Daily ITCZ States over the East Pacific in Observations, Reanalyses, and CMIP6 Models

The persistence of tropical precipitation biases in and near the ITCZ for nearly 30 years continues to puzzle many scientists. These biases can be highly sensitive to the region and/or season of interest, with strong commonalities in the east Pacific and Atlantic Ocean basins possibly due to their similar climatological northern hemisphere ITCZ position in observations. In this study, we devise and apply an algorithm that determines a region’s dominant daily ITCZ configuration, its “ITCZ state,” based on a snapshot of the precipitation field over the east Pacific and Atlantic basins. The five ITCZ states (Haffke et al. 2016) include: northern hemisphere (nITCZ), southern hemisphere (sITCZ), double (dITCZ), equatorial (eITCZ), and absent (aITCZ).

We compare results between observational data sets (TRMM, IMERG) and reanalyses (ERA5, MERRA2, and CFSR) for various overlapping time periods from 1980–2022. Preliminary results suggest that the second most common ITCZ configuration over the east Pacific after nITCZ is either sITCZ or dITCZ, both of which occur during the months of February–April. At the same time, strong interannual variability exists for the eITCZ over the east Pacific, with strong east Pacific El Niño years dominated by eITCZ. Over the Atlantic, the second most common ITCZ configuration after nITCZ is either sITCZ or eITCZ, which occur during the months of January–April. Early results from CMIP6 models show a plethora of different biases in ITCZ configurations and seasonal timing compared to observations and reanalyses, e.g., with some models overproducing eITCZ and sITCZ, and others underproducing eITCZ and dITCZ and overproducing sITCZ. Almost all models underproduce nITCZs and overproduce dITCZ and/or sITCZ during the boreal winter and spring months, which are the months with the largest double ITCZ biases in monthly climatological data.