Dynamical and Thermodynamical Causes of Large-Scale Changes in the Hydrological Cycle over North America in Response to Global Warming*

Abstract

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.

Projected changes in US rainfall erosivity

“…Although barely significant (see the limited extent of the stippling), there is general drying in Mexico and Texas, extending into the southwest US, and more positive anomalies in the eastern gulf and Florida. The pattern of yearly anomalies shown here is consistent with the expectations reported in the literature for both CMIP3 and CMIP5 models (see for example Seager et al, 2007Seager et al, , 2014Biasutti et al, 2011;Christensen et al, 2013;Maloney et al, 2014). It can be interpreted as the sum of a weak US-wide summertime drying and the wintertime pattern of increase in rainfall north of 40 • N and decrease in precipitation in the southwest and Texas.…”

Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part II: The Meridian Moves East

“…While the plains have experienced a never-ending variation between times of drought and times of pluvial Stahle et al 2007;Seager et al 2005;Herweijer et al 2006;Schubert et al 2004;Forman et al 2001) in which the aridity gradient would have changed in intensity and location, it appears that agriculture has evolved in a way that the size of farms, and how the farm land is used, responds to the mean aridity gradient. However, in recent decades, humaninduced climate change has been added onto this natural variability.…”

“…There is considerable model agreement on these changes. These patterns are somewhat different to changes in P alone (see Seager et al 2014) because of the seasonal weighting by PET, which provides preferential weighting to the cooler, low PET, seasons. The P-induced changes in AI only modestly weaken the AI gradient across the southern plains.…”

“…All model data were regridded to a common 28 3 28 grid. The numerical methods for evaluating moisture transport in the models and ERA-Interim are as in Seager and Henderson (2013), and computations were done on 6-hourly data. Computations are first done for each model run before averaging in time for each model, as needed, and then across the models to create the multimodel means shown here.…”

“…Based on the most recent climate model projections from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and analyzed by the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5), many workers have reported that North America will see, over the coming decades, a marked transition in hydroclimate. Precipitation is expected to decline in southwestern North America but increase in the northeast, and temperature will rise everywhere (Seager et al 2013;Maloney et al 2014;Seager et al 2014). Cook et al (2015) used the CMIP5 model ensemble to show that these changes combine to cause a quite alarming increase in aridity in the plains as measured either by the Palmer drought severity index (utilizing the Penman-Monteith formulation for potential evapotranspiration) or in the actual modeled soil moisture.…”