Effect of water vapor feedback on internal and anthropogenic variations ofthe global hydrologic cycle

Using two versions of the GFDL coupled ocean-atmosphere model, one where wa ter vapor anomalies are allowed to affect the longwave radiation calculatio n and one where they are not, we examine the role of water vapor feedback i n internal precipitation variability and greenhouse-gas-forced intensificat ion of the hydrologic cycle. Without external forcing, the experiment with water vapor feedback produces 44% more annual-mean, global-mean precipitati on variability than the one without. We diagnose the reason for this differ ence: In both experiments, global-mean surface temperature anomalies are as sociated with water vapor anomalies. However, when water vapor interacts wi th longwave radiation,the temperature anomalies are associated with larger anomalies in surface downward longwave radiation. This increases the temper ature anomaly damping through latent heat flux, creating an evaporation ano maly. The evaporation anomaly, in turn, leads to an anomaly of nearly the s ame magnitude in precipitation. In the experiment without water vapor feedb ack, this mechanism is absent. While the interaction between longwave and w ater vapor has a large impact on the global hydrologic cycle internal varia tions, its effect decreases as spatial scales decrease, so water vapor feed back has only a very small impact on grid-scale hydrologic variability. Wat er vapor feedback also affects the hydrologic cycle intensification when gr eenhouse gas concentrations increase. By the 5th century of global warming experiments where CO2 is increased and then fixed at its doubled value, the global-mean precipitation increase is nearly an order of magnitude larger when water vapor feedback is present. The cause of this difference is simil ar to the cause of the difference in internal precipitation variability: Wh en water vapor feedback is present, the increase in water vapor associated with a warmer climate enhances downward longwave radiation. To maintain sur face heat balance, evaporation increases, leading to a similar increase in precipitation. This effect is absent in the experiment without water vapor feedback. The large impact of water vapor feedback on hydrologic cycle inte nsification does not weaken as spatial scales decrease, unlike the internal variability case. Accurate representations of water vapor feedback are the refore necessary to simulate global-scale hydrologic variability and intens ification of the hydrologic cycle in global warming.