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