| Abstract: Using two versions of the GFDL
coupled ocean-atmosphere model, one where water vapor anomalies are
allowed to affect the longwave radiation calculation and one where they
are not, we examine the role of water vapor feedback in internal
precipitation variability and greenhouse-gas-forced intensification of
the hydrologic cycle. Without external forcing, the experiment
with water vapor feedback produces 44% more annual-mean, global-mean
precipitation variability than the one without. We diagnose the
reason for this difference: In both experiments, global-mean
surface temperature anomalies are associated with water vapor
anomalies. However, when water vapor interacts with longwave
radiation, the temperature anomalies are associated with larger
anomalies in surface downward longwave radiation. This increases
the temperature anomaly damping through latent heat flux, creating an
evaporation anomaly. The evaporation anomaly, in turn, leads to an
anomaly of nearly the same magnitude in precipitation. In the
experiment without water vapor feedback, this mechanism is absent.
While the interaction between longwave and water vapor has a large impact
on the global hydrologic cycle internal variations, its effect decreases
as spatial scales decrease, so water vapor feedback has only a very
small impact on grid-scale hydrologic variability. Water vapor
feedback also affects the hydrologic cycle intensification when
greenhouse 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 similar
to the cause of the difference in internal precipitation
variability: When water vapor feedback is present, the increase in
water vapor associated with a warmer climate enhances downward longwave
radiation. To maintain surface 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
intensification does not weaken as spatial scales decrease, unlike the
internal variability case. Accurate representations of water vapor
feedback are therefore necessary to simulate global-scale hydrologic
variability and intensification of the hydrologic cycle in global
warming. |