Hydrologic Implications of 20th Century Climate Variability and Global Climate Change in the Western U.S. Alan F. Hamlet, Philip W. Mote, Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington

Hydroclimatology of the Western U.S.

Cool Season Climate of the Western U.S. PNW GB CA CRB DJF Temp (°C) NDJFM Precip (mm)

(mm) Cool Season Precipitation Warm Season Precipitation

Hydrologic Characteristics of PNW Rivers

A Seasonal Water Balance for the Naches River in Eastern Washington

Climatic Variations Associated with ENSO and PDO

Pacific Decadal Oscillation El Niño Southern Oscillation A history of the PDO A history of ENSO warm warm cool 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Effects of the PDO and ENSO on Columbia River Summer Streamflows PDO Cool Cool Warm Warm high high low low Ocean Productivity

PDO Effects ENSO Effects PDO/ENSO Effects Naturalized Streamflow for the Columbia River at The Dalles OR PDO Effects ENSO Effects PDO/ENSO Effects

PDO Effects ENSO Effects PDO/ENSO Effects Inflows to Chester Morse Lake in the Cedar River Basin PDO Effects ENSO Effects PDO/ENSO Effects

Cool Season Precipitation Anomalies Compared to the PDO -0.845 -0.264 -0.438 -0.053 (Regional to PDO Correlation R )

X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003 Fig 8 Warm, neutral, cool ENSO 100-year flood DJF Avg Temp (C) DJF Avg Temp (C) DJF Avg Temp (C) X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003

Changing Climate in the 20th Century

A Time Series of Temporally Smoothed, Regionally Averaged Met Data for the West

(% per century for precip, degrees C per century for temperature) Linear Trends in Cool and Warm Season Climate for 1916-2003 and 1947-2003 (% per century for precip, degrees C per century for temperature)

Tmax Tmin Figure 4

Differences in cool and warm season precipitation trends suggest different mechanisms (large-scale advective storms vs. smaller scale convective storms) and differing sensitivity to regional warming. Trends in warm season precipitation in the CRB are very different than the other regions and may function more like cool season precipitation (e.g. related to circulation rather than locally generated storms)

Regionally Averaged Cool Season Precipitation Anomalies

Trends in April 1 SWE 1950-1997 Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): 39-49

Overall Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

Temperature Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

Precipitation Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

Wide-Spread Glacial Retreat has Accompanied 20th Century Warming. Loss of glacial mass may increase summer flow in the short term and decrease summer flow in the long term. 1902 2002 The recession of the Illecillewaet Glacier at Rogers Pass between 1902 and 2002. Photographs courtesy of the Whyte Museum of the Canadian Rockies & Dr. Henry Vaux.

Snowpack is melting earlier Hamlet A.F.,Mote P.W, Clark M.P., Lettenmaier D.P., 2005: Effects of temperature and precipitation variability on snowpack trends in the western U.S., J. of Climate, 18 (21): 4545-4561 Simulated Change in Date of 90% Melt 1916-2003

spring flows rise and summer flows drop As the West warms, spring flows rise and summer flows drop Stewart IT, Cayan DR, Dettinger MD, 2005: Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8): 1136-1155 Spring snowmelt timing has advanced by 10-40 days in most of the West, leading to increasing flow in March (blue circles) and decreasing flow in June (red circles), especially in the Pacific Northwest.

June March Trends in simulated fraction of annual runoff in each month from 1947-2003 (cells > 50 mm of SWE on April 1) March June Relative Trend (% per year)

Flood risks have increased in many coastal areas with warm winter temperatures, whereas colder inland areas show decreases in flood risk. Simulated Changes in the 20-year Flood Associated with 20th Century Warming DJF Avg Temp (C) Fig 3 20 year flood A spatial scale X20 2003 / X20 1915 X20 2003 / X20 1915

Regionally Averaged Cool Season Precipitation Anomalies

20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime DJF Avg Temp (C) X20 ’73-’03 / X20 ’16-’03 X20 ’73-’03 / X20 ’16-’03

Forecasting the Impacts of Global Climate Change

Natural AND human influences explain the observations of global warming best. Natural Climate Influence Human Climate Influence Red: observations of global average temperature Grey: simulations with a climate model (huge computer program, like weather prediction model only run for hundreds of years) Natural influence: volcanoes, solar variations – guesses before ~1970, better since then Human influence: greenhouse gases, sulfate aerosols it is possible to simulate the climate of the last 100 years, and the conclusion is that humans didn’t matter much before 1960 – the early warming and the cooling were largely natural, and the late-century warming was largely human-caused All Climate Influences

Observed 20th century variability Curves are fits to ln(CO2) for A2 (solid) and B1 (dashed) Warming ranges are shown for 2020s, 2040s and 2090s relative to 1990s. Central estimates: 0.7C by 2020s, 1.7C by 2040s, 3.2C by 2090s. Pink box shows +/- 2 sigma for annual average temperature (sigma=0.6C). Red lines show previous generation of change scenarios. Until mid-century, emissions scenarios play a minor role in the temperature impacts. Towards the end of the century they play a big role. Conclusions: 1) Adaptation will be an essential component of the response to warming over the next 50 years. 2) Mitigation of greenhouse gas emissions will play an important role in determining the scope of late 21st century impacts. 0.4-1.0°C Pacific Northwest

Observed 20th century variability % -1 to +3% +6% +2% +1% Curves are fits to ln(CO2) for A2 (solid) and B1 (dashed) Precip changes are shown for 2020s, 2040s and 2090s relative to 1990s. Central estimates: 1% by 2020s, 3% by 2040s, 6% by 2090s. Pink bar shows +/- 2 sigma for PNW annual precip. Observed 20th century variability -1 to +9% -2 to +21% Pacific Northwest

Schematic of VIC Hydrologic Model and Energy Balance Snow Model PNW CA CRB GB Snow Model

Shasta Reservoir inflows

Seasonal Water Balance Naches River Current Climate 2040s Scenario More runoff in winter and early spring, less in summer 2040s Scenario (+ 2.5 C)

For areas that accumulate snowpack in winter, the warmest locations are most sensitive to warming +4.5% winter precip

Changes in Simulated April 1 Snowpack for the Canadian and U. S Changes in Simulated April 1 Snowpack for the Canadian and U.S. portions of the Columbia River basin (% change relative to current climate) 20th Century Climate “2020s” (+1.7 C) “2040s” (+ 2.25 C) -3.6% -11.5% -21.4% -34.8% April 1 SWE (mm)

Simulated Changes in Natural Runoff in the Snowmelt-Dominant Naches River Basin Associated with 2 C Warming Impacts: Increased winter flow Earlier and reduced peak flows Reduced summer flow volume Reduced late summer low flow

Simulated Changes in Natural Runoff in the Rain Dominant Chehalis River Basin Associated with 2 C Warming

Conclusions Cool season precipitation and temperature play a crucial role in the hydrologic behavior of mountain watersheds in the western U.S. Cool season climate is not stationary and is influenced by cyclical patterns like ENSO, decadal variability associated with the PDO, and by long term trends related to global warming and other unknown factors. ENSO and PDO forecasts provide useful information for predicting streamflow and other hydrologic variables at seasonal to interannual time scales. Regional expressions of global warming in the West are expected to result in systematically warmer temperatures, streamflow timing shifts in snowmelt and transient basins, increases in winter flows, and losses of summer water availability. Systematic changes in cool season precipitation volumes appear to be modest both in observations and GCM simulations, but changes in cool season precipitation variability since 1975 suggest another possible impact pathway.