While CMIP5 models robustly project drying of the subtropics and more precipitation in the tropics and subpolar latitudes by the end of the century, the magnitude of these changes in precipitation varies widely across models: for example, some models simulate no drying in the eastern Mediterranean while others simulate more than a 50% reduction in precipitation relative to the model-simulated present-day value. Furthermore, the factors leading to changes in local subtropical precipitation remain unclear. The importance of zonal-mean changes in atmospheric structure for local precipitation changes is explored in 42 CMIP5 models. It is found that up to half of the local intermodel spread over the Mediterranean, northern Mexico, East Asia, southern Africa, southern Australia, and southern South America is related to the intermodel spread in large-scale processes such as the magnitude of globally averaged surface temperature increases, Hadley cell widening, polar amplification, stabilization of the tropical upper troposphere, or changes in the polar stratosphere. Globally averaged surface temperature increases account for intermodel spread in land subtropical drying in the Southern Hemisphere but are not important for land drying adjacent to the Mediterranean. The factors associated with drying over the eastern Mediterranean and western Mediterranean differ, with stabilization of the tropical upper troposphere being a crucial factor for the former only. Differences in precipitation between the western and eastern Mediterranean are also evident on interannual time scales. In contrast, the global factors examined here are unimportant over most of the United States, and more generally over the interior of continents. Much of the rest of the spread can be explained by variations in local relative humidity, a proxy also for zonally asymmetric circulation and thermodynamic changes.
Earth's water cycle has already begun to change, and these changes will intensify as the climate warms (Allen & Ingram, 2002;Cubasch et al., 2001;Manabe & Wetherald, 1980;Mitchell, 1983), impacting societies and ecosystems throughout the world. The net water flux at the surface -precipitation minus evapotranspiration over land or precipitation minus evaporation over ocean (P − E) -is a key aspect of the water cycle as it regulates oceanic salinity and continental aridity (Allan et al., 2020;Durack et al., 2012;Hartmann, 2016). While globally averaged P − E must be zero both in the present climate and in the future, regional variability in P − E can arise from a range of dynamic and thermodynamic processes which are, in turn, affected by climate change.Over oceans, projected changes in P − E on large scales appear to scale with changes in surface temperature in both the tropics (
<p>End of century projections from Coupled Model Intercomparison Project (CMIP) models show a decrease in precipitation over subtropical oceans that often extends into surrounding land areas, but with substantial intermodel spread. Changes in precipitation are controlled by both thermodynamical and dynamical processes, though the importance of these processes for regional scales and for intermodel spread is not well understood. The contribution of dynamic and thermodynamic processes to the model spread in regional precipitation minus evaporation (P-E) is computed for 48 CMIP models. The intermodel spread is dominated essentially everywhere by the change of the dynamic term, including in most regions where thermodynamic changes dominate the multi-model mean response. The dominant role of dynamic changes is insensitive to zonal averaging which removes any influence of stationary wave changes, and is also evident in subtropical oceanic regions. Relatedly, intermodel spread in P-E is generally unrelated to climate sensitivity.</p>
<p>Observations from the past century and projections for the end of this century show a decrease in precipitation over the eastern Mediterranean Sea and surrounding land areas. Changes in precipitation are controlled by both thermodynamic and dynamic processes, but the relative contributions of these processes, in particular on regional scales, is not well understood. Models included in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP5 and CMIP6) exhibit a wide spread in the magnitude of expected drying in the eastern Mediterranean region, as well as in other meteorological variables. By decomposing projected changes in the moisture budget in 48 models into mean dynamic and mean thermodynamic components, we explore the contribution of each of these components to the model spread in regional drying. In the eastern Mediterranean, the dynamic component explains 64% and the thermodynamic component explains 9% of the variance in net precipitation change. We further examine the relation of the regional components to changes in five large-scale mechanisms: tropical vertical stratification, global near-surface temperature, latitude of the eddy-driven jet, stratospheric polar vortex, and arctic amplification. Of these, we find that a decrease in the dynamical contribution in the eastern Mediterranean, causing regional drying, is most strongly related to a northward shift of the eddy-driven jet and a rise in global near-surface temperature.</p>
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