Inter-model spreads of the climatological mean Hadley circulation in AMIP/CMIP6 simulations

We study inter-model spreads of the climatological annual mean Hadley circulation (HC), using 37 models from Coupled Model Intercomparison Project phase 6 (CMIP6) Atmospheric Model Intercomparison Project (AMIP). Our results show significant inter-model spreads of the climatological annual mean HC although the models are driven with identical sea surface temperatures (SSTs). Two leading modes of inter-model HC spreads are identified, using the method of inter-model empirical orthogonal function (EOF) decomposition. The EOF1 mode exhibits an equatorial symmetric dipole pattern, explains 40.5% of the total inter-model variance of mean meridional mass streamfunction (MMS), and reflects inter-model spreads of the HC strength and latitudinal locations of the ascending branch in AMIP simulations. The EOF2 mode explains 23.1% of the MMS variance, and mainly reflects inter-model spreads of latitudinal locations of the ascending branch and poleward edges of the Hadley cells. Regression of tropical precipitation on PC1 and PC2 shows that both two leading modes are closely related to model performance in simulating tropical convective precipitation. It suggests that inter-model spreads of the HC are due to different cloud-convection parameterization schemes among the AMIP models. Models that simulate heavier tropical convective precipitation generate concentrated equatorial convective heating and stronger, but narrower, Hadley cells, and the associated ascending branch is located more southward. Models that simulate stronger tropical convective precipitation in the south of the equator exhibit southward biases of the latitudinal position of the ascending branch and a narrower Northern-Hemispheric cell. This study indicates that improvements of cloud-convection parameterizations are of critical importance in simulating the HC.

Automated summary

This study investigates the inter-model spreads of the climatological annual mean Hadley circulation in a set of 37 models. The results reveal significant differences in the simulation of the Hadley circulation among these models, even though they are driven by the same sea surface temperatures. The findings suggest that the discrepancies are mainly attributed to the cloud-convection parameterization schemes used in the models. Improvements in the cloud-convection parameterizations are crucial for simulating the Hadley circulation.