The Double‐ITCZ Bias in CMIP3, CMIP5, and CMIP6 Models Based on Annual Mean Precipitation

• Published in: Geophysical research letters Vol. 47; no. 8
• Main Authors: Tian, Baijun, Dong, Xinyu
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-04-2020
• Subjects: Annual; Annual precipitation; Bias; Climate Models; Climate prediction; CMIP3; CMIP5; CMIP6; Convergence; Convergence zones; Double‐ITCZ bias; Errors; Future climates; Intercomparison; Intertropical convergence zone; Mean precipitation; Northern Hemisphere; Precipitation; Tropical climate; Tropical environments
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL087232

The double‐intertropical convergence zone (ITCZ) bias is one of the most outstanding errors in all previous generations of climate models. Here, the annual double‐ITCZ bias and the associated precipitation bias in the latest climate models for Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) are examined in comparison to their previous generations (CMIP Phase 3 [CMIP3] and CMIP Phase 5 [CMIP5]). All three generations of CMIP models share similar systematic annual multi‐model ensemble mean precipitation errors in the tropics. The notorious double‐ITCZ bias and its big inter‐model spread persist in CMIP3, CMIP5, and CMIP6 models. Based on several tropical precipitation bias indices, the double‐ITCZ bias is slightly reduced from CMIP3 or CMIP5 to CMIP6. In addition, the annual equatorial Pacific cold tongue persists in all three generations of CMIP models, but its inter‐model spread is reduced from CMIP3 to CMIP5 and from CMIP5 to CMIP6.
Plain Language Summary
The double‐intertropical convergence zone (ITCZ) bias is one of the most outstanding errors in all previous generations of climate models that may reduce the reliability of future climate prediction based on these models. The models have two ITCZs (i.e., zonally elongated narrow belt of high precipitation) in both hemispheres, over the equatorial central and eastern Pacific and Atlantic, instead of one ITCZ over the northern hemisphere in observations except for a short period in March and April. Here, we examine such bias in the latest models for Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) based on long‐term annual mean precipitations in observations and models and compare the biases among the recent three generations of CMIP models (CMIP3, CMIP5, and CMIP6). We find that the double‐ITCZ bias with a big inter‐model spread persists in all CMIP models and still remains a serious problem in the latest CMIP6 models. However, the bias is slightly reduced in CMIP6 models from CMIP3 or CMIP5 models based on several precipitation bias indices. In addition, the annual equatorial Pacific cold tongue bias also persists in all CMIP models, but its inter‐model spread is reduced from CMIP3 to CMIP5 and from CMIP5 to CMIP6.
Key Points
CMIP3, CMIP5, and CMIP6 models share similar systematic annual multi‐model ensemble mean precipitation errors in the tropics
The annual double‐ITCZ bias and its big inter‐model spread persist in CMIP3/5/6 models but are slightly reduced from CMIP3/5 to CMIP6
The annual equatorial Pacific cold tongue bias persists in CMIP3/5/6, but its inter‐model spread is reduced from CMIP3 to CMIP5 and to CMIP6