Abstract
Abstract. The spurious double Intertropical Convergence Zone (ITCZ) is one of the most prominent systematic biases in coupled atmosphere–ocean general circulation models (CGCMs), and the underestimated marine stratus over eastern subtropical oceans has been recognized as a possible contributor. Rather than modifying the cloud scheme itself, this study significantly ameliorates the marine stratus simulation through improving parameterizations of boundary-layer turbulence and shallow convection in the medium-resolution Beijing Climate Center Climate System Model version 2 (BCC-CSM2-MR). The University of Washington moist turbulence scheme is implemented in BCC-CSM2-MR to better represent the stratocumulus, and a decoupling criterion is also introduced to the shallow convection scheme for improving the simulation of the stratocumulus-to-cumulus transition. Results show that the simulated precipitation in the eastern Pacific south of the Equator is largely reduced, alleviating the double ITCZ problem. The tropical precipitation asymmetry index increases from −0.024 in the original BCC-CSM2-MR to 0.147 in the revised BCC-CSM2-MR, which is much closer to the observation. The study suggests that improving parameterizations of boundary-layer turbulence and shallow convection is effective for mitigating the double ITCZ syndrome in CGCMs.
Highlights
The coupled atmosphere–ocean general circulation models (CGCMs) have been widely used in the studies of climate variability, change, and prediction
A prominent tropical bias in generations of CGCMs is the double Intertropical Convergence Zone (ITCZ) syndrome, which is characterized by two parallel zonal bands of annual precipitation straddling the Equator over the central and eastern Pacific, while it is absent in the observations (Mechoso et al, 1995; Lin, 2007; Zuidema et al, 2016; Zhang et al, 2019)
The spurious double ITCZ affects the intensity of the Hadley circulation and the distribution of the trade winds directly related to the simulation of El Niño events and creates biases of latent heating in the tropics that can impact
Summary
The coupled atmosphere–ocean general circulation models (CGCMs) have been widely used in the studies of climate variability, change, and prediction. A prominent tropical bias in generations of CGCMs is the double Intertropical Convergence Zone (ITCZ) syndrome, which is characterized by two parallel zonal bands of annual precipitation straddling the Equator over the central and eastern Pacific, while it is absent in the observations (Mechoso et al, 1995; Lin, 2007; Zuidema et al, 2016; Zhang et al, 2019). The observed convergence zone south of the Equator extends southeastward from the western Pacific, whereas most CGCMs simulate a southern zonal rainfall band extending too far eastward. This bias is often associated with excessive warm sea surface temperatures (SSTs) in the southeastern Pacific (SEP). The spurious double ITCZ affects the intensity of the Hadley circulation and the distribution of the trade winds directly related to the simulation of El Niño events and creates biases of latent heating in the tropics that can impact
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