The Cycle of Large-Scale Aggregation in Tropical Radiative-Convective Equilibrium

In numerical experiments in tropical radiative-convective equilibrium with interactive sea surface temperature (SST), the interaction between SST gradients and circulation can lead to an inter-annual oscillation between aggregated and relatively disaggregated convection. We investigate these oscillations in a global aquaplanet model with uniform insolation, no rotation, and slab ocean. The large-scale circulation organizes into warm convective regions and cooler subsiding regions. The cycle is characterized by variations in the surface temperature contrast between these warm and cold regions and the fraction of the domain that is subsiding. The temperature contrast grows as the warm region warms, the cool region cools, and the subsiding region expands. When the temperature contrast has reached its maximum, the large-scale circulation becomes much more effective at removing energy from the warm region, which enables evaporative cooling and cloud shortwave reflection to rapidly cool the warm region. The efficiency of atmospheric transport in cooling the warm region is associated with positive skewness of the SST distribution and a minimum in the area of rising motion. During the decline of SST contrast, evaporative cooling declines in the subsiding region and the cold pool begins to warm. Low clouds in the subsiding region decline due to a decrease in longwave cooling above the boundary layer associated with decreased subsidence drying as the subsiding region expands. At temperatures warmer than the current tropics, the response of the hydrologic cycle to SST contrast becomes stronger, so that the oscillation speeds up, weakens in amplitude, and vanishes at sufficiently high temperatures.