What do moisture recycling estimates tell? Lessons from an extreme global land-cover change model experiment
Authors: Goessling H. F. , Reick C. H. 
Published: April 2011

Abstract. Moisture evaporated from the continents (recycled moisture) contributes up to 80% to the total atmospheric moisture content and, hence, precipitation in some regions. Recycling estimates are traditionally used to indicate a region's rainfall-dependence on land-surface evaporation. Accordingly, recycling estimates are employed to deduce the hydrological consequences of land-cover change. However, moisture is not a passive but an active constituent of the atmosphere. Recent studies indicate that at small scales (up to 1000 km) local to regional evaporation-precipitation coupling by far dominates the atmospheric precipitation response, while the water-balance effect from moisture recycling in the traditional sense seems to be of minor importance. The value of moisture recycling estimates as indicator for consequences of land-cover change is therefore questionable. However, since atmospheric moisture is still subject to mass conservation, the relevance of moisture recycling may come into play at the continental scale. To explore the relevance of recycling estimates regarding land-cover change at the continental scale, we conduct two global experiments with an atmospheric general circulation model: (I) with present-day conditions and (II) with extreme land-cover change conditions, namely with totally suppressed continental evaporation. Using the simulated fields of moisture, wind, and evaporation from the present-day experiment, we quantify continental moisture recycling with a vertically integrating tracing scheme. We then compare the computed recycling patterns with the hydrological changes that follow the suppression of continental evaporation. While under present-day conditions the fraction of recycled moisture increases from continental upstream to downstream regions with respect to the prevailing winds, the suppression of continental evaporation leads to severe precipitation loss in almost all continental regions, no matter if situated upstream or downstream. Over the ocean the hydrological response is ambigious, even where under present-day conditions large fractions of the atmospheric moisture stem from continental evaporation. This suggests that continental moisture recycling can not act across large ocean basins. Over land the absence of evaporative cooling at the surface leads to substantial warming which acts to suppress precipitation. In large parts of the continents the precipitation decrease compensates for much of the missing evaporation, such that the continental moisture-sink is not much amplified. Consequently, the atmospheric moisture content is not systematically reduced in the evaporation-free experiment, as would be necessary for the traditional moisture recycling mechanism to be active. Noteworthy exceptions are continental regions that are substantial moisture sources for some time of the year, first of all tropical wet-dry climates during the dry season. Apart from these exceptions, our results challenge the relevance of moisture recycling estimates for the hydrological consequences of land-cover change even at the continental scale.