4 Key Points: 5 • The SST contrast increases with warming, primarily because the clear-sky green-6 house effect feedback is stronger in the warm region. 7 • As the climate warms, the integrated cooling rate of the atmosphere increases by 8 moving upward into lower pressures and increasing in strength, giving a more top-9 heavy cooling profile. 10 • The more top-heavy cooling rate profile results in increased cloud ice as the cli-11 mate warms. Abstract 13 Warming experiments with a uniformly insolated, non-rotating climate model with a slab 14 ocean are conducted by increasing the solar irradiance. As the climate warms, the sur-15 face temperature contrast between the warm, rising and cooler, subsiding regions increases, 16 mostly as a result of the stronger greenhouse effect in the warm region. The convective 17 heating rate becomes more top-heavy in warmed climates, producing more cloud ice, prin-18 cipally because the radiative cooling rate moves to lower pressures and increases. To pro-19 duce this more top-heavy convective heating, precipitation shifts from the convective to 20 the stratiform parameterization. The net cloud radiative effect becomes more negative 21 in the warm region as the climate warms. At temperatures above about 310K surface 22 temperature contrast begins to decline, and the climate becomes more sensitive. The re-23 duction in SST contrast above 310K again appears to be initiated by clear-sky radiative 24 processes, although cloud processes in both the rising and subsiding regions contribute. 25 The response of clear-sky outgoing longwave to surface warming begins to accelerate in 26 the region of rising motion and decline in the region of subsidence, driving the SST con-27 trast to smaller values. One-dimensional simulations are used to isolate the most rele-28 vant physics. 29 Plain Language Summary 30 A global model of a non-rotating Earth with an ocean that stores heat but does 31 not transport it is run to equilibrium with different values of globally uniform solar heat-32 ing. Despite the complete uniformity of the system, it still develops regions of warm sea 33 surface temperature where rain and rising motion occur, and regions with downward, 34 subsiding air motion where rainfall does not occur. These contrasts look very similar to 35 what is observed in the present-day tropics. As the climate is warmed from current tem-36 peratures toward warmer temperatures, the warm regions warm faster, mostly because 37 the rising regions contain more water vapor. The clouds rise to higher altitudes in the 38 warmer climates, and produce more cloud ice. These changes are shown to arise from 39 well-understood physical processes that are expected to operate in nature. 40

Warming experiments with a uniformly insolated, non-rotating climate model with a slab ocean are conducted by increasing the solar irradiance. As the global mean surface temperature warms from the current global mean surface temperature of 289K, the surface temperature contrast between the warm-rising and cool-subsiding regions decreases to a small value at around 298K, then increases with further warming. The growing surface temperature contrast is associated with reduced climate sensitivity, mostly due to reduced strength of the greenhouse effect in the subsiding region. The clouds in the convective region are always more reflective than those in the subsiding region and this difference increases as the climate warms, acting to reduce the surface temperature contrast. At lower temperatures between 289K and 298K the shortwave suppression of SST contrast increases faster than the longwave enhancement of SST contrast. At warmer temperatures between 298K and 309K the longwave enhancement of SST contrast with warming is stronger than the shortwave suppression of SST contrast, so that the SST contrast increases. Above 309K the greenhouse effect in the subsiding region begins to grow, the SST contrast declines and the climate sensitivity increases. The transitions at 298K and 309K can be related to the increasing vapor pressure path with warming. The mass circulation rate between warm and cool regions consists of shallow and deep cells. Both cells increase in strength with SST contrast. The lower cell remains connected to the surface, while the upper cell rises to maintain a roughly constant temperature.