The responses of tropical clouds and water vapor to SST variations are investigated with simple numerical experiments. The Pennsylvania State University (PSU) / National Center for Atmospheric Research (NCAR) mesoscale model is used with doubly periodic boundary conditions and a uniform constant sea surface temperature (SST). The SST is varied and the equilibrium statistics of cloud properties, water vapor and circulation at different temperatures are compared.

The top of the atmosphere (TOA) radiative fluxes have the same sensitivities to SST as in observations averaged from 20N - 20S over the Pacific, suggesting that the model sensitivities are realistic. As the SST increases, the temperature profile approximately follows a moist adiabatic lapse rate. The rain rate and cloud ice amounts increase with SST. The average relative humidity profile stays approximately constant and the upper tropospheric relative humidity increases slightly.

The high cloud amount increases with SST and the high cloud OLR decreases with increasing SST. These effects contribute to the outgoing longwave radiation decreasing with SST which is a positive feedback. The increase in high cloud amount with increasing SST causes a negative shortwave cloud feedback with half the magnitude of the longwave cloud feedback.

The clear-sky mean temperature and water vapor feedbacks have similar magnitudes and opposite signs. The lapse rate feedback is negative and the same magnitude as the positive net clear-sky feedback (~2 W m-2 K-1). The combination of feedbacks results in TOA energy fluxes that are very insensitive to SST, implying a very sensitive climate.