A quantitative assessment of precipitation associated with the ITCZ in the CMIP5 GCM simulations

Abstract

According to the Intergovernmental Panel on Climate Change 5th Assessment Report, the broad-scale features of precipitation as simulated by Phase 5 of the Coupled Model Intercomparison Project (CMIP5) are in modest agreement with observations, however, large systematic errors are found in the Tropics. In this study, a new algorithm has been developed to define the North Pacific Intertropical Convergence Zone (ITCZ) through several metrics, including: the centerline position of the ITCZ, the width of the ITCZ, and the magnitude of precipitation along the defined ITCZ. These metrics provide a quantitative analysis of precipitation associated with the ITCZ over the equatorial northern Pacific. Results from 29 CMIP5 Atmospheric Model Intercomparison Project (AMIP) Global Circulation Model (GCM) runs are compared with Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) observations. Similarities and differences between the GCM simulations and observations are analyzed with the intent of quantifying magnitude-, location-, and width-based biases within the GCMs. Comparisons show that most of the GCMs tend to simulate a stronger, wider ITCZ shifted slightly northward compared to the ITCZ in GPCP and TRMM observations. Comparisons of CMIP and AMIP simulated precipitation using like-models were found to be nearly equally distributed, with roughly half of GCMs showing an increase (decrease) in precipitation when coupled (decoupled) from their respective ocean model. Further study is warranted to understand these differences.