Climate variability and projected change in the western United States: regional downscaling and drought statistics

Abstract

Climate change in the twenty-first century, projected by a large ensemble average of global coupled models forced by a mid-range (A1B) radiative forcing scenario, is downscaled to Climate Divisions across the western United States. A simple empirical downscaling technique is employed, involving model-projected linear trends in temperature or precipitation superimposed onto a repetition of observed twentieth century interannual variability. This procedure allows the projected trends to be assessed in terms of historical climate variability. The linear trend assumption provides a very close approximation to the time evolution of the ensemble-average climate change, while the imposition of repeated interannual variability is probably conservative. These assumptions are very transparent, so the scenario is simple to understand and can provide a useful baseline assumption for other scenarios that may incorporate more sophisticated empirical or dynamical downscaling techniques. Projected temperature trends in some areas of the western US extend beyond the twentieth century historical range of variability (HRV) of seasonal averages, especially in summer, whereas precipitation trends are relatively much smaller, remaining within the HRV. Temperature and precipitation scenarios are used to generate Division-scale projections of the monthly palmer drought severity index (PDSI) across the western US through the twenty-first century, using the twentieth century as a baseline. The PDSI is a commonly used metric designed to describe drought in terms of the local surface water balance. Consistent with previous studies, the PDSI trends imply that the higher evaporation rates associated with positive temperature trends exacerbate the severity and extent of drought in the semi-arid West. Comparison of twentieth century historical droughts with projected twenty-first century droughts (based on the prescribed repetition of twentieth century interannual variability) shows that the projected trend toward warmer temperatures inhibits recovery from droughts caused by decade-scale precipitation deficits.

Appendix: Sensitivity studies of PDSI calculations

The algorithm used to calculate of the palmer drought severity index follows the US operational practice (Guttman 1998) derived from monthly temperature and precipitation. Several local, tunable parameters are embedded in the code describing boundary values that determine the persistence of the surface water budget from month to month. In a nonstationary climate, these boundary values may change significantly.

We examined the sensitivity of the projected PDSI values to two internal parameters. First, the soil moisture constant wcbot, which represents the soil moisture capacity in each Climate Division, was varied. The nationwide standard deviation of wcbot is 1.28 in NCDC’s formulation. We repeated twenty-first century calculations after artificially inflating or deflating wcbot by one unit, and found little difference in the resulting PDSI values. We conclude that any changes to this parameter in the twenty-first century would have to be unusually large to significantly affect the results of this study.

Second, we adjusted the period of record used to define the surface water budget that is climatologically appropriate for existing conditions (CAFEC). Although the calibration period for PDSI calculations is usually the entire period of the record, this need not be the case (Heim 2002) and may not be desirable in a nonstationary climate. The projected climate change used in this study is characterized by a continuously shifting definition of “normal” conditions so it is difficult to determine the most appropriate calibration period to use for the twenty-first century. As Lockwood (1999) and Burke et al. (2006) have pointed out, the calibration period determines the relationship between temperature and evapotranspiration that is implicit in the PDSI calculation used operationally (and in the calculations presented here), leading to probable overestimates in the decrease of PDSI associated with upward temperature trends.

All scenarios discussed in the body of the paper use a calibration period of 1931–1990, the same calibration period used by NCDC for ongoing operational drought monitoring. An additional scenario was generated for New Mexico using a calibration period of 2001–2100. This scenario demonstrated patterns similar to those of the scenario illustrated in Figs. 4, 6, 7, 8, 9 and 10. However in this case 11-year PDSI running averages from 2001 to 2045 were higher (more positive) than the analogous running averages for both the twentieth century and the 1931–1990-calibrated scenario. These observed increases in PDSI during the first half of the twenty-first century reflect artificial elevation of surface water availability as the result of normalizing increased drought severity in the twenty-first century and do not make statistical sense in the near term when juxtaposed with the twentieth century PDSI records.

Nevertheless, post-2052 running averages of PDSI using twenty-first century calibration were still lower than the corresponding post-1952 twentieth century running averages, indicating that even if “normal climate” is based on twenty-first-century projections, then the western US exhibits increased drought severity, frequency, and duration (as depicted by PDSI). However, these running averages are, by construction, less negative than those shown in Figs. 4, 6, 7, 8, 9 and 10, indicating that if 2001–2100 calibration is used, the resulting drought scenario is of intermediate severity between conditions of the twentieth century and those prominent in the future scenario based on a 1931–1990 calibration.