What causes the excessive response of clear-sky greenhouse effect to El Niño warming in Community Atmosphere Models?

Abstract

To diagnose the causes of an excessive response of the clear-sky greenhouse effect to El Niño warming in the Community Atmosphere Models (CAMs), the response of both water vapor and temperature to El Niño warming in the models is examined as a function of height. The percentage response of water vapor to El Niño warming in the models is considerably stronger than the response in the NCEP reanalysis in the middle and upper troposphere (700-300 mbar). The maximum discrepancy with NCEP data at 500 mbar reaches 18%/K in CAM3. The discrepancy in the temperature response between the models and NCEP data at all tropospheric levels is within 0.3 K/K, with the maximum discrepancy occurring in the immediate neighborhood of 600 mbar. The comparison between the models and ERA-40 reanalysis leads to the similar results. Employing a radiative model, we have calculated the contributions of the excessive water vapor response in the middle and upper troposphere and the contributions from the differences in the lapse rate response to the discrepancies seen in the clear-sky greenhouse effect. The results confirm that the main cause of the excessive response of the clear-sky greenhouse effect is an excessive response of water vapor in the middle and upper troposphere. The excessive response of upper tropospheric water vapor is found to be accompanied with an excessive response in the upper cloud cover and vertical motion. Biases in both phases of ENSO contribute to these excessive responses to ENSO. © 2008 by the American Geophysical Union.