The equatorial Pacific is a region with strong negative feedbacks. Yet coupled general circulation models
(GCMs) have exhibited a propensity to develop a significant SST bias in that region, suggesting an unrealistic
sensitivity in the coupled models to small energy flux errors that inevitably occur in the individual
model components. Could this “hypersensitivity” exhibited in a coupled model be due to an underestimate
of the strength of the negative feedbacks in this region? With this suspicion, the feedbacks in the equatorial
Pacific in nine atmospheric GCMs (AGCMs) have been quantified using the interannual variations in that
region and compared with the corresponding calculations from the observations. The nine AGCMs are the
NCAR Community Climate Model version 1 (CAM1), the NCAR Community Climate Model version 2
(CAM2), the NCAR Community Climate Model version 3 (CAM3), the NCAR CAM3 at T85 resolution,
the NASA Seasonal-to-Interannual Prediction Project (NSIPP) Atmospheric Model, the Hadley Centre
Atmospheric Model (HadAM3), the Institut Pierre Simon Laplace (IPSL) model (LMDZ4), the Geophysical
Fluid Dynamics Laboratory (GFDL) AM2p10, and the GFDL AM2p12. All the corresponding coupled
runs of these nine AGCMs have an excessive cold tongue in the equatorial Pacific.
The net atmospheric feedback over the equatorial Pacific in the two GFDL models is found to be
comparable to the observed value. All other models are found to have a weaker negative net feedback from
the atmosphere—a weaker regulating effect on the underlying SST than the real atmosphere. Except for the
French (IPSL) model, a weaker negative feedback from the cloud albedo and a weaker negative feedback
from the atmospheric transport are the two leading contributors to the weaker regulating effect from the
atmosphere. The underestimate of the strength of the negative feedbacks by the models is apparently linked
to an underestimate of the equatorial precipitation response. All models have a stronger water vapor
feedback than that indicated in Earth Radiation Budget Experiment (ERBE) observations. These results
confirm the suspicion that an underestimate of the regulatory effect from the atmosphere over the equatorial
Pacific region is a prevalent problem. The results also suggest, however, that a weaker regulatory
effect from the atmosphere is unlikely solely responsible for the hypersensitivity in all models. The need to
validate the feedbacks from the ocean transport is therefore highlighted.