Detecting global hydrological cycle intensification

Global warming is expected to intensify the global hydrological cycle, but the magnitude of the predicted response is uncertain. The proposed zonal mean thermodynamic response is enhanced horizontal moisture transport associated with increased saturation vapor presand additional net evaporation in the subtropics. Sea surface salinity (SSS) anomalies are forced from above by changes in evaporation minus precipitation (E-P) and thus will respond to changes in the surface freshwater flux, opening the possibility of using historical SSS anomalies to diagnose the global water cycle response to warming. We estimate zonal mean SSS trends from 1955-2015 to test whether historical changes in the global hydrological cycle are consistent with a primarily thermodynamic response. Rising global mean sea surface temperature (SST) decreases SSS in the tropics and increases SSS in the subtropics. Motivated by this observation, we calculate the sensitivity of zonal mean SSS anomalies to SST forcing as a function of timescale to estimate the signal-to-noise ratio of the thermodynamic signal. We find the zonal mean SSS sensitivity pattern of hydrological cycle amplification is present at all frequencies over a 61 year period. Our results suggest that changes in the global water cycle are dominated by thermodynamic processes driven by global mean SST.