Expected Changes in Key Weather-related Extreme Events in California

1. Expected Changes in Key Weather-related Extreme Events in California Presented by: Alexander Gershunov (Scripps/UCSD) Co-authors: Mike Dettinger (USGS) Ed Maurer (Santa Clara University) Michael Mastrandrea (Stanford) Alex Hall (UCLA) Norm Miller (LBNL) Lisa Sloan (UC Santa Cruz) Kelly Redmond (WRCC/DRI) Guido Franco (CEC) Robin Webb (NOAA) David Pierce (Scripps/UCSD) Dan Cayan (USGS and Scripps/UCSD) Kristen Guirguis (Scripps/UCSD and UCAR-PACE)

2. Heat Waves are Already Changing in California INDIVIDUAL HEAT WAVES The heat wave of July 2006 was an unprecedented deadly event California heat wave activity is increasing Specifically, humid, nighttime-accentuated heat waves are on the rise HEAT WAVES ARE BECOMING MORE HUMID! TOTAL HEAT WAVE ACTIVITY Day > Night magnitude Night > Day magnitude Gershunov, Cayan and Iacobellis, 2009, Journal of Climate

3. California Heat Waves and Global Climate Change: CNRM ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO PAST PRESENT FUTURE observations Day > Night magnitude Night > Day magnitude

4. California Heat Waves and Global Climate Change: CNRM ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO SUGGESTS THAT THE OBSERVED CHANGE IS A TIP OF THE ICEBERG PAST PRESENT FUTURE observations Day > Night magnitude Night > Day magnitude

5. SUMMERTIME DAILY TMAX ºC Gershunov and Guirguis, work in progress Mean Warming and Temperature Extremes Along the coast, extreme heat is projected to become more extreme relative to mean temperatures.

6. Mean Warming and Temperature Extremes JANUARY TMIN • The high end of the distribution would experience more pronounced warming • Very cold temperatures (low end of the distribution) would not change • Average temperatures (e.g., Tmin average for Jan) are not a good indicator of change Warm Tmin extremes are projected to become more extreme relative to mean temperatures everywhere Pierce et al. 2011. Probabilistic estimates of California climate change by the 2060s using statistical and dynamical downscaling

7. Snow-to-Rain conversion changes the frequency, intensity and timing of floods Water Resources and Flood Regimes MOST PRECIPITATION FALLS IN THE MOUNTAINS AND CURRENTLY AS SNOW: HERE WE HAVE A TREND!

8. historically: “Cool” storms contribute immediate runoff from smaller areas of the river basin (the rest goes into snowpack for later) Warming affects the type of precipitation runoff Less snow, more rain In a warmer climate: Warm storms contribute immediate runoff from larger areas of the river basin runoff 1°C (1.8 °F) warming causes snowlines to rise 500 feet

9. More frequent and intense rainfall-driven floods in winter, fewer snowmelt-driven floods in spring Das et al. (2011) project increases in both the magnitudes of annual-peak floods and in the frequencies of generally high flows from both the Northern and South-Central Sierra. The reasons for those increases reflect some combination of effects from increased storm  intensities, warmer storms, more rain, and even wetter winter soils.  Das et al., 2011, Climatic Change

10. COLORADO RIVER DROUGHT • Drought, as expressed in Colorado River flow, is projected to become more frequent, more intense and longer-lasting, resulting in water deficits not seen during the instrumental record. This is due primarily to the effects of warming, e.g. declining snowpack, earlier melt, and enhanced evapotranspiration. • The Southern 2/3 of California is expected to be more drought-prone than in the recent past. • Northern watersheds in the Sierra Nevada, however, may become wetter with climate change. Cayan et al. 2010, PNAS

11. Santa Ana Winds Santa Ana winds of coastal Southern California are expected to diminish in frequency and intensity, but at the same time become drier and hotter. Hughes, Hall and Kim, 2011, Climatic Change

12. Looking ahead • The concept of frequency and intensity of extremes is key. • Extreme impacts often involve a convergence of different causal mechanisms, e.g. heat and humidity. • Global change does not automatically equate to increased storminess or increases in other measures.   • It is crucial to sustain high quality observations, not only of physical climate measures but also of key impact variables. • Models are invaluable tools in looking forward. Careful scrutiny of model simulations is needed to ascertain if models make realistic results for the "right" reason. This requires mechanistic understanding. • Finer scale climate and atmospheric models, which perform realistically, are needed to elucidate processes and occurrence of extreme impact events. These models are still being developed.

13. Summary of Extremes • Heat waves: A tendency towards not only stronger, longer lasting and more intense, but also more humid heat waves and specifically accentuated nighttime temperatures is observed and projected for California. In coastal regions, heat waves may become more intense even relative to the mean seasonal warming. • Cold spells might not necessarily decline much in intensity. • Storms: Global change does not automatically equate to increased storminess or increases in other measures. Taking a global view, storminess changes vary considerably, and along the low mid latitudes of the North Pacific and the southern 2/3 of the California coast storminess may actually decline. However, the intensity and frequency of very intense Atmospheric Rivers is projected to increase. • Floods generated by atmospheric rivers on the western slopes of the Sierra Nevada range are expected to increase in intensity and in frequency in winter. It is also expected that snowmelt-driven spring and summertime floods will diminish in both frequency and intensity. • Drought, as expressed in Colorado River flow, is projected to become more frequent, more intense and longer-lasting, resulting in water deficits not seen during the instrumental record. This is a high-confidence result. The Southern 2/3 of California is expected to be more drought-prone than in the recent past. Northern watersheds in the Sierra Nevada, however, may become wetter with climate change. • Santa Ana winds of coastal Southern California are expected to diminish in frequency and intensity, but at the same time become drier and hotter. • Sea level rise is expected to cause more frequent and intense coastal erosion and salt-water intrusion and extreme impacts on coastal infrastructure, tourism and water resources.