Robustness of Double ITCZ Biases to the Use of Explicit Convection in the Superparameterized Community Earth System Model

Double ITCZ biases (such as a spurious east Pacific rainfall band south of equator and excessively extended equatorial SST cold tongue) have been a hard problem in many coupled global climate models (GCMs) for more than two decades. While there is no complete theory on their causes and underlying physical mechanisms, traditional cumulus parameterizations have been attributed as one of the major potential bias sources. To explore the effect of replacing traditional cumulus schemes with explicit embedded convection, we conducted multi-member ensemble hindcast simulations using a superparameterized version of Community Earth System Model. Our results show the double ITCZ biases are not eliminated by superparameterization, but do somewhat improve insofar as the SST cold tongue bias becomes weaker and the meridional location of the ITCZ is becomes more realistically close to the equator. Despite these improvements, the superparameterized simulations exhibit degraded Pacific warm pool precipitation, surface wind stress, and net shortwave flux at surface. The persistence of double ITCZ biases in superparameterized simulations may implicate systematic errors beyond deep convective parameterization as important contributors to the double ITCZ.