variable-temperature 3 1/2-layer system. Its atmospheric component
consists of two basic parts: an empirical model, constructed from patterns
obtained by the singular value decomposition (SVD) statistical technique,
that determines wind stress anomalies from model sea-surface temperature
(SST), and a simple representation of the planetary boundary layer to
calculate the surface heat-flux anomalies. A third part specifies stochastic
wind-stress forcing from observed variability. In addition, the model is
specifically designed to separate tropical and extratropical interactions,
such that the tropics can force the extratropics through the atmosphere but
the extratropics can only feedback to the tropics through the ocean.
Solutions develop two types of oscillations: an ENSO-like interannual mode
and a decadal mode. As in many models of ENSO, the interannual mode is
driven by positive, ocean-atmosphere feedbacks near the equator, and
time-delayed negative feedback is provided by off-equatorial Rossby waves.
For parameter choices that amplify midlatitude coupling by 30% (
phi_o=1.3), a self-sustained decadal oscillation develops in the North
Pacific without any tropical interactions. Diagnostic analyses show that it
is maintained by ocean-to-atmosphere feedbacks in the northwest and
subtropical northeast Pacific, and by atmospheric teleconnections from those
regions to the northeast ocean. For weaker coupling (phi_o=1.2), the
decadal mode is damped. In this case, the mode can be sustained by
atmospheric teleconnections from the tropics associated with the interannual
mode, but not by extratropical stochastic forcing. Although including
stochastic forcing does generate variability at decadal time scales, a
distinct decadal spectral peak only exists when the decadal mode is active.
Decadal variability is carried to the equator by variations in the
transport, rather than temperature, of the North Pacific Subtropical Cell.
These variations modulate near-equatorial SST by altering the amount of
cool, thermocline water that upwells in the eastern equatorial Pacific,
which in turn feeds back to the interannual mode.
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