What Drives the Asymmetrical Structure of the Intertropical Convergence Zone?

Around one-third of the world’s precipitation occurs within a narrow tropical rain belt, the Intertropical Convergence Zone (ITCZ), supplying hundreds of millions of people with freshwater. However, the processes inside the ITCZ and the mechanisms ultimately determining its position are poorly understood. In the ITCZ, the trade winds from the northern and southern hemispheres meet. This convergence causes moist air masses to rise, leading to strong cloud formation and intense rainfall. But contrary to what this picture might imply, observations show that the ITCZ has an asymmetric internal structure. Using global storm-resolving simulations, researchers at the Max Planck Institute for Meteorology (MPI-M) have investigated how this asymmetry arises.

In their study, Divya Sri Praturi and Bjorn Stevens considered the ITCZ over the Atlantic and East Pacific basins during boreal summer. During this period, the ITCZ is located north of the equator. As previous work by MPI-M scientists Julia Windmiller and Bjorn Stevens showed, the rising and precipitating of air masses happens across a relatively wide area, but that the structure of the ITCZ is asymmetric in that the most intense rainfalls occur near the southern edge. The new study by Praturi and Stevens now provides an explanation for this structure using high-resolution simulations created with the ICON climate model as part of the nextGEMS project, which was funded by the European Union and led by the MPI-M.

“This is a very exciting simulation to work with, as it does not employ convective parameterization.  As a result, atmospheric circulation and convection can interact directly with each other, which allows us to better infer the causal relationships. We take the perspective of winds and force balances to explain the observed asymmetry,” says Praturi.