Global warming driven by rising greenhouse-gas concentrations is expected to cause wet regions of the tropics and mid to high latitudes to get wetter and subtropical dry regions to get drier and expand polewards 1-4 . Over southwest North America, models project a steady drop in precipitation minus evapotranspiration, P − E, the net flux of water at the land surface 5-7 , leading to, for example, a decline in Colorado River flow [8][9][10][11] . This would cause widespread and important social and ecological consequences 12-14 . Here, using new simulations from the Coupled Model Intercomparison Project Five, to be assessed in Intergovernmental Panel on Climate Change Assessment Report Five, we extend previous work by examining changes in P, E, runoff and soil moisture by season and for three different water resource regions. Focusing on the near future, 2021-2040, the new simulations project declines in surface-water availability across the southwest that translate into reduced soil moisture and runoff in California and Nevada, the Colorado River headwaters and Texas.The global climate models used in this study include all simulations for all models that were continuous from 1950 to 2040 and that provided all of the data required. Historical simulations to December 2005 and future projections to 2040 using the Representative Concentration Pathway (RCP)85 scenario whereby anthropogenic radiative forcing equals 8.5 W m −2 by 2100 (refs. 15, 16) were analysed (see Methods). The RCP85 scenario involves stronger anthropogenic radiative forcing than the Special Report on Emissions Scenario A1B for Coupled Model Intercomparison Project (CMIP) 3/Intergovernmental Panel on Climate Change Assessment Report Four analysed in ref. 5, and was chosen to reflect the present lack of any international action to limit CO 2 emissions. The models are being evaluated as part of CMIP5 and have been shown to model important aspects of North American hydroclimate with fidelity but with biases, for example, to excess P in the interior of western North America (see Supplementary Information).In both the models and the observations, California gets almost all of its annual P in the winter, whereas inland regions such as the Colorado headwaters and Texas have a more even distribution of P throughout the year (see Supplementary Information). However, in all cases, winter P has a disproportionate importance from the water-resource perspective because E is lowest during winter and P is effective at increasing soil moisture, streamflows and reservoir storage, often with a delay until snowmelt 8 . Summer rains are however very important for dry farming, rangelands and ecosystems and for influencing fire risk 17 . Figure 1 shows the changes in the multimodel ensemble mean P and P − E for North America for 2021-2040 minus 1951-2000 To the north, in central and northern California, Nevada, Utah and Colorado the models project increased winter P. In spring (April-June, AMJ) the region of reduced P extends to include all of California and most of...

Mediterranean-type climates are defined by temperate, wet winters, and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter storm tracks and summer subtropical anticyclones. The climatology, variability, and long-term changes in winter precipitation in Mediterranean-type climates, and the mechanisms for model-projected near-term future change, are analyzed. Despite commonalities in terms of location in the context of planetary-scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Ocean forcing of variability is a notable influence only for California and Chile. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in phase 5 of the Coupled Model Intercomparison Project. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the Northern Hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific storm track equatorward. In the Southern Hemisphere, mixed dynamic–thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland.

The mechanisms of model-projected atmospheric moisture budget change across North America are examined in simulations conducted with 22 models from phase 5 of the Coupled Model Intercomparison Project. Modern-day model budgets are validated against the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis. In the winter half year transient eddies converge moisture across the continent while the mean flow wets the west from central California northward and dries the southwest. In the summer half year there is widespread mean flow moisture divergence across the west and convergence over the Great Plains that is offset by transient eddy divergence. In the winter half year the models project drying for the southwest and wetting to the north. Changes in the mean flow moisture convergence are largely responsible across the west but intensified transient eddy moisture convergence wets the northeast. In the summer half year widespread declines in precipitation minus evaporation (P − E) are supported by mean flow moisture divergence across the west and transient eddy divergence in the Great Plains. The changes in mean flow convergence are related to increases in specific humidity but also depend on changes in the mean flow including increased low-level divergence in the U.S. Southwest and a zonally varying wave that wets the North American west and east coasts in winter and dries the U.S. Southwest. Increased transient eddy fluxes occur even as low-level eddy activity weakens and arise from strengthened humidity gradients. A full explanation of North American hydroclimate changes will require explanation of mean and transient circulation changes and the coupling between the moisture and circulation fields.