Evaluation of precipitation and ITCZ characteristics in CMIP6 models

The Intertropical Convergence Zone (ITCZ) is a band of low pressure near the equator, where the trade winds converge. It is usually accompanied by cloudiness and heavy precipitation, and it migrates northward and southward, following the sun in different seasons. Climate models often misrepresent key atmospheric processes, including ITCZ's position, width and strength. As a result, biases in the modeled precipitation are also common in tropical and sub-tropical regions, such as the Indian subcontinent and parts of South America. Here, we assess the skill of four state-of-the-art Earth System Models in representing key ITCZ characteristics and the associated precipitation. The four ESMs under investigation are EC-EARTH, CNRM-ESM, IPSL-ESM, and UKESM. Besides the CMIP6 version of the aforementioned models, we also aim to evaluate post-CMIP6 simulations, which are currently under development in the framework of the OptimESM Horizon Europe project (https://optimesm-he.eu/). These post-CMIP6 models include advancements in the representation of physical, biogeochemical and biophysical processes. As a reference dataset, we use the ERA5 reanalysis data. Firstly, we divide the world into eight longitudinal zones and then calculate the zonal averages. For each season, we define the ITCZ location as the latitude where there is a peak in the 500hPa vertical velocity, while we consider the edges of the ITCZ at the latitudes where the 500hPa vertical velocity becomes zero. The strength of the ITCZ is defined as the value of the rainfall peak associated with the peak in the vertical velocity field. The analysis is performed on an annual basis, for each year from 1981 through 2010, and the corresponding peaks are clustered. The long-term characteristics of the ITCZ from the ESMs are compared to those from the ERA5 to understand the processes that drive precipitation biases in the global tropics.