Abstract
A change in the cycle of water from dry to wet regions of the globe would have far reaching impact on humanity. As air warms, its capacity to hold water increases at the Clausius-Clapeyron rate (CC, approximately 7% °C−1). Surface ocean salinity observations have suggested the water cycle has amplified at close to CC following recent global warming, a result that was found to be at odds with state-of the art climate models. Here we employ a method based on water mass transformation theory for inferring changes in the water cycle from changes in three-dimensional salinity. Using full depth salinity observations we infer a water cycle amplification of 3.0 ± 1.6% °C−1 over 1950–2010. Climate models agree with observations in terms of a water cycle amplification (4.3 ± 2.0% °C−1) substantially less than CC adding confidence to projections of total water cycle change under greenhouse gas emission scenarios.
Highlights
Understanding and quantifying observed global water cycle change is key to predicting future climate[1]
Coupled Model Intercomparison Project Phase 3 (CMIP3) historical simulations showed considerably lower salinity Pattern Amplification (PA) values than observations, and presented much less consistent results regarding the relationship between Sea Surface Salinity (SSS) patterns, water cycle change and warming
(a) Historical mean volumetric distribution of sea-water in salinity anomaly space (m3/pss; colours: Coupled Model Intercomparison Project Phase 5 (CMIP5); black: En4 observations); Bars show the mean deviation of salinity (W; pss). (b) Historical mean P+R−E in sea surface salinity anomaly space (Sv/pss; colours: as in (a); solid black: observations CORE2-Dai, dashed black: observations GPCP-OAFLUX-Dai)
Summary
We quantify recent water cycle change using a new method developed by Zika et al.[17] based on water mass transformation theory[18,19]. Seawater can only change salinity due to the addition/removal of freshwater, i.e. through precipitation (P), evaporation (E), runoff (R), ice melt/freeze etc., or by mixing with waters of differing salinities. The integral of P−E + R over the salinities lower than the global mean (S) (equal and opposite to the integral over the more saline side of the distribution in a balanced system) is defined as the oceanic water cycle (Fcycle) for the purposes of this study. It is important to note that this definition considers only the geographical redistribution of water by the atmosphere and patterns of E-P It does not necessarily scale with changes in P and E individually as such changes can compensate leading to no change in E-P locally and no change in salinity. The mixing parameterization of equation (1) is tested against output from ten state-of-the-art models from the Coupled Model Intercomparison Project Phase 5 (CMIP5)[22] including pre-industrial, historical and Representative Concentration Pathways 4.5 (RCP4.5) and 8.5 (RCP8.5) scenarios (see Methods and Table S1)
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