Journal
CLIMATE DYNAMICS
Volume 41, Issue 11-12, Pages 2909-2920Publisher
SPRINGER
DOI: 10.1007/s00382-013-1839-0
Keywords
Precipitation distribution; Tropical climate; CMIP5; Double ITCZ; Cold tongue bias
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Funding
- Green Network of Excellence (GRENE) Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan
- Environment Research and Technology Development Fund of the Ministry of the Environment, Japan [A-1201]
- U.S. Department of Energy
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Reproducibility of precipitation distribution over the tropical oceans for the recent dataset of the Coupled Model Intercomparison Project phase 5 (CMIP5) is investigated and compared to CMIP3. The Taylor skill score for the reproducibility of the CMIP5 multi-model ensemble mean (0.64) is slightly higher than that of CMIP3 (0.60), but the difference is not statistically significant. Still, there is some evidences that the double intertropical convergence zone (ITCZ) bias is mitigated from CMIP3 to CMIP5, whereas the cold tongue bias remains similar. An inter-model empirical orthogonal function analysis shows that these two biases are closely related to the dominant inter-model discrepancies of precipitation patterns. The two biases are attributed to two factors, respectively. In the CMIP5 models with the prominent double ITCZ, the deep convection is not sensitive enough to environmental air humidity at the lower-mid troposphere, as is in CMIP3. Thus, the deep convection is not suppressed even over the dry subsidence region of the southeastern Pacific, forming the double ITCZ bias. Conversely, models with the severe cold tongue bias have lower ocean model resolution with too strong equatorial trades. Therefore, proper representation of the sensitivity of deep convection to humidity and higher resolution of the ocean models with better equatorial trades are important for reducing the double ITCZ and the cold tongue biases.
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