According to the standard bulk formula, evaporation increases with sea surface temperature (SST) but is also controlled by atmospheric parameters including surface wind speed, surface relative humidity (SRH), and air-sea temperature difference. In the IPCC AR4 simulations these atmospheric surface parameters reduce the response of the hydrological cycle from 7% K-1 to 2% K-1, with a 1% increase of SRH during the 21st century contributing 1/3 of this reduction.
In the present study, we use the Community Atmosphere Model 3.0 coupled to a slab ocean model to conduct a set of equilibrium sensitivity tests, in which latent heat flux calculations over the oceans use SRH prescribed from either present day or CO2 doubling climatology. The results show that a 1% increase in RH contributes 0.49 K to the SST warming, or 23% of the total warming due to CO2 doubling (2.11 K). This suggests an SRH feedback, in which global warming-induced SRH increase inhibits latent heat release, which in turn warms the ocean. When increased SRH is prescribed, strong surface warming occurs in subtropical stratocumulus regions. This constitutes a positive shortwave radiation feedback, in which SRH-induced SST warming reduces low cloud fraction and increase surface insolation. On the other hand, SST warming and low cloud reduction act to increase surface longwave output and thus nearly balance the enhancement of solar insolation globally, but not locally. In the stratocumulus regions, solar enhancement dominates.