WHOI PO

Abstract:

The persistence of precipitation biases in and near the Inter-Tropical Convergence Zone (ITCZ) for nearly 30 years continues to puzzle many scientists. These biases are 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 observed climatological northern hemisphere ITCZ. However, the colloquial name “double ITCZ bias” comes from a time and/or zonal mean and many studies do not acknowledge that these biases are most prominent during boreal winter and spring when models overestimate a southern hemisphere ITCZ, not a double ITCZ. 

In this study, we explore high resolution characteristics of single and double ITCZs in observations, four reanalyses, and 25 models from the sixth Coupled Model Intercomparison Project (CMIP6). 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 Ocean basin. The five ITCZ states (Haffke et al. 2016) include: northern hemisphere (nITCZ), southern hemisphere (sITCZ), double (dITCZ), equatorial (eITCZ), and absent (aITCZ). Our new ITCZ state dataset computes important daily statistics about the ITCZ, including mean precipitation rate, spatial variability, latitude, and width. We show that all CMIP6 models and reanalyses underestimate dITCZs and overestimate sITCZs over the east Pacific Ocean. 

To better understand the convective environment of single ITCZs vs. dITCZs during boreal winter and spring, we examine all four reanalyses and eight CMIP6 models that have a significant percentage of dITCZs alongside observations. We find that all reanalyses are able to reproduce the correct ranking of mean precipitation rates – nITCZ (largest), sITCZ, and dITCZ (smallest). However, five out of the eight CMIP6 models produce too much precipitation in the southern hemisphere ITCZ band. We hypothesize that most CMIP6 models are unable to capture the favorable convective environment for dITCZs, which could have implications for improving longstanding tropical precipitation biases in all models.