Response of water vapor and clouds to El Niño warming in three National Center for Atmospheric Research atmospheric models

[1] The response of water vapor and clouds to El Niño warming over the tropical Pacific in the most recent three versions of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM2, T42 CAM3, and T85 CAM3) is quantified and compared to observations. It is found that all three models have a stronger response in the greenhouse effect of water vapor than that indicated in Earth Radiation Budget Experiment (ERBE) observations. The overestimate is most severe in the T85 CAM3 (up to 36%). Compared with National Centers for Environmental Prediction (NCEP) reanalysis, all three models have an excessive response of atmospheric moisture content in the upper troposphere, suggesting a cause for the excessive response in the greenhouse effect of water vapor. The response in the greenhouse effect of clouds (the longwave forcing of clouds) in two of the models (CAM2 and T42 CAM3) is significantly weaker than that indicated in ERBE observations, but it is comparable to the observed in the T85 CAM3. The improvement in the T85 CAM3 in simulating the response in the greenhouse effect is helped by a stronger response in the middle level clouds in the model (compared to CAM2 and T42 CAM3). In contrast to the strong negative response in the shortwave forcing seen in the ERBE observations, the response of the shortwave forcing of clouds in the CAM2 and the T42 CAM3 is weak and even has a sign opposite to observations when it is averaged over the entire region of surface warming. The simulation of the response of the shortwave forcing in the T85 CAM3 is considerably improved, though the negative response over the equatorial Pacific is still not strong enough compared to ERBE observations. The stronger negative response in the shortwave forcing in the T85 CAM3 is apparently linked to a stronger response in the middle level cloud cover. All three models fail to simulate the observed pattern of the response in the low cloud cover over the central and eastern Pacific, resulting in deficiency in the response of the shortwave cloud forcing in that region. The deficiency in the low cloud cover response suggests that the low cloud cover scheme has room for improvement.