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An Imminent Transition to Drier
Conditions in the United States?
Aiguo Dai
Department of Atmospheric & Environ. Sci., University at Albany, SUNY And
National Center for Atmospheric Research (NCAR)
Invited Talk at Water Forum II: Texas Drought and Beyond
Austin, Texas, 22-23 October, 2012
Questions to: [email protected]
Related Papers:
Dai, A., 2012: Increasing drought under global warming in observations and models. Nature Climate Change, published online on Aug. 5, 2012 (available here).
Dai, A., 2011: Characteristics and trends in various forms of the Palmer Drought Severity Index (PDSI) during 1900-2008. J. Geophys. Res., 116, D12115, doi:10.1029/2010JD015541. (PDSI Data)
Dai, A., 2011: Drought under global warming: A review. Wiley Interdisciplinary Reviews: Climate Change, 2, 45-65.
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• Introduction: - Drought Indices, PDSI
- Global warming & its potential impact on drought
• Observed global changes of drought
• Model Predictions
• U.S. precipitation & drought changes
• Conclusions
Outline
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Some towns in western
Texas went more than 60
straight days over 100F
65 days >100
Texas
JJA 2011
Hottest
on
record
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Drought Indices are used to monitor and quantify the areal extent and severity of drought
Many drought indices are based on precipitation alone
Some are based on P minus Potential Evaporation (PE)
A few are based on a surface water balance model, such as the Palmer Drought Severity Index (PDSI).
P E
R W
dW/dt=P-E-R
How do we quantify drought?
•Temperature affects PDSI through E
• Negative PDSI = dry conditions;
• PDSI< -3 : Severe drought
• Self-calibrated PDSI with Penman-
Monteith PE (sc_PDSI_pm)
Two soil layers in the PDSI model
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Monthly anomaly data
4/2004 -12/2008
Significant
PDSI correlates with observed soil moisture, streamflow, and water storage over land (Dai 2011)
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IPCC 2007
Observed Global Temperature Series
1850−2006
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• Increased surface radiation provides new energy for evaporation
• Higher temperatures increase atmospheric demand for water vapor
• Reduced precipitation frequency means more dry spells
• Larger warming over land than over ocean leads to
- larger PE increases over land than over ocean - increases in water vapor transport from oceans can
not match atmospheric demand over land drier . conditions over land.
Drying Processes under Global Warming
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Has drought increased during the recent warming?
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PDSI < -3 :
Severe drought
Precipitation and PDSI Trends: 1950-2010
PDSI Trend
Precip. Trend
Stippling: the trend is significant at the 5% level
Red=Drying
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Other Observational Evidence of Drying
Dai et al. (2009) Runoff Trend
Precip. Trend
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Effect of Surface Warming on Drought Area
Dai (2012a)
With warming
Without warming
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Do models predict increasing drought under GHG-induced global
warming?
Climate models are imperfect, but useful for
studying what MIGHT happen in the future.
GHG = greenhouse gas
CMIP5 = the Fifth Coupled Model Inter-comparison Project, used for IPCC AR5
We use CMIP5 multi-model ensemble mean to represent the GHG-
induced change, as natural variations are small in the ensemble mean.
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PDSI in 21st Century: PDSI<−3 = Severe Drought
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Different PDSI Trends over the U.S.: Obs. vs. Model
Historical
Changes
Model
Prediction
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What Caused the Historical Wetting Trend in the U.S.?
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U.S. PDSI Time Series: 1923-2010
Smoothed PDSI
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U.S. Precipitation Time Series: 1923-2010
Smoothed Precipitation
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Observed Monthly Precipitation vs. Nino3.4 SST Correlation Data Period: 1920-2010 (land) and 1979-2010 (ocean)
Nino3.4 = 5oS-5oN, 120oW-170oW
Dotted area: significant at 5% level 22
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Leading Modes of Global SST Variability: 1920-2011 Defining the Inter-decadal Pacific Oscillation (IPO) Index
EOF = Empirical Orthogonal Function SST data source: HadISST data set from U.K. Met Office Hadley Centre
Global Warming Mode
ENSO & IPO Mode
IPO Index
Smoothed PC
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Precip
IPO
1977-1998 1946-1976
IPO vs. Southwest U.S. Precip Correlation
1924-1945
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Observed Precip IPO
Simulated Precip
CanAM4 - Canadian Atmospheric Model version 4 (dx~2.8o) forced with observed SST from 1950-2009
IPO vs. Southwest U.S. Precip Correlation
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Change Patterns: 1999-2010 minus 1977-1998 Observed Temperature Change Observed Precip Change
Model Simulated P Change: HadGEM2-A Model Simulated P Change: CanAM4
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Change Patterns: 1999-2010 minus 1977-1998
Atmos. Reanalysis
ERA-Interim
Model Simulation
CanAM4
Colors:
850mb geopotential
height difference
Arrows:
850mb wind difference
H
H
H
H
160oE 160oW 120oW 80oW 40oW 0o
160oE 160oW 120oW 80oW 40oW 0o
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Meehl & Hu (2006)
IPO Warm Phase:
IPO Cold Phase:
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Decadal Difference: 1999-2010 minus 1977-1998
Drying from 1977-1998 to 1999-2010 resulted mainly from IPO and other natural variations. 30
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What is the outlook for the near future?
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Future Trends in Soil Moisture and PDSI due to Global Warming Only
soil moisture
PDSI
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Prediction of the IPO
- What controls the IPO phase change is not well understood
- Models still have difficulties in predicting the IPO
- If it follows previous cycles Cold phase 1999-2029, Warm phase 2030-2051 33
Precip
IPO
1977-1998
22yrs
1946-1976
31yrs
2020 2030
1999-2029?
31yrs
1924-1945
22yrs
?
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Model-simulated Soil Moisture Changes
Global Warming Effect Only
Future PDSI Changes
With/wihtout IPO Effect
IPO +
Global Warming
Global Warming
only
Future Changes Relative to 1977-1998
2011-2029
2030-2051
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Global Warming Mode: 1950-2099
SST
PDSI
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Available on YouTube: www.youtube.com/NcarUcar
PDSI Due to GHG-induced Global Warming Only
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Is this our future? Worst wild fire in Texas history!
Bastrop State Park: 14,000 acres, Nearly 600 homes destroyed. 5 Sept 2011
Sixty separate wildfires, whipped by strong winds, were burning
across Texas on Monday, destroying hundreds of homes and killing
at least two people: TIME 37
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• Precipitation and streamflow data, together with PDSI, show a drying trend
over most Africa, South and East Asia, southern Europe, eastern Australia
and other regions since 1950;
• Recent warming appears to have enhanced drying over many land areas
during the last 30 years;
• Model projections suggest severe drying in the 21st century over most land
areas (including the U.S.), except northern high-latitudes and most Asia;
• Warm tropical Pacific SSTs led to the wet period from 1977-1998 over the
U.S.;
• The switch to cold tropical Pacific SSTs since 1999 caused drier
conditions over the U.S. during 1999-2011;
• The current dry conditions may last and worsen during the next 20 yrs as
the IPO cold phase persists and global warming continues; and
• Even if the IPO switches back to a warm phase after 2030, the U.S. may
still face dry conditions as the drying from global warming becomes large.
Summary
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