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?