Controls of Zonal Convective Self-Aggregation in an Idealized Tropical Rain Belt

Self-aggregation of deep moist convection has been widely studied in idealized radiative-convective equilibrium. While its relevance to the real tropical climate remains debated, one potential link is zonal convective aggregation within tropical rain belt. However, the mechanisms controlling zonal aggregation are still not well understood. Here, to investigate how convection interacts with the large-scale environment in a zonally symmetric setting, we conduct a series of idealized cloud-resolving simulations in which a meridionally varying, sinusoidal sea surface temperature (SST) distribution is systematically controlled. In particular, we vary the SST amplitude and SST maximum value.

We find that the system selects either a zonally uniform or a zonally aggregated state depending on the following SST parameters: zonal aggregation occurs when both the SST amplitude and the SST maximum are large. We explain this behavior as follows.

For large SST amplitude but low SST maximum, a narrow convergence zone can be maintained by the strong meridional pressure gradient and the associated circulation. In contrast, for large SST amplitude and high SST maximum, the narrow convergence zone cannot be sustained because the circulation weakens as free-tropospheric static stability increases, consistent with convection sitting on a warmer (and more stable) moist adiabat by warmer SST. As a result, convection over the high-SST region cannot maintain surface convergence solely via the meridional overturning circulation driven by subsidence over the low-SST region. Instead, it selects a zonally aggregated state that also extends the circulation in the zonal direction.