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Updated 12 October, 2003
National Assessment of
HADCM2 seems to show better ENSO signatures than CGCM1 in terms of several
sea surface temperature (SST) indices ( NINO
), these signatures are the result of variance in the tropical oceans that
is too high, both compared to observations and every other GCM, including
HADCM3. As part of the Coupled Model Intercomparison Project (CMIP),
power spectra of globally and annually averaged surface air temperatures
from several CMIP models (unforced control simulations), including HADCM2
( spectra ), show
that the ENSO-time period variances (< 10 years) in HADCM2 are too high
compared to observational variance at those time periods. Notice
also that this error has been corrected in HADCM3.
Mark Stevens from CCCma has calculated
several correlation coefficients to highlight tropical variances in
several models. All of these experiments are unforced control
simulations, which can be used to understand the model variability.
The upper left panel in each figure is the observations.
Figure 1 (above): The
correlation between the global annual mean and the annual mean at each
grid point shows that
Figure 2 (above): The
correlation between the NINO3 annual mean and the annual mean at each
grid point shows that all the models reproduce the NINO3 ENSO
signature. As Legler and O'Brien have shown (
) the amplitude of the NINO3 signature is too weak in CGCM1, but the
strength of the NINO3 signature in HADCM2 is due to an overprediction of
variance in the entire tropical Pacific, rather than a specific ENSO
Figure 3 (above): The standard deviation of the annual mean surface temperature anomalies confirms that the tropical variance in CGCM1 is less than that in HADCM2.