Atmospheric Feedbacks Over the Pacific Cold-Tongue: Results From Models and Observations

Abstract

The equatorial Pacific cold-tongue plays a fundamental role in the heat and carbon balance of the coupled ocean-atmosphere system and is a major source region of climate variability. The processes and feedbacks that control the cold-tongue, however, are not well understood. The state-of-the-art coupled general circulation models (GCMs) have a tendency to develop an excessive cold-tongue in the equatorial Pacific. To better understand the feedbacks in this region and in particular to test the hypothesis that the tendency for the coupled GCMs to develop an excessive cold-tongue is due to a weak regulating effect from the model atmosphere, we have quantified the feedbacks in the equatorial Pacific in nine leading atmospheric GCMs using the ENSO signal in that region, and compared with the corresponding calculations from observations.

We find that:

• The net atmospheric feedback over the equatorial Pacific in two GFDL models is comparable to the observed value.
• All other models have a weaker net atmospheric feedback-- a weaker regulating effect on the underlying SST than the real atmosphere.
• Most of the models underestimate the strength of the negative feedbacks: the cloud albedo feedback and the feedback from the atmospheric transport.
• All models overestimate the positive feedback from water vapor.

The underestimate of the strength of the negative feedbacks by the models is apparently linked to an underestimate of the equatorial precipitation response. These results confirm the suspicion that an underestimate of the regulating effect from the atmosphere over the equatorial Pacific is a prevalent problem. The results also suggest, however, that a weaker regulating effect from the atmosphere is unlikely solely responsible for the development of an excessive cold-tongue in all models. The need to evaluate the feedbacks from the ocean dynamics is therefore highlighted.