Download In California

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
End of July 2006
INDIVIDUAL HEAT WAVES
The heat wave of
July 2006 was an
unprecedented
deadly event.
0
2
4
6
daytime magnitude
nighttime magnitude
1950
1960
1970
1980
1990
2000
147 total
deaths
99% of cases lived in zip
codes where > 50% of
residents live below
Poverty Guide Line
8
TOTAL HEAT WAVE ACTIVITY
6
Day > Night magnitude
Night > Day magnitude
4
California heat wave activity is increasing.
0
2
Specifically, nighttime-accentuated heat
waves are on the rise…
1950
1960
1970
1980
1990
2000
Is this a feature of Mediterranean climates in
general?
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
Working definition of extreme heat:
Daytime Intensity
Sacramento Tmax
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2006 Tmax
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threshold (42.2C/108F)
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th percentile in C of JJA Tmax over
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99
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the base period 1950 – 1999.
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Temperatures have a climatological
20
25
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45
• Locally extreme temperature
exceeding a high percentile
threshold t* (e.g. 99%-ile)
99th percentile thresholds
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42.3
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41.7
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40.6
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39.4
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38.7
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36.2
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32.4
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22.2
15
Mean Tmax
5
10 15 20 25 30
Jun
5
10 15 20 25 30
Jul
5
0.01 probability of exceeding these
thresholds and should be considered
locally extreme.
10 15 20 25 30
Aug
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
Working definition of extreme heat:
Nighttime Intensity
Sacramento Tmin
99th percentile thresholds
• Locally extreme temperature
exceeding a high percentile
threshold t* (e.g. 99%-ile)
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2006 Tmin
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Mean Tmin
5
10 15 20 25 30
Jun
5
10 15 20 25 30
Jul
5
10 15 20 25 30
Aug
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.
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
An Index of California Regional
Heat Wave Activity
Nighttime Heat Wave Activity
2.0
1+nights
2+nights
3+nights
1.0
2006
2003
0.0
4
4
0.5
Daytime heat wave activity has
increased somewhat
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6
1+day s
2+day s
3+day s
1.5
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8
Daytime Heat Wave Activity
1960
1970
1980
1990
2000
0
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2
1950
1950
1960
1970
1980
1990
2000
1950
1960
1970
1980
1990
2000
Nighttime heat wave activity is ramping up
for all local durations
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
Tmin
Tmax
Seasonal Maxima of Regional
Heat Wave Components
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
GREAT HEAT WAVES AND THEIR OVERALL REGIOINAL MAGNITUDES
Local intensity + duration + spatial extent
in degree days summed over the region
for the six greatest daytime and six greatest nighttime events
6
daytime magnitude
nighttime magnitude
4
2003
1972
1960
2
1961 jun
1961 aug
2002
1981
1983
1990
1992
2001
1990
2000
0
degree days per station
2006
1950
1960
1970
1980
Gershunov, Cayan and Iacobellis, Journal of Climate, 2009
What is the meteorological difference
between daytime and nighttime heat waves?
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 resampled 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 24hour averaged fields.
Moisture is what makes the difference between
day and nighttime heat waves in this arid region
WHY THIS CHANGE
IN HEAT WAVE ACTIVITY?
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4
6
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IS IT NATURAL?
IS IT ANTHROPOGENIC?
Day > Night magnitude
IS IT REGIONAL?
Night > Day magnitude
IS IT GLOBAL?
IS IT ALL OF THE
ABOVE?
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•
1950
1960
1970
1980
1990
2000
Is there a trend in humidity?
Where does the moisture come from?
a) Linear trend in Precipitable Water (Reanalysis 1)
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0.0
b) PWTR Trend off Baja California
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50.0
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40.0
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35.0
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30.0
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27.5
25.0
-160.0
-147.5
-135.0
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-110.0
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-77.5
-67.5
o
1950
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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 [132.5125W, 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
Linear trend in sea surface temperature (C/decade)
-0.1
0.0
0.1
0.2
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-1.0
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1.0
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1.5
-0.2
SST Trend off Baja California
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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.
GLOBAL TEMPERATURE
Variations of the Earth’s surface temperature
Departures in temperature in oC (from the 1990 value)
• As temperature around the globe continues
to rise, we can expect changes in specific
regional weather extremes:
Projections
• Heat waves
• Cold spells
• What changes do models show in
summertime heat waves in California?
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C
Proxy
Instrument
California Heat Waves and
Global Climate Change: CNRM
PAST
PRESENT
FUTURE
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50
ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO
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0
2
30
4
40
6
observations
1950
1960
1970
1980
1990
2000
Night > Day magnitude
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10
Day > Night magnitude
1900
1950
2000
2050
2100
California Heat Waves and
Global Climate Change: CNRM
PAST
PRESENT
FUTURE
8
50
ONE CLIMATE MODEL UNDER THE MILD EMISSIONS “B1” SCENARIO
SUGGESTS THAT THE OBSERVED CHANGE IS A TIP OF THE ICEBERG
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0
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4
40
6
observations
1950
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1980
1990
2000
Night > Day magnitude
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Day > Night magnitude
1900
1950
2000
2050
2100
California Heat Waves and
Global Climate Change
WHAT DO OTHER MODELS SHOW?
California Heat Waves and
Global Climate Change: GFDL
80
ANOTHER CLIMATE MODEL UNDER THE MILD EMISSIONS “B1”
SCENARIO SUGGESTS INCREASE, BUT IN THE OPPOSITE SENSE
PRESENT
FUTURE
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PAST
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2
4
6
60
observations
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1950
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Day > Night magnitude
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Night > Day magnitude
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2050
2100
What’s happening in other
Mediterranean Climate Regimes?
SPAIN
Tmax
Tmin
At least one, two and three day/night local duration events
Courtesy of Maria Jose Ortiz Bevia
Conclusions
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