On the Radiative and Dynamical Feedbacks over the Equatorial Pacific Cold Tongue

Abstract

An analysis of the climatic feedbacks in the NCAR Community Climate Model, version 3 (CCM3) over the equatorial Pacific cold tongue is presented. Using interannual signals in the underlying SST, the radiative and dynamical feedbacks have been calculated using both observations and outputs from the NCAR CCM3. The results show that the positive feedback from the greenhouse effect of water vapor in the model largely agrees with that from observations. The dynamical feedback from the atmospheric transport in the model is also comparable to that from observations. However, the negative feedback from the solar forcing of clouds in the model is significantly weaker than the observed, while the positive feedback from the greenhouse effect of clouds is significantly larger. Consequently, the net atmospheric feedback in the CCM3 over the equatorial cold tongue region is strongly positive (5.1 W m?2 K?1), while the net atmospheric feedback in the real atmosphere is strongly negative (?6.4 W m?2 K?1). A further analysis with the aid of the International Satellite Cloud Climatology Project (ISCCP) data suggests that cloud cover response to changes in the SST may be a significant error source for the cloud feedbacks. It is also noted that the surface heating over the cold tongue in CCM3 is considerably weaker than in observations. In light of results from a linear feedback system, as well as those from a more sophisticated coupled model, it is suggested that the discrepancy in the net atmospheric feedback may have contributed significantly to the cold bias in the equatorial Pacific in the NCAR Climate System Model (CSM).