Understanding the Sensitivity of North American Drought in the Present and Past
Climate to the Tropical SSTs
29th Annual Climate Diagnostics & Prediction Workshop(October 2004, Madison WI)
Sang-Ik Shin, Robert S. Webb, Prashant D. Sardeshmukh, Robert J. Oglesby, Joshep J. Barsugli
NOAA-CIRES Climate Diagnostics Center, NASA/Marshall Space Flight Center
Questions
Do the tropical Pacific SST changes explain the
paleoclimate records of North American Drought at
the mid-Holocene (about 6,000 years ago) ?
Was sensitivity of the North American drought to the
tropical Pacific SSTs changed at 6ka?
Outline
Paleoclimate Evidence
Model & Experimental
Setup
6ka Changes in
Hydroclimate
Sensitivity Issue
Summary &
Conclusions
Present-day ClimateDai’s Leading EOF of the PDSI(Dai et al., Geophys. Res. Lett. [1998])
dry
wet
Leading EOF of the PDSISOI (r=0.61)Darwin SLP index (r=0.67)
Paleoclimate Records6ka
Paleoclimate Records (Tropical Pacific)Variability Weaker ENSOMean State La Niña-like
condition
Tudhope et al., (Science, 2001)
0ka
6ka
Cooler SSTs (~-2°C) at 6ka
Koutavas et al., (Science,2002)
Overpeck and Webb (PANS, 2000)
Warmer Colder
Model & Experimental Setup
• NCAR-CCM v3.10 (T42) / Slab Mixed Layer Ocean / Prescribed Tropical Pacific SST Conditions
(by using the 50-year composite of present-day observation)
• Sensitivity Experiments
orbital forcing (A-O coupling) amplified (weakened) seasonal cycle in Northern (Southern) Hemisphere
reduced greenhouse gases e.g., CO2
0ka : 355 ppmv (year 1950)
6ka : 280 ppmv
tropical Pacific conditions
Average (10S - 10N)
50-year Composite
Monsoon ChangesP
recip
. (m
m
day-
1)
North America
North Africa
Asia
Present-day
Month
P
recip
. (m
m
day-
1)
6ka-0ka
Month
1. 6ka Changes in Hydroclimete
Annually-Averaged Changes
Seasonal Cycle of Changes
Orbital Forcing (Enhanced Seasonal Cycle)Coupled
WES feedback ITCZ shift
Uncoupled (PMIP-I type)Temp.(C)
Precip.(mmday-1)
V(m/s)
PSL(hPa)
land/sea contrast
Greenhouse Gases
Temp.(C)
V(m/s)
PSL(hPa)
Precip.(mmday-1)
Radiative Forcing by
Reduced Greenhouse Gases
= -2.12 Wm-2
CO2 -1.30 Wm2
CH4 -0.47 Wm2
N2O -0.12 Wm2
CFCs -0.23 Wm2
Opposite to the Reconstructed Precipitation Changes
general coolingplus
reversed WES feedback
La Niña-Like Tropical Pacific Condition
Temp.(C)
V(m/s)
PSL(hPa)
Precip.(mmday-1)
500 hPa Height(m)
CI = 5 m
Other Tropical Pacific SST Conditions
Temp. (C) Precip. (mmday-1) V (m/s) and PSL (hPa)
La N
iña
plu
s 1
C
La N
iña
Tim
es
-1
El
Niñ
o
Seasonal Cycle over N. America
Month
Tem
p.
(
C)
Pre
cip
. (m
m
day-
1)
20N
50N
120W 70W
2. Sensitivity Issue
Was sensitivity of the North American drought to the tropical Pacific SSTs changed at 6ka? Sensitivity Experiments by using “CCM-MLM-Prescribed Tropical Pacific SSTs” - two model runs with prescribed tropical Pacific SSTs
observed present-day climatologyobserved 50-year La Niña composite
- in three different climate settingspresent-day climateclimate under the 6ka orbital forcingclimate under the 6ka forcings (orbital plus greenhouse
gases)
Effect of La Niña-Like Condition on the N. American Drought
Present-day Climate
Orbitally-Forced (6ka) Climate
6ka Climate
Temp. (C) Precip. (mmday-1)
Springtime (MAM) Precipitation Sensitivity Map
Target
Sensitivity Map
Spline Smoothing (SNR)
Area-averaged Precipitation over TargetSensitivity = Area-weighted SST over Patch
Patch Experiments
20N
50N
120W 70W
Drought
1C
Springtime (MAM) Combined EOFs
Sensitivity
PCs
Precip. (mmday-1)
(CI = 1 m)and
Surface Temp. (C)
500 hPa Heightand
Surface Temp.
Precip.
1st EOF (30.06%) 2nd EOF (15.96%)
Fraction of Local Variability in EOFs (Springtime)
Precip.
500 hPa Height
Temp.
1st EOF (30.06%)2nd EOF (15.96%)
%
Conclusions
Similar sensitivity of the springtime North American drought at 6ka and the present-day climate
Cooler Tropical Pacific SSTs (La Niña) Springtime Drought
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