Spontaneous zonal symmetry breaking of tropical rain belt

Abstract

Abstract The intertropical convergence zone (ITCZ) is a central component of tropical climate, yet the conditions under which a tropical rain belt remains zonally extended or becomes unstable to zonal organization remain poorly understood. Here we investigate this problem using idealized non‐rotating kilometer‐scale simulations forced by a prescribed sea surface temperature (SST) distribution that varies only in the meridional direction. This setup produces an ITCZ‐like rain belt while allowing spontaneous zonal convective self‐aggregation (ZCSA) to emerge. A parameter sweep shows that ZCSA occurs preferentially when both the peak SST and the meridional SST amplitude are large. All ZCSA cases exhibit a temporary weakening of the meridional near‐surface inflow that maintains the zonally elongated rain belt. Boundary‐layer momentum and thermodynamic analyses show that this weakening is associated with enhanced lower‐tropospheric stability, which leads to a shallower boundary layer and stronger effective frictional damping of the meridional inflow. However, weak meridional inflow alone does not determine whether ZCSA develops. Cases that develop ZCSA are additionally characterized by a large meridional contrast in moist‐static‐energy forcing, implying a strong demand for meridional energy transport. During the transition, relatively dry zonal sectors develop larger moisture deficits together with stronger radiative cooling, suppressed convective latent heating, and stronger zonal surface divergence. In the mature ZCSA state, meridional transport of moist static energy (MSE) is also reorganized, including enhanced stationary‐eddy export from the warm region. These results suggest that the imposed SST distribution controls the susceptibility of an ITCZ‐like rain belt to zonal symmetry breaking through its effects on the meridional low‐level circulation and the large‐scale demand for meridional energy transport, while the transition itself involves coupled local changes in moisture, radiation, convection, and circulation.

The authors' abstract, as published at the source. Quarterly Journal of the Royal Meteorological Society, 2026 · DOI ↗