Abstract
Abstract The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.
Highlights
The rainfall decrease and devastating droughts in the Sahel region during the last three decades of the twentieth century are among the largest recent climate changes recognized by the climate research community (e.g., Dai et al 2004)
This study investigates the role of dust radiative forcing in the Sahelian drought observed in the last three decades of twentieth century using simulations with an atmospheric general circulation model (GCM)
This is mainly due to the differences in the refractive index and the particle size distribution of dust; compared to many existing studies, we use less absorbing dust and more large particles and fewer small particles than previous modeling studies (e.g., Woodward 2001; Miller et al 2004) following suggestions from recent observations (Grini and Zender 2004; Hand et al 2004)
Summary
Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance of Dust Compared to Sea Surface Temperature Variations, Vegetation Changes, and Greenhouse Gas Warming. Donald Bren School of Environmental Science and Management, and Institute of Computational Earth System Science, University of California, Santa Barbara, Santa Barbara, California, and Climate and Global Dynamics Division, National Center for Atmospheric. Climate and Global Dynamics Division, National Center for Atmospheric Research,* Boulder, Colorado. Climate and Global Dynamics Division, National Center for Atmospheric Research,* and Program in Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado. Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder,* Colorado (Manuscript received 26 January 2006, in final form 3 July 2006)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
