Abstract
The authors explore the use of the "neutral vectors'' of a linearized
version of a global quasigeostrophic atmospheric model with realistic
mean flow in the study of the nonlinear model's low-frequency variability.
Neutral vectors are the (right) singular vectors of the linearized
model's tendency matrix that have the smallest eigenvalues; they
are also the patterns that exhibit the largest response to forcing
perturbations in the linear model. A striking similarity is found
between neutral vectors and the dominant patterns of variability
(EOFs) observed in both the full nonlinear model and in the real
world. The authors discuss the physical and mathematical connection
between neutral vectors and EOFs. Investigation of the öptimal forcing
patterns''-the left singular vectors-proves to be less fruitful.
The neutral modes have equivalent barotropic vertical structure,
but their optimal forcing patterns are baroclinic and seem to be
associated with low-level heating. But the horizontal patterns of
the forcing patterns are not robust and are sensitive to the form
of the inner product used in the singular vector decomposition analysis.
Additionally, applying öptimal'' forcing patterns as perturbations
to the full nonlinear model does not generate the response suggested
by the linear model.
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