Daily precipitation ITCZ states for observations, reanalyses, and 25 CMIP6 models for “We need to simulate more northern ITCZs and less southern ITCZs over the east Pacific Ocean in coupled climate models.”

Tropical precipitation biases have persisted since the very first generations of climate models. These biases are highly sensitive to the region and/or season of interest, with commonalities in the east Pacific and Atlantic Ocean basins, possibly due to their similar observed climatological northern hemisphere intertropical convergence zone (ITCZ). However, the colloquial name “double ITCZ bias” comes from a time and/or zonal mean, and may be missing important information about errors at smaller time and space scales. In this study, we explore daily characteristics of the ITCZ in observations, reanalyses, and 25 Coupled Model Intercomparison Project 6 (CMIP6) models over the east Pacific Ocean. We devise and apply an algorithm that determines a region's dominant daily ITCZ configuration, its “ITCZ state,” based on the daily mean precipitation field. The five ITCZ states include: northern hemisphere (nITCZ), southern hemisphere (sITCZ), double (dITCZ), equatorial (eITCZ), and absent (aITCZ). We find that nearly all CMIP6 models gravely underestimate nITCZs and overestimate sITCZs during January through May, in contrast with what “double ITCZ bias” suggests. Surprisingly, all reanalyses also underestimate nITCZs and overestimate eITCZs. Errors in ITCZ state interannual variability are consistent with mean errors in reanalyses, while sITCZ interannual variability is far too low relative to the mean in most CMIP6 models. Lastly, all reanalyses and CMIP6 models overestimate precipitation rates in the southern hemisphere ITCZ band for dITCZs and sITCZs, suggestive of errors with atmospheric origins.