On the meridional asymmetry of the poleward‐displaced intertropical convergence zone

We analyze the mean dynamical structure of the lower atmosphere over the tropical East Pacific and Atlantic basins during boreal summer, when the sea‐surface temperature (SST) is maximized well poleward of the Equator. We use the data from a global, coupled storm‐resolving simulation performed using the ICOsahedral Nonhydrostatic (ICON) model with an explicit representation of convection (ICON Sapphire) for our analysis. It has previously been hypothesized that inertial instability plays a role in determining the latitude of the intertropical convergence zone (ITCZ). Seasonal mean composites of absolute vertical vorticity and convergence about the latitude of maximum precipatable water suggest that convergence emerges well within the region of inertial stability. We show that the dynamical structure can be explained better using the mean momentum balance. Equatorial pressure gradients near the surface must be balanced by advection, leading to the acceleration of southerlies and advection of negative absolute vertical vorticity north of the Equator. The strengthening westerlies, due to negative relative vorticity in the southerly flow, balance the pressure gradients south of the ITCZ, leading to deceleration of southerlies and dynamic convergence. The dynamical nature of convergence in the southerlies is fundamentally different from the convergence observed in the northerlies, where the pressure gradients can be balanced by easterlies owing to their more poleward location. The resulting dynamical structure is asymmetrical, with a concentration of convergence in the southerly trade‐wind regime. We also observe that the relative vorticity in the ITCZ depends on the ITCZ latitude, and increases as the ITCZ moves to the north.