GISS Lunch Seminar
Speaker: Alex Gonzalez (WHOI)
Title: We need to simulate more double ITCZs and less southern ITCZs in reanalyses and coupled climate models

Abstract:
Model tropical precipitation biases in the Inter-Tropical Convergence Zone (ITCZ) have persisted for nearly 30 years, colloquially known as the "double ITCZ bias." While the double ITCZ bias phrase comes from a time and/or zonal mean of seasonal or annual precipitation data, biases are most prominent during boreal winter and spring when models overestimate a southern hemisphere ITCZ, but not always a double ITCZ. In this study, we explore high-resolution characteristics of the ITCZ in three observations, four reanalyses, and 25 Coupled Model Intercomparison Project 6 (CMIP6) models over the east Pacific Ocean with a goal of improving understanding of the building blocks of what makes up seasonal and annual ITCZ biases.

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 dITCZs and nITCZs and overestimate sITCZs during January through May, in contrast with what the double ITCZ bias phrase suggests. Surprisingly, all reanalyses also underestimate dITCZs and overestimate sITCZs. 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 the seven CMIP6 models that can produce dITCZs overestimate precipitation rates in dITCZs, while nearly all CMIP6 models overestimate precipitation rates in sITCZs.