Transcript Jim Hurrell, NCAR Attribution of Atlantic climate variability and change

The Great 20th Century Drying of Africa
Jim Hurrell, Marty Hoerling, Jon Eischeid
Adam Phillips, Taiyi Xu, Gary Bates
Climate Analysis Section, NCAR
Climate Diagnostics Center, NOAA
Ninth Annual CCSM Workshop
Climate Variability Working Group
9 July 2004, Santa Fe
Motivation
Increasing evidence of the controlling effect of global SST variations
on regional climate change over the 20th Century
• “On the cause of the 1930s Dust Bowl”
(Schubert, Suarez, Pegion, Koster, Bacmeister: Science 2004)
• “The Perfect Ocean for Drought”
(Hoerling and Kumar: Science 2003)
• “Tropical Origins for Recent North Atlantic Climate Change”
(Hurrell, Hoerling, Phillips, Bates, Xu: Climate Dynamics: 2004)
• “Oceanic Forcing of Sahel Rainfall …”
(Giannini, Saravanan, Chang: Science: 2003)
• Others …
Atmospheric GCM Integrations
Monthly evolving global SST and sea ice
(fixed solar insolation, aerosols and trace-gas composition)
80 members: NCAR (CAM2)
ARPEGE (3)
NASA (NSIPP-1)
ECHAM (3)
ECHAM (4.5)
(15)
(8)
(23)
(10)
(24)
Coupled Integrations
Unforced Control
CCSM2:
Last 650 of 1000 years
Coupled Integrations
Unforced Control
CCSM2:
Last 650 of 1000 years
Forced (“Dutch Challenge Experiment”)
CCSM1.4:
62 member ensemble
1940-2000: Prescribed GHG, sulfate aerosols,
solar radiation, volcanic aerosols
2001-2080: Business as usual GHG
Seasonal Cycle of Mean African Rainfall
First Harmonic
Observed
January
July
NCAR CCM3
Seasonal Cycle of Mean African Rainfall
First Harmonic
Observed
January
July
Multi-AGCM
Seasonal Cycle of 50-yr Seasonal Rainfall Trends
(1950-1999)
Observed
AGCM Simulated
(3-month overlapping seasons)
Seasonal Cycle of 50-yr Seasonal Rainfall Trends
(1950-1999)
Observed
AGCM Simulated
FMA
JAS
Time Series of Regional African Rainfall
(1950-1999)
South Africa: FMA
Sahel: JAS
Observed
Observed
AGCM simulated
AGCM simulated
PDF of 50-yr Seasonal Rainfall Trends
(1950-1999)
South Africa: FMA
Sahel: JAS
Observed
AGCM simulated
Unforced CCSM control
PDF of 50-yr Seasonal Rainfall Trends
(1950-1999)
South Africa: FMA
Sahel: JAS
Observed
Forced CCSM
Preliminary Conclusions
• African drying since 1950 has followed the seasonal migration of
monsoon rains
•The drying has been attributable to the temporal evolution of global SSTs
• Sub-Saharan drought has been strongly driven by the warming of the
South Atlantic relative to the North Atlantic Ocean
• The progressive warming of the Indian Ocean has played a major role
in the austral summer drought over southern Africa
• Experiments performed to date do not sample all SST influences known
to be important for African rainfall variability
• Coupled climate models do not produce 50-year drying trends as
large as observed