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.