Abstract

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.