How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the 1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.

SUMMARYThe patterns of precipitation anomalies forced by the El Niño-Southern Oscillation during northern hemisphere winter and spring are remarkably hemispherically symmetric and, in the midlatitudes, have a prominent zonally symmetric component. Observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically-forced changes in the subtropical jets. During El Niño events the jets strengthen in each hemisphere and shift equatorward. Changes in the subtropical jet influence the transient-eddy momentum fluxes and the eddy-driven mean meridional circulation. During El Niño events, eddy-driven ascent in the midlatitudes of each hemisphere is accompanied by low-level convergence and brings increased precipitation. These changes in the transient-eddy and stationary-eddy moisture fluxes almost exactly cancel each other and, in sum, do not contribute to the zonal-mean precipitation anomalies. Propagation of anomalous stationary waves disrupts the zonal symmetry. Flow around the deeper Aleutian Low and the eastward extension of the Pacific jet stream supply the moisture for increased precipitation over the eastern North Pacific and the western seaboard of the United States, while transient-eddy moisture convergence supplies the moisture for increased precipitation over the southern United States. In each case, increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes.

[1] Recent eddy-permitting simulations of the North Pacific Ocean have revealed robust patterns of multiple zonal jets that visually resemble the zonal jets on giant planets. We argue that this resemblance is more than just visual because the energy spectrum of the oceanic jets obeys a power law that fits spectra of zonal flows on the outer planets. Remarkably, even the non-dimensional proportionality coefficient, C Z , determined by data under that spectral law, appears to be constant for all cases and approximately equal to 0.5. These results indicate that the multiple jet sets in the ocean and in the atmospheres of giant planets are governed by the same dynamics characterized by an anisotropic inverse energy cascade, i.e., the flow of energy from isotropic small-scale eddies to anisotropic large-scales structures, as well as the unique anisotropic spectrum. Implications of these results for climate research and future designs of observational missions are discussed.