Greenhouse warming and the 21st century hydroclimate of southwestern North America

Abstract

Climate models robustly predict that the climate of southwestern North America, defined as the area from the western Great Plains to the Pacific Ocean and from the Oregon border to southern Mexico, will dry throughout the current century as a consequence of rising greenhouse gases. This regional drying is part of a general drying of the subtropics and poleward expansion of the subtropical dry zones. Through an analysis of 15 coupled climate models it is shown here that the drying is driven by a reduction of winter season precipitation associated with increased moisture divergence by the mean flow and reduced moisture convergence by transient eddies. Due to the presence of large amplitude decadal variations of presumed natural origin, observations to date cannot confirm that this transition to a drier climate is already underway, but it is anticipated that the anthropogenic drying will reach the amplitude of natural decadal variability by midcentury. In addition to this drop in total precipitation, warming is already causing a decline in mountain snow mass and an advance in the timing of spring snow melt disrupting the natural water storage systems that are part of the region's water supply system. Uncertainties in how radiative forcing will impact the tropical Pacific climate system create uncertainties in the amplitude of drying in southwest North America with a La Niña-like response creating a worst case scenario of greater drying.

Fig. 1.

Time series for 1900–2099 of the median of 24 IPCC AR4 models’ simulated and projected change in P - E (Red), with 25th and 75th percentiles of the distribution (Shading), P (Blue), and E (Green) for the winter (Oct–Mar, Top) and summer (Apr–Sep) half years and averaged across the SWNA land region, relative to 1950–1999 climatologies. Units are mm/day.

Fig. 2.

The 24-model mean change in P - E, 2021 to 2040 minus 1950 to 1999, for winter and summer half years. Units are mm/day.

Fig. 3.

The 15-model mean climatology change (2046–2065 minus 1961–2000) in October to March half year P - E (Top Left) and terms in the vertically integrated moisture budget due to the mean flow divergence term (, Top Right), the mean flow advection term (, Bottom Right), and the transient eddy convergence (, Bottom Left). All units are mm/day.

Fig. 4.

The observed (Top, from combined satellite-gauge GPCP data), simulated with an atmosphere GCM forced by observed SSTs in the tropical Pacific ocean (Middle) and IPCC 24 model mean simulated and projected (Bottom) trend in precipitation for 1979 to 2006. Units are mm/day.

Fig. 5.

The annual change in surface temperature (Left) and P - E (Right) for 2021 to 2040 minus 1950 to 1999 for models that have a strengthening equatorial Pacific SST gradient (Top, six models) and a weakening gradient (Bottom, six models). Units are degrees centigrade and mm/day.