key points

•Drought and Flood are a common occurrence in California, 2012-2015 Drought is intense and warm as well as dry

•The absence of very large storms drives California dry (and wet) spells

• Climate change will likely exacerbate stresses on water resources

Climate Change Will Stress California’s Already Volatile Water Supply

Dan Cayan with Mike Dettinger, Mary Tyree, Sam IacobellisScripps Institution of Oceanography and USGS

UC Davis Oct 2015

Sierra discharge highly variable stddev/mean = 0.44 lowest 1977 highest 1983

But ,Columbia dischargeis quite steady, stddev/mean = 0.19

Annual discharge varies considerably in the Sierra Nevada

and Colorado dischargeis intermediately variable, stddev/mean = 0.33

Cayan et al 2003

California and much of western region has been more-or-less dry since 1999 observed precipitation departure (% of average), 1998-99 thru 2013-14

% departure from1981-2010 average

U.S. Drought Monitor/National Weather Service Hanford

Dettinger and Cayan Drought and the Delta—A Matter of Extremes accepted, San Francisco Estuary and Watershed Science, April 2014

a few large storms (or their absence) account for a disproportionate amount of California’s precipitation variability

5yr smoother

LARGE STORM CONTRIBUTION

95%ile

Recent dry years have mostly been warmer than normal. 2013-14 was exceptionally warm.

Precipitation Departure during dry years

Temperature Departure during dry years

2013-14

Anomalous warmth during winter 2014-15

virtually all climate simulations project warming, but with a wide envelope of temperature change

+3°C

CMIP5 GCMs project +3°C summer warming by 2060, under mid and high RCPs

14 GCMs X 3 RCP Emissions Scenarios IPCC 5th Assessment (CMIP5) models

July Temperature change Sacramento

Need to kmow

Which emissions pathway will we take?

How much summer amplification of warming?

How will temperature change inNear term?

Sierra Snowpack has been extremely low 2015 and 2014 spring snowpack disproportionately light

Dettinger and Anderson SFEWS 2015

Loss of California Spring Snowpack from 21st Century warming

•Under this scenario, California loses half of its spring (April 1) snow pack due to climate warming. Less snow, more rain, particularly at lower elevations. The result is earlier run-off, more floods, Less stored water. This simulation by Noah Knowles is guided by temperature changes from PCM’s Business-as-usual coupled climate simulation. (this is a low-middle of the road emissions and warming scenario)

Knowles, N., and D.R. Cayan, 2002: Potential effects of global warming on the Sacramento/San Joaquin watershed and the San Francisco estuary. Geophysical Research Letters, 29(18), 1891.

Declining odds of median or higher snowpack

Source: Cayan et al. (2011)

Drier Summer Landscapesincreased warming and diminished snowcauses successively greater soil drying

throughout 21st Century

(this picture could change somewhat under more recent CMIP5 simulations)

early 21st

middle 21st

late 21st

Cayan et al. Ch 6 Southwest Climate Assessment

Projected patterns of precipitation changes2090-2099 versus 1980-1999

Globally, dry regions become drier?

Although the number of wet days is projected

to decrease with climate change,

the intensity of the largest wet days is projected to increase !

Suraj Polade Polade, S.D., Pierce, D.W., Cayan, D.R., Gershunov, A., and Dettinger, M.D., 2014, The key role of dry days in changing regional climate and precipitation regimes: Nature Scientific Reports, 4:4364, 8 p., doi:10.1038/srep04364.

28 CMIP5 RCP 8.5 GCMs (2060–2089) vs. (1960-1989)

• California’s climate is prone to year-to-year and longer term variation in precipitation—drought is an expected part of our climate..

• California’s current dry spell has built up over multiple years, a more/less dry pattern has been in place since 1999.

• The absence of a few very large storms is often a key driver of dry years. And large storms are frequently involved in “busting” drought.

.

• Continued, substantial climate warming is highly likely but changes in annual precipitation is not well understood in California. However, climate change may shift precipitation characteristics—fewer overall wet days but more intense heavy events.

• In addition to the present ongoing dry spell, recurrent wet and dry events and episodes, in the midst of warm temperatures will provide future learning opportunities.

Summary

Whole CONUS PDSI

CA/NV PDSI

California/Nevada droughtonly moderately correlated w whole U.S.

PDSI from NCDC divisional dataset

ground waterwithdrawl increases

greatly in dry years

Dry Year

Typical Year

Faunt et al 2009 USGS

pumping

How are droughts made?

>80%

Mike Dettinger

California’s large yearly variation in precipitation is dictated by the presence or absence of storms during a

narrow seasonal window

California has only about 120 days to accumulate two thirds of its annual precipitation. When Pacific winter stormsAre diverted from California (as last winter) dry conditions result.

California year-to-year variation in precipitation is has the most volatile!of the entire U.S.

Cayan et al.2003

Multi-year dry spells are quite common in California Sacramento Drainage Divi standardized precip index 1900-2014 showing Intensity and Duration

Anne Steinemann et al. 2015 J. HydroMet. in press

Calif dought footprint is quite broad in western U.S. Sierra flows often in same phase (wet or dry)

as the Columbia or the Colorado

Low Sierra flows associated with large regional pattern

Cayan et al.2003

Sierra Nevada Annual PrecipitationCoef of Variation ~31.5%

mean 39.1 inches std dev 12.3 inches

2012-2014 dry spell is characteristic of California’s volatile precipitation climate

WY 2014 precip estimated at 65% of LT mean

From California Climate TrackerWestern Regional Climate Center

California has a narrowseasonal window to generate its annual water supply.

If atmospheric conditions areunfavorable during that period, a dry year results

prevalence of anomalously warm daytime temperatures

during cool season monthsof drought years

Sacramento Drainage DivisionDark bars are from 20% driest years

Pale bars are all years 1895-2014

CA1

ACCESS 1.0

CMCC CM

CMCC CMS

Climate Models indicate occasional

drought for CaliforniaIn 21st Century

Recent Water Loss over Western U.S. Surface displacements from GPS network show progressive uplift due to water mass loss in drought

GPS estimate by Borsa et al showing deficit of ~240km**3 water in March 2014 relative to 2003-2014 baseline over western U.S., west of 109°W. This is in general agreement with GRACE satellite observations, S. Castle et al 2014 GRL found ~65km**3 water water loss in Colorado Basin during 2004-2013.

Borsa et al. 2014

Meko, D.M., Woodhouse, C.A., and Touchan, R., 2014, Klamath/San Joaquin/Sacramento hydroclimatic reconstructions from tree rings: Report to California DWR 4600008850, 117 p.

1100 years of Sacramento River Discharge little overall change but considerable variability and multi-year spells reconstructed from tree ring records bu Dave Meko and colleagues, Univ Arizona

prevalence of anomalously warm daytime temperatures

during cool season monthsof drought years

Sacramento Drainage DivisionDark bars are from 20% driest years

Pale bars are all years 1895-2014

Co- occurrences of high (h) and low (l) annual Columbia, Sierra, Colorado discharge anomaly magnitudes > 0.7 1906-1999; anomalies +0.7 standard deviations above long-term means are counted as high, more than –0.7

standard deviations below normal are counted as low.

Sierra

h l

Columbia

h 8 3

l 2 13

How often do extreme annual flows co-occur in Sierra, Columbia, Colorado?

Sierra

h l

Coloradoh 13 4

l 2 16

Sierra and Columbia High and especially low flows co-occur. Relatively few Years when they have Opposing anomalies

Sierra and Colorado High and especially low flows co-occur. Relatively few Years when they have Opposing anomalies

Cayan et al.2003

34

Atmospheric Rivers West Coast flood generators

SummaryDrought Characteristics •California’s climate is prone to year-to-year and longer term variation in precipitation—drought is expected..

•California’s current dry spell has built up over multiple years, a more/less dry pattern has been in place since 1999.•Temperature during cool seasons of drought years—in recent decades, has more often been warmer than cooler than average. As with precipitation,, anomalous temperature strongly controlled by atmospheric circulation

Drought prediction is a challenge:•The absence of a few very large storms is often a key driver of dry years. And large storms are frequently involved in “busting” drought.

•there is not a unique atmospheric drought-circulation pattern..

•Anomalous Pacific SST patterns, tropical and extratropical appear in advance, during and after strong drought circulation months, but signals are somewhat weak.

500mb height anomalies averaged, all months during Dry years

ENSO vs. California PrecipitationSouthern California

K. Redmond et alWestern Regional Climate Center

K. Redmond et alWestern Regional Climate Center

Nino 3.4 hatched regionNino 3.4 SST

Number dry yrs Drought years occur duringAll phases of ENSO

PDO SST Anomaly Pattern

K. Redmond et alWestern Regional Climate Center

Number dry yrs

PDO Index

Drought years occur duringboth phases of PDO

over 21st Century occurs a marked decline of chances of reaching or exceeding historical median

Snow Water Equivalent Sierra Nevada+

10%

….and, chances of historical 10th percentile or less SWE increases greatly

40%

Median Apr 1 SWE 11.9cm

10th % Apr 1 SWE 3.6cm