heat waves in mediterranean climate regimes: focus on california alexander gershunov climate,...
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Heat Waves in Mediterranean climate regimes:
focus on California
Alexander Gershunov
Climate, Atmospheric Science and Physical Oceanography
(CASPO)
Scripps Institution of Oceanography
La Jolla, California
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
Regional Heat Waves are Changing
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daytime magnitudenighttime magnitude
1950 1960 1970 1980 1990 2000
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End of July 2006
Day > Night magnitude
Night > Day magnitude
INDIVIDUAL HEAT WAVES
TOTAL HEAT WAVE ACTIVITY
California heat wave activity is increasing.
Specifically, nighttime-accentuated heat waves are on the rise…
Is this a feature of Mediterranean climates in general?
The heat wave of July 2006 was an unprecedented deadly event.
99% of cases lived in zip codes where > 50% of
residents live below Poverty Guide Line
147 total deaths
Quantifying Regional Heat Waves
• Heat Wave Index, related to impacts:
MAGNITUDE = intensity + duration + spatial extent
• Day and nighttime heat waves
• Defined regionally from daily weather station data: local temperatures exceeding a threshold
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o 43.3o 42.3o 41.7o 40.6o 39.4o 38.7o 36.2o 35o 32.4o 22.2
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Working definition of extreme heat:Daytime Intensity
• Locally extreme temperature exceeding a high percentile threshold t* (e.g. 99%-ile)
Sacramento Tmax
99th percentile in C of JJA Tmax over the base period 1950 – 1999.
Temperatures have a climatological 0.01 probability of exceeding these
thresholds and should be considered locally extreme. 15
2025
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5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 30
Jun Jul Aug
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Mean Tmax
2006 Tmax
Deg
rees
C
threshold (42.2C/108F)
99th percentile thresholds
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
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o 26.5o 24o 22.8o 21.1o 18.9o 18.3o 17.2o 16.1o 15o 12.8
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Working definition of extreme heat:Nighttime Intensity
• Locally extreme temperature exceeding a high percentile threshold t* (e.g. 99%-ile)
Mean Tmin
2006 Tmin
Deg
rees
C
Sacramento Tmin
99th percentile in C of JJA Tmin over the base period 1950 – 1999.
Temperatures have a climatological 0.01 probability of exceeding these
thresholds and should be considered locally extreme.
29C/84F
threshold (22.8C/73F)
99th percentile thresholds
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
1950 1960 1970 1980 1990 2000
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1+nights2+nights3+nights
1950 1960 1970 1980 1990 2000
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1+days2+days3+days
An Index of California Regional Heat Wave Activity
Daytime Heat Wave Activity Nighttime Heat Wave Activity
Nighttime heat wave activity is ramping up for all local durations
2006
2003
Daytime heat wave activity has increased somewhat
1950 1960 1970 1980 1990 2000
0.0
0.5
1.0
1.5
2.0
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
Seasonal Maxima of Regional Heat Wave Components
Tm
axT
min
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
GREAT HEAT WAVES AND THEIR OVERALL REGIOINAL MAGNITUDESLocal intensity + duration + spatial extentin degree days summed over the region
for the six greatest daytime and six greatest nighttime events2006
2003
19721960
1961 jun
1961 aug
19812002
1983 19901992
2001deg
ree
day
s p
er s
tati
on
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
1950 1960 1970 1980 1990 2000
02
46
daytime magnitudenighttime magnitude
What is the meteorological difference between daytime and nighttime heat waves?
Gre
at d
ayti
me
even
tsG
reat nigh
ttime even
ts
Moisture is what makes the difference between day and nighttime heat waves in this arid region
Surface circulation (wind at sigma level 995, arrows in m/s) and mean sea level
pressure in millibars (a,b), 500mb
geopotential height in meters (c,d), and
precipitable water kg/m2 (e,f) anomalies
with respect to JJA mean. Anomalies are
composited for the peak days of the
largest five daytime events (a,c,e) and the largest five nighttime
events (b,d,f). The data are from the
NCEP/NCAR Reanalysis I. Red rectangles outline
regions used for evolution plots
presented below.
Contours and colors represent the same anomalies, but only values statistically significant with 95% confidence (two-tailed test) determined via bootstrap re-sampling (performed with 1000 re-sampled 5-date composite anomaly maps) are plotted in color. Low level wind vectors are colored blue where significant according to similar re-sampling test performed for the u- and v-components separately. Significance is everywhere a function of magnitude and location. The reference period for computing anomalies is 1950 – 1999, as elsewhere. The anomalies are computed from 24-hour averaged fields.
WHY THIS CHANGE IN HEAT WAVE ACTIVITY?
• IS IT NATURAL?• IS IT ANTHROPOGENIC? • IS IT REGIONAL?• IS IT GLOBAL?• IS IT ALL OF THE
ABOVE?
1950 1960 1970 1980 1990 2000
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Day > Night magnitude
Night > Day magnitude
Is there a trend in humidity?Where does the moisture come from?
a) Linear trend in Precipitable Water (Reanalysis 1) b) PWTR Trend off Baja California
-160.0 -147.5 -135.0 -122.5 -110.0 -97.5 -87.5 -77.5 -67.5
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-0.5 0.0 0.5
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1950 1960 1970 1980 1990 200018
2022
2426
Linear trend computed at each pixel of the PRWTR averaged for July. Significant trends (95% significance level in a two-tailed test) are colored. (b) July PRWTR in the box [132.5-125W, 25-35N] and linear trend significant well above the 99% level for both the full and base (not shown) periods.
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
Why this moisture trend?Regional SST warming
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-0.2 -0.1 0.0 0.1 0.2
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1950 1960 1970 1980 1990 2000-1
.0-0
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Linear trend in sea surface temperature (C/decade) SST Trend off Baja California
As this region becomes warmer and more moist, the atmospheric circulation associated with heat waves, tends to bring humid air into California, giving heat waves a stronger nighttime expression.
Variations of the Earth’s surface temperatureDepartures in temperature in oC (from the 1990 value)
Proxy Instrument
Projections
oF
oC
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GLOBAL TEMPERATURE
• As temperature around the globe continues to rise, we can expect changes in specific regional weather extremes: • Heat waves• Cold spells
• What changes do models show in summertime heat waves in California?
California Heat Waves and Global Climate Change: CNRM
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02
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ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO
PAST FUTUREPRESENT
observations
Day > Night magnitude
Night > Day magnitude
1900 1950 2000 2050 2100
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1900 1950 2000 2050 2100
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California Heat Waves and Global Climate Change: CNRM
1950 1960 1970 1980 1990 2000
02
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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
California Heat Waves and Global Climate Change
WHAT DO OTHER MODELS SHOW?
California Heat Waves and Global Climate Change: GFDL
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PAST FUTUREPRESENT
observations
Day > Night magnitude
Night > Day magnitude
ANOTHER CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO SUGGESTS INCREASE, BUT IN THE OPPOSITE SENSE
1900 1950 2000 2050 2100
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What’s happening in other Mediterranean Climate Regimes?
At least one, two and three day/night local duration events
Tmax Tmin
Courtesy of Maria Jose Ortiz Bevia
SPAIN
In California: • Great heat waves are primarily day or nighttime events caused by specific
pressure, wind and moisture patterns
• Tendency towards more humid heat waves and unprecedented nighttime temperatures have implications for energy demand, agriculture, public and animal health…
• Climate models suggest trends will continue and accelerate – BUT NOT ALWAYS IN THE OBSERVED SENSE
• Is the observed trend over California and Nevada reflected over other world regions with similar “Mediterranean” climates?
• Spanish heat waves are increasing somewhat consistently with their Californian sisters, but without a disproportionate nighttime intensification
• Climate change is playing out with regional idiosyncrasies
Conclusions