The Transition to Double‐Celled Circulations in Mock‐Walker Simulations.

Mock‐Walker simulations have the potential to play a key role in a tropical model hierarchy, bridging small‐scale Radiative‐Convective Equilibrium simulations and global models of tropical circulations. We demonstrate that mock‐Walker simulations transition from single‐ to double‐celled overturning circulations as mean Sea Surface Temperature (SST) is increased, with the transition occurring near 300 K. The transition is robust to domain geometry and microphysical scheme, and is favored by larger SST gradients. The transition is associated with the development of a mid‐tropospheric minimum in the radiative‐subsidence velocity over the cold pool of the simulations, and is likely reinforced by zonal moisture and temperature fluxes between the warm and cold pools. Several methods of suppressing the transition are investigated, but all set‐ups produce a double‐cell at sufficiently warm mean SSTs. The striking dynamical transition of mock‐Walker simulations dominates their response to warming, though its relevance for observed tropical climate change is unclear. Plain Language Summary: Untangling the coupled interactions between clouds and large‐scale atmospheric flows is one of the "Grand Challenges" of climate science. Large‐scale flows are the main control on the spatial distribution of cloud‐types in the tropics, but clouds in turn play a key role in setting the strengths and spatial structures of these flows. Here, we investigate "mock‐Walker" simulations as a potential idealized modeling set‐up for investigating the two‐way interactions between clouds and tropical circulations. Mock‐Walker simulations include the zonal sea‐surface temperature (SST) gradient needed to generate realistic tropical circulations, while using grid resolutions sufficient to partially resolve clouds. However, in this study we document a robust transition in the large‐scale flow in mock‐Walker simulations from a single overturning cell to two vertically‐stacked overturning cells (a double‐celled flow) as the mean SST is increased. This transition occurs for a mean SST close to the observed SSTs in the equatorial Pacific, and dominates the response of mock‐Walker simulations to warming. The transition is robust to domain geometry, microphysical scheme and to fixing the radiation. While we are unable to provide a complete explanation of the transition, it does seem to be associated with extreme dryness over the cold pools of the simulations. Key Points: Mock‐Walker simulations transition from single‐to a double‐celled circulation as the mean SST is increasedThis is associated with a mid‐tropospheric minimum in the cold pool radiative‐subsidence velocityElevated stability maxima cause transitions in mock‐Walker simulations with fixed radiative cooling and artificial moisture sources [ABSTRACT FROM AUTHOR]