Mechanisms of drought in present and future climate

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Mechanisms of drought in present and future
climate

• Gerald A. Meehl and Aixue Hu

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Definition of drought (premise if its not
raining it will be dry must be simple to apply
to model analysis) Megadrought (long-lived
multi-decadal extreme drought often mentioned in
a paleoclimate context) 11 year running mean
area-averaged precipitation anomalies over
western U.S. less than zero for at least 20
consecutive years (Meehl and Hu, J. Climate,
2006) (one megadrought every 170 years in a 1360
year global coupled climate model control run, or
0.6 megadroughts per century) Is there an
analogous simple definition for notorious
shorter-lived decadal droughts that have had
severe impacts in the U.S. (e.g. the 1930s and
1950s) for Great Plains and western U.S. areas?
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Severe decadal drought definition based on
historical precedents (1930s and 1950s droughts)
negative values of 11 year running mean
area-averaged precipitation that exceed one
standard deviation for at least 5 consecutive
years (rate of 2 per century)
1930s 1950s
1930s 1950s
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In another 150 year long dataset, the 1880s
drought also qualifies (occurrence rate remains
at 2 droughts per century)
1880s 1930s 1950s
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Correlations of (11 year running mean) observed
western U.S. and Great Plains precipitation with
observed SSTs confirms connections to tropical
Pacific and Atlantic
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Observed decadal pattern (1871-2000), IPO, gt13
yr low pass
Pacific pattern strongly resembles the
Interdecadal Pacific Oscillation (IPO, Power et
al., 1999)
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IPO correlated with observed low pass precip,
1901-2000
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Multi-decadal IPO pattern from long model control
run
EOF1 of low pass filtered (13 yr) SST EOF1 of
band pass filtered (2-7 yr) SST
ENSO
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Correlation of IPO (multidecadal EOF1 SST) with
Precipitation Sea level pressure (over North
America, connected to convective heating
anomalies in Pacific)
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To look at future changes in decadal droughts
analyze stabilized forcing multi-hundred year
global coupled climate model simulations (PCM and
CCSM3) present-day 1990 control (300
years) future 2100-2300 from stabilized (at
year 2100) A1B scenario (200 years) Severe
drought 11 year running mean precipitation
anomalies exceeding 1 s for at least 5
consecutive years Extreme drought precipitation
anomalies exceeding 2 s for at least one year
during a severe drought, or severe drought
conditions for at least 10 consecutive years
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Great Plains area (wrt respective climatology 1
s 0.05-0.07) PCM Present 2 droughts per
century average duration 7.8 years extreme 0.7
per century (obs 2/century, 9.0 yrs,
1.0/century) Future 0.5 droughts per century
average duration 9.0 years extreme 0.5 per
century CCSM3 present 1.1 droughts per
century average duration 11.0 yrs extreme
0.9 per century Future 0.5 droughts
per century average duration 9.0 years
extreme 0 per century
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Western U.S. area (wrt respective climatology 1
s 0.05-0.06) PCM Present (350 yrs) 2
droughts per century average duration 7.0 yrs
extreme 1.0 per century (obs 2/century, 9.5
yrs, 0.5/century) Future (200 yrs) 2 droughts
per century average duration 5.3 years
extreme 1.0 per century CCSM3 present (300
yrs) 2.0 droughts per century average
duration 8.0 yrs extreme 0.6 per century
Future (200 yrs) 0.5 droughts per
century average duration 8.0 years extreme
0.5 per century
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Great Plains precipitation correlated with SSTs
(11 yr running means) dominant connection with
tropical Pacific (cold tropical Pacific dry
Great Plains) Future pattern somewhat weakened
compared to present, but qualitatively similar
present-day
future
CCSM3 PCM
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Western U.S. precipitation correlated with SSTs
(11 yr running means) dominant connection with
tropical Pacific (cold tropical Pacific dry
Western U.S.) Future pattern somewhat weakened
compared to present, but qualitatively similar
CCSM3 PCM
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There are significant contributions of
multi-decadal Pacific SST anomalies (IPO/PDO) to
severe drought over both the Western U.S. and
Great Plains Therefore, understanding the
mechanism of severe drought over North America
requires (at least) understanding the mechanism
of the IPO/PDO that produces the multidecadal SST
anomalies
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Mechanism for multi-decadal SST and precipitation
variability in the Indo-Pacific region (Meehl, G.
A., and A. Hu, 2006, Journal of Climate, 19,
16051623.)
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______
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CMIP3 multi-model future changes of
season-averaged precipitation show large seasonal
changes in the pattern over North America (dry in
far southwest U.S. and Mexico during winter dry
over the Pacific northwest and southern Great
Plains in summer)
Fig. SPM-6
Stippled areas are where more than 90 of the
models agree in the sign of the
change Precipitation increases very likely in
high latitudes Decreases likely in most
subtropical land regions This continues the
observed patterns in recent trends
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Average future JJA surface temperature change
shows relatively greater warming in western
U.S. due to changes in circulation (more ridging
over western U.S.) associated with convective
heating anomalies in tropical Pacific
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Proportionately greater increases of western U.S.
temperature (and decreased precipitation)
increase vulnerability to future drought in terms
of precipitation and evaporation (P minus E
becomes more negative there in future)
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Western U.S. P minus E mean base state climate
change for JJA PCM -0.21 mm day-1 CCSM3 -0.08
mm day-1 1 s for both about 0.01 to 0.02 mm
day-1 JJA base state change roughly an order of
magnitude greater than one standard deviation, so
virtually every year in future climate would be a
severe drought year by present-day standards
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Great Plains P minus E mean base state climate
change for JJA PCM -0.12 mm day-1 CCSM3
-0.02 mm day-1 1 s for both about 0.04 to 0.06
mm day-1 JJA base state change produces a
tripling of severe drought in PCM, and a doubling
of severe drought in CCSM3
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• Conclusions
• Severe droughts over two areas of North America
  (western US and Great Plains) defined in relation
  to historical severe droughts of 1930s and 1950s
  in terms of multi-decadal precipitation anomalies
• Similar patterns of SSTs that drive present-day
  droughts (relative cool tropical Pacific SSTs)
  also drive future droughts a warmer base state
  appears to qualitatively reduce drought severity
  somewhat with respect to the new future climate
• A mechanism proposed to explain the decadal
  pattern of SSTs in the tropical Pacific
  associated with droughts over North America
  involves coupled interactions between
  tropics-midlatitudes and ocean-atmosphere with
  wind-forced ocean Rossby waves near 25N and 25S
  providing the decadal timescale
• In relation to present-day droughts, future
  summertime droughts more severe due to warmer
  mean temperatures and enhanced evaporation (P
  minus E becomes more negative)
• In western U.S. every year is a severe drought
  year in future compared to present drought
  regime over Great Plains there is a doubling to
  tripling of severe droughts in future compared to
  present

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• Can we predict North American droughts 10 to 30
  years in advance?
• Decadal prediction regional skill could come
  from three sources
• Climate change commitment from the forcing
  already in the system
• Climate change from the forcing from ongoing
  increases of GHGs
• Predicting time-evolving regional decadal
  phenomena whose mechanisms could be captured in
  an initialized climate state (e.g. PDO/IPO, MOC,
  AMO, etc.)

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An example EOF1 from a model 20th century
simulation is the forced trend Lets see if we
can predict EOF2 (the internally generated IPO
pattern associated with North American drought)
in a perfect model ensemble experiment
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Decadal predictions of IPO index for the Pacific
9 out of 29 members (31) show some predictive
skill out to 20 years (CCSM3.0, T42, atmospheric
initial state perturbed with same ocean initial
state at year 2000 one reference, 29 ensemble
members)
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• CMIP5 Decadal Predictability/Prediction core
  model runs
• 1.1 10 year integrations with initial dates
  towards the end of 1960, 1965, 1970, 1975, 1980,
  1985, 1990, 1995 and 2000 and 2005
• Ensemble size of 3, optionally to be increased
  to O(10)
• Ocean initial conditions should be in some way
  representative of the observed anomalies or full
  fields for the start date
• Land, sea-ice and atmosphere initial conditions
  left to the discretion of each group
• Model run time 300 years (optionally, an
  additional 700 years)
• 1.2 Extend integrations with initial dates near
  the end of 1960, 1980 and 2005 to 30 yrs.
• Each start date to use a 3 member ensemble,
  optionally to be increased to O(10)
• Ocean initial conditions represent the observed
  anomalies or full fields.
• Model run time 180 years (optionally, an
  additional 420 years)

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