Sea Surface Temperature Contrast in Tropical World: Part 1 Mean State
Hartmann, Dennis L
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Warming experiments with a uniformly insolated, non-rotating climate model with a slab ocean are conducted by increasing the solar irradiance. As the climate warms, the surface temperature contrast between the warm, rising and cooler, subsiding regions increases, mostly driven by the stronger greenhouse effect in the warm region. Cloud changes act to decrease the surface temperature contrast. The clouds in the convective region become much more reflective as the climate warms, acting to reduce the surface temperature contrast. It is argued that the increase in cloud ice in the convective region is associated with stronger clear-sky radiative cooling in the upper troposphere in warmer climates. This more top-heavy radiative cooling rate can be explained with simple theory. The mass circulation rate between warm and cool regions also increases with warming. At temperatures above about 310K surface temperature contrast begins to decline, and the climate becomes more sensitive. The reduction in SST contrast above 310K again appears to be initiated by clear-sky radiative processes, although cloud processes in both the rising and subsiding regions contribute. The response of clear-sky outgoing longwave to surface warming begins to accelerate in the region of rising motion and decline in the region of subsidence, resulting in a smaller SST contrast.