Idealized Models of Intrinsic Midlatitude Atmosphere-Ocean Interaction
Joseph J. Barsugli
Ph.D. Dissertation
Department of Atmospheric Sciences
University of Washington
28 March 1995
Abstract
An efficient two-level global general circulation model with simplified
physical parameterizations is coupled to a 50 m constant-depth mixed-layer
ocean. This model is used to study the effect of coupling on the
intraseasonal and interannual variability of the atmosphere-ocean system in
the midlatitudes on an all-ocean planet in the absence of tropical forcing.
The coupled model circulation is compared with the atmosphere model forced
by constant zonal mean sea surface temperatures (SST's) and prescribed
time-dependent SSTs. Multidecadal integrations with constant annual-mean
insolation are used for simplicity of interpretation and to ensure
statistical significance of the results. It is found that coupling enhances
SST anomaly variance and persistence, and leads to slow eastward propagation.
These effects cannot be explained solely by the direct forcing of the
atmosphere by SST anomalies. Linear regressions are used to diagnose a
possible mechanism for these phenomena. The natural nolinear variability of
the atmosphere is the main source of low frequency variability in this
model. The feedback due to coupling with SST anomalies localizes this
variabilty in such a way as to markedly enhance persistence, and produce
propagation. The tentative explanation for the localization is found in the
interaction between the barotropic structure of the natural variability and
the baroclinic response of the atmosphere to low-level heating in this
model. In order to simply explain the role of coupling in the three
numerical model runs, a one-dimensional stochastically-forced coupled energy
balance model is developed. SST and atmosphere spectra, total variance, and
lag correlations are predicted.
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