On the interpretation of AGCM response to prescribed
time varying SST anomalies
(A. Czaja and J. Marshall, MIT)
Geophysical Research Letters, vol 27, No 13, 1927-1930, July 1, 2000
Abstract:
Recently, Bretherton and Battisti (1999) have presented an interesting
interpretation of ensemble experiments with atmospheric general
circulation models (AGCMs) forced by observed sea surface temperature
(SST) whose mean successfully simulates the decadal evolution
of the observed North Atlantic Oscillation (NAO) index.
Using a linear model of
atmosphere/ocean interaction, they plausibly argue
that this hind-cast skill, as measured by low-pass correlations
between observed and simulated indices, is
consistent with the ocean mixed layer merely
integrating stochastic surface heat
flux forcing governed by the natural variability of the atmosphere.
They go on to suggest, however, that predictability associated with
middle-latitude SST anomalies is limited to timescales associated with the
thermal inertia of the oceanic mixed layer (perhaps a year).
Here, we include ocean circulation in a simple coupled ocean-
atmosphere model and also consider hypothetical limits in which the coupled
system is highly predictable at low frequencies. We find that low pass
correlations between observed and simulated NAO indices, obtained from
ensembles of SST-forced AGCMs, are insensitive to the predictability
of the system. Thus inferences about predictability of the atmosphere-ocean
system cannot be made on the basis of this measure of the
hindcast skill of atmosphere-only simulations.
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