Effects of Saharan dusts on weather and climate over the Atlantic:

How Nature foiled the 2006 hurricane forecasts

William K. M. Lau

NASA/GSFC

Laboratory for Atmospheres

Co-author: K. M. Kim/GEST

MODIS Images

Effects of Saharan dusts on weather and climate over the Atlantic

• Annual emission of Saharan dust is estimated to be 4 billion tons

• Increased Saharan dust outbreak since the 1970’s, associated with the prolonged drought in Sahel and increased dust storm over the Sahara desert

Possible weather and climate impacts:

• SAL dry air may suppress tropical cyclogenesis, and/or hurricane formation

• Microphysics effects: suppress warm rain; increase ice-nucleation and deep convection

• Solar Dimming Effect Solar Dimming Effect • “ “ Elevated Heat Pump” EffectElevated Heat Pump” Effect

2005 15H, 12TS, 2TD, 1STS, 1STD

2006 5H, 4TS

How did nature foil the2006 hurricane forecasts?

Dust appears to be the primary cause for SST cooling in the MDR; El Nino is associated with reduced SST (and increased vertical shear) in the eastern equatorial Atlantic, suppresses cyclogensis

El Nino-SSTeffect

Dust-SST“Solar Dimming”effect

More dust means lesser number of days of tropical cyclones

Correlations of HC/TS statistics with a) Barbados dust and b) El Nino SST, 1980-2007

•

TS = sustained wind speed > 34 knts (17 m/s)HC = sustained wind speed >64 knts (33 m/s)

The “Elevated Heat Pump” effect : fvGCM simulation of radiativeEffect on circulation and climate of the West African region

Caribbean Africa

Gulf of Guinea Africa

Estimate of anomalous (aerosol+ water cycle feedback) surface energy flux over the N.

Atlantic/Caribbean region

Short wave flux = -30.4 Wm-2

Longwave flux = 5.9 Wm-2

Sensible heat flux = 2.1 Wm-2

Latent heat flux = 8.6 Wm-2

Net surface flux = -14.1 Wm-2

reducedSW

Anomalous Overturning Circulation induced by“Elevated Heat Pump” effect for WAM

cools ocean

increasedmoisture influx

induced subsidence suppresses convection

reduced SWcools land

dustsource

increased low level clouds

reduced upper level cloudsincreased upper level clouds

Africa

Caribbean Region

West Africa

East Atlantic

Caribbean