Expected Changes in Key Weather-related Extreme Events

in CaliforniaPresented 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)

Heat Waves are Already Changing in California

1950 1960 1970 1980 1990 2000

02

46

daytime magnitudenighttime magnitude

1950 1960 1970 1980 1990 2000

02

46

8

Day > Night magnitude

Night > Day magnitude

INDIVIDUAL HEAT WAVES

TOTAL HEAT WAVE ACTIVITY

California heat wave activity is increasing

Specifically, humid, nighttime-accentuated heat waves are on the rise

The heat wave of July 2006 was an unprecedented deadly event

Gershunov, Cayan and Iacobellis, 2009, Journal of Climate

HEAT WAVES ARE BECOMING MORE HUMID!

California Heat Waves and Global Climate Change: CNRM

1950 1960 1970 1980 1990 2000

02

46

8

ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO

PAST FUTUREPRESENT

observations

Day > Night magnitude

Night > Day magnitude

1900 1950 2000 2050 2100

01

02

03

04

05

0

1900 1950 2000 2050 2100

01

02

03

04

05

0

California Heat Waves and Global Climate Change: CNRM

1950 1960 1970 1980 1990 2000

02

46

8

PAST FUTUREPRESENT

observations

Day > Night magnitude

Night > Day magnitude

ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO SUGGESTS THAT THE OBSERVED CHANGE IS A TIP OF THE ICEBERG

Mean Warming and Temperature

Extremes

Along the coast, extreme

heat is projected to

become more extreme

relative to mean

temperatures.

SUMMERTIME DAILY TMAX

Gershunov and Guirguis, work in progressºC

Mean Warming and Temperature

Extremes

Warm Tmin extremes are projected to

become more extreme

relative to mean

temperatures everywhere

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 changePierce et al. 2011. Probabilistic estimates of California climate change by the 2060s using statistical and dynamical downscaling

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!

historically:historically:““Cool” storms Cool” storms contribute contribute immediate runoff immediate runoff from smaller areas from smaller areas of the river basin of the river basin (the rest goes into (the rest goes into

snowpack for later)snowpack for later)

In a warmer climate:In a warmer climate:

Warm storms Warm storms contribute contribute immediate runoff immediate runoff from larger areas from larger areas of the river basin of the river basin

runoff

runoff

Warming affects the type of precipitation

Less snow, more rain

1°C (1.8 °F) warming causes snowlines to rise 500 feet

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-peakfloods 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

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

Santa Ana Winds

Hughes, Hall and Kim, 2011, Climatic 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.

• 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.

Looking ahead

• 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.

Summary of Extremes