We Need to Simulate More Northern ITCZs and Less Southern ITCZs Over the East Pacific Ocean in Coupled Climate Models
Year: 2025
Type: article
Abstract: 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.
DOI: 10.1029/2025jd043650
Language: English
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences
SDG: Climate action
Open Access status: gold
Estimated APC paid: $4,070
Awards: Nonlinear Dynamics of Daily-weekly Boreal Spring InterTropical Convergence Zone (ITCZ) Shifts over the Eastern Pacific Ocean, Collaborative Research: The Relationship between the Trade Wind Inversion Layer and the Seasonal Development of the Southeast Pacific Inter-Tropical Convergence Zone (ITCZ)
OpenAlex ID: W4417442006