Research

Regional Climate

Key Findings

Research at the Climate Impacts Group (CIG) has made significant contributions to our understanding of Pacific Northwest (PNW) climate variability and change. Key findings from this research include the following.

Future climate in the Pacific Northwest

The CIG applies the most updated global climate models to determine the regional effects on temperature and precipitation:

• The fourth report from Intergovernmental Panel on Climate Change (IPCC 2007) describes six future greenhouse gas emissions scenarios, based on such factors as future socioeconomic changes, technological advances and population growth. These scenarios are used to estimate projections of radiative forcing, and consequent warming, through the 21st century. The radiative forcing under each scenario increases, however significant differences among the scenarios are not apparent until after the year 2020 (Figure 1). The CIG applies two of these scenarios to downscaled regional climate models to project temperature and precipitation trends through the 21st century for the Pacific Northwest (Figure 2) in Mote and Salathé 2010.

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Figure 1 Globally averaged radiative forcing by greenhouse gases and sulfate aerosols, for four of the six illustrative scenarios plus the older IS92a scenario, from IPCC (2001). In this study we use A1B and B1.

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Figure 2 Smoothed traces in temperature (top) and precipitation (bottom) for the twentieth and twenty-first century model simulations for the PNW, relative to the 1970–1999 mean. The heavy smooth curve for each scenario is the REA value, calculated for each year and then smoothed using loess. The top and bottom bounds of the shaded area are the 5th and 95th percentiles of the annual values (in a running 10-year window) from the ~20 simulations, smoothed in the same manner as the mean value. Mean warming rates for the twenty-first century differ substantially between the two SRES scenarios after 2020, whereas for precipitation the range is much wider than the trend and there is little difference between scenarios.

Regional climate model projections

The CIG strives to boost the resolution of global climate models to determine the regional effects of a changing climate, with the following results:

• The projections of temperature and precipitation for the Pacfic Northewest vary significantly between climate models depending on their resolution. The global models are coarser and represent wider geographic areas, whereas the high resolution climate models provide more regionally detailed climate projections. Salathé et al. 2008 show that finer scale models perform better when simulating regional climatic conditions because they capture topography, circulation patterns and oceanic processes, all of which influence regional climate. The warming trend is intensified in the regional projections, in part because the regional loss of snow pack and the resulting lower snow-albedo effect, is well represented in the regional model. Additionally, the regional model incorporates the local topography more realistically than the coarser global models, resulting in higher projections of fall season precipitation.
• Further work to resolve how global climate changes could specifically affect the Pacific Northwest region was completed by Zhang et al. (in press) and Salathé et al. (2009). The regional climate models capture the effects of the large scale weather patterns from the global climate models, incorporate the local terrain and represent the interactions between the weather and the topography. Projections from the regional models indicate the changes in temperature, precipitation, frequency of extreme events, snowpack and the seasonality of these trends. Figure 3 shows the change in snow water equivalent on April 1 projected by two regional climate models for the 2040s compared to the 1980s.

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Figure 3 Projected changes (from 1980 to 2040 averages) in April 1 snow water equivalent (mm) from the regionally downscaled CCSM3-WRF (left) and the ECHAM5-WRF (right) models. The CCSM3-WRF model projects a 71% decline, compared to a 32% decline projected by the ECHAM5-WRF model.

Projections of extreme events

The CIG has examined how global climate change may change the frequency and intensity of extreme events in the Pacific Northwest:

• Several global climate models agree on a northward shift in the North Pacific storm track and the Aleutian Low, two systems that heavily influence precipitation patterns in the North Pacific. Salathé et al (2006) statistically downscales global climate models to determine the shift in precipitation intensity and frequency under one future emission scenario. The results indicate that precipitation grows in intensity due to a projected increase in large-scale storm intensity and the interactions of large storms with the local terrain.
• Shifts in the magnitude of precipitation extremes as projected by regional climate models, downscaled from global climate models, are a key components to designing and managing stormwater infrastructure. Rosenberg et al. (2009) apply the precipitation output from two downscaled climate models to simulate streamflow in three urban watersheds in Washington State. The results indicate differences in precipitation extremes dependent on geographic location; an indication that adaptations in stormwater design will have to be region-specific and not a sweeping overhaul of infrastructure.