Convective self-aggregation over land versus ocean: insights from diurnally oscillating sea surface temperature experiments

Atmospheric convection grows differently over land and ocean. We address land-ocean contrast in convective organization using idealized cloud-resolving model simulations in a non-rotating radiative-convective equilibrium framework. In our ocean forced simulations, we mimic ocean and land forcing, exploiting their difference in heat capacities by fixing and oscillating sea surface temperature (SST), respectively, keeping temporal mean SST the same and also having a spatially homogeneous SST at a given time. Examining the speed of transition to an aggregated state we found that, an oscillating SST forces a faster onset of an aggregated state. Spatial heterogeneities of long-wave cooling are known to favor aggregation. With oscillating SST, we find that spatial anomalies of outgoing long-wave radiation are stronger, thus favoring aggregation. During the warm SST phase, long-wave cooling is enhanced in dry regions compared to neighboring moist convective regions. Stronger long-wave cooling allows stronger subsidence which allows low-level circulation to more efficiently transport moisture and energy up-gradient, driving convection to aggregate faster. We also note a sensitivity of our experimental setup to initial conditions, more so at warmer SST. This stochastic behavior, we suspect, might be critical in reconciling the differences of opinion regarding the response of convection aggregation to oscillating SST forcing.