Download Mechanisms of drought in present and future climate

Mechanisms of drought in
present and future climate
Gerald A. Meehl and Aixue Hu
Definition of drought (premise: if it’s 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?
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
1930s
1950s
1950s
In another 150 year long dataset, the 1880s
drought also qualifies (occurrence rate
remains at 2 droughts per century)
1880s
1930s
1950s
Correlations of (11 year running mean) observed western
U.S. and Great Plains precipitation with observed SSTs
confirms connections to tropical Pacific and Atlantic
Observed decadal pattern (1871-2000), “IPO”, >13 yr low pass
Pacific pattern strongly resembles the “Interdecadal
Pacific Oscillation” (IPO, Power et al., 1999)
IPO correlated with observed low pass precip, 1901-2000
Multi-decadal IPO pattern from long model control run
EOF1 of
low pass
filtered (13
yr) SST
ENSO
EOF1 of
band pass
filtered (2-7
yr) SST
Correlation of IPO
(multidecadal
EOF1 SST) with:
Precipitation
Sea level pressure
(over North
America,
connected to
convective heating
anomalies in
Pacific)
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 σ for at least 5 consecutive years
Extreme drought: precipitation anomalies exceeding 2 σ
for at least one year during a severe drought, or severe
drought conditions for at least 10 consecutive years
Great Plains area (wrt respective
climatology; 1 σ = 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
Western U.S. area (wrt respective
climatology; 1 σ = 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
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
CCSM3
PCM
future
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
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
______
Mechanism for multi-decadal SST and precipitation
variability in the Indo-Pacific region (Meehl, G. A., and A. Hu,
Fig. 20
2006, Journal of Climate, 19, 1605–1623.)
______
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
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
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)
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 σ 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
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 σ 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
Conclusions
1. 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
2. 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
3. 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
4. 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)
5. 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
Can we predict North American droughts 10 to 30 years in
advance?
“Decadal prediction” regional skill could come from three
sources:
1. Climate change commitment from the forcing already in
the system
2. Climate change from the forcing from ongoing increases of
GHGs
3. Predicting time-evolving regional decadal phenomena
whose mechanisms could be captured in an initialized
climate state (e.g. PDO/IPO, MOC, AMO, etc.)
An example:
EOF1 from a model 20th
century simulation is the
forced trend
Let’s see if we can
predict EOF2 (the
internally generated IPO
pattern associated with
North American drought)
in a perfect model
ensemble experiment
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)
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)