Antropogenic changes of Sahel rainfall Reindert Haarsma Nanne Weber Frank Selten
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Transcript of Antropogenic changes of Sahel rainfall Reindert Haarsma Nanne Weber Frank Selten
Antropogenic changes of Sahel rainfall
Reindert Haarsma
Nanne Weber
Frank Selten
Royal Netherlands Meteorological Institute
Dutch Challenge ProjectImpact of global warming on extreme events and other questions that require good statistical sampling: NAO, ENSO, THC, hydrological cycle, etc.
Period of investigation 1940-2080
• State-of-the-Art climate model: CCSM version 1.4, T30, 18 levels
• Large ensemble (62 members). Different initial conditions
Until 2000: Observed forcing (GHG, Solar, Vulcanos, Aerolsols)After 2000: GHG scenario: BAU (SRES A1b)
Experiment done on TERAS supercomputer SARA AmsterdamStart experiment: 01-06-2003 Last run: 22-09-2003
Project leaders: Frank Selten (VO/KNMI), Henk Dijkstra (IMAU/UU)
Experiment was motivated by earlier studies with ECBilt(Schaeffer et al. 2003, vd Brink et al. 2003).
Large number of scientists at KNMI, including Meteorological Research, and at other CKO institutes are involved in analyzing the runs.
Global mean temperature
Blue dots: observed (Clim, Res. Unit)
Red dots: ensemble members
Black line: ensemble mean
Ensemble mean difference
2050-2080 minus 1950-1980
Surface air temperature (SAT)
Precipitation
Sahel Seasonal Cycle
Change in precipitation in August Seasonal cycle (1951-1980)-(2051-2080) 62 members for each period
month
Red: 2051-2080
Blue: 1951-1980
Precipitation
Annual cycle precipitation in West Africa
Model 62 ensembles
NCEP
1950-1980
EOF’s Precipitation JAS Model Giannini et al., Scienceexpress, 2003
25% obs.
15% obs.
Correlation SAHEL precipitation with SAT.
Member 55
Member 56
Inter-member variability
Projection of precipitation EOF’s on SAHEL rainfall
Difference SAT
2050-2080 minus 1950-1980
during JAS
Correlation EOF 1 with SAT
Detrended 21 yearrunning mean
Model ensemble mean
Model ensemble mean
Correlation EOF 1 with MSLP
Detrended 21 year running mean
Difference MSLP
2050-2080 minus 1950-1980
during JAS
Model ensemble mean
Correlation EOF 1 withSoil moisture
Detrended 21 year running mean
Model ensemble mean
PDF of seasonal mean (JAS) rainfall
2050-20801950-1980
rainfall mm/day
CONCLUSIONS
Increase of Sahel rainfall due to greenhouse warming
This increase of Sahel rainfall is connected with dominant mode Sahel precipitation
Mechanism is due to:
enhanced warming of Sahara
decrease MSLP over Sahara
increased advection of moisture
QUESTIONS
Explained variance of EOF’s and correlation with SST’s: Hindcast run with observed SST’s.
Role of clouds
How crucial is the heating of the Sahara?