Advances in Research on the ITCZ: Mean Position, Model Bias, and Anthropogenic Aerosol Influences

The zonal-mean position of the intertropical convergence zone (ITCZ) and its shift in the meridional direction significantly influence both the tropical and even global climate. This work reviews three aspects of the progress in ITCZ-relevant research: 1) the mechanism behind the asymmetry of the ITCZ annual- and zonal-mean positions relative to the equator; 2) causes of the double-ITCZ problem (pervasive in climate models) and the efforts to solve it; and 3) the physical mechanisms by which anthropogenic aerosols affect the location of the zonal-mean ITCZ. According to recent studies, the north-of-the-equator location of the annual- and zonal-mean ITCZ is mainly driven by the cross-equatorial energy transports in the ocean, induced by the Atlantic overturning circulation. A quantitative relationship between the ITCZ shift and the anomalous cross-equatorial energy transport in the atmosphere has been found. Presently, the double-ITCZ problem is still the most common and pronounced bias in tropical precipitation simulations with climate models. Recently, some studies have found that simply correcting the biases in hemispheric energy contrast does not improve the simulation of the ITCZ with climate models; whereas others have found that improving model resolutions and convective parameterizations in climate models, such as entrainment rate, rain-droplet re-evaporation, and convection triggering function, can alleviate the double-ITCZ bias. Therefore, it seems that the double-ITCZ problem in climate models is rooted in the complex physics of the models, which is not yet well-understood. In addition, anthropogenic aerosols are suggested to be able to induce meridional shifts of the ITCZ, but through various physical mechanisms. Absorbing aerosols like black carbon influence the ITCZ position basically via instantaneous absorption of shortwave radiation in the atmosphere, whereas scattering aerosols like sulfate affect the location of the ITCZ through the cloud lifetime effect and the subsequent response of surface evaporation.