Geophysical Research Letters, 37 (2010) L09706; doi: 10.1029/2010GL043038
Physical mechanism of long-term drying trend over tropical North Africa
Hiroaki Kawase, Manabu Abe, Yukiko Yamada (National Institute for Environmental Studies, Tsukuba, Japan), Toshihiko Takemura (Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan), Tokuta Yokohata (Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan) and Toru Nozawa (National Institute for Environmental Studies, Tsukuba, Japan)
Abstract
Based on an approximated moisture budget equation, we investigate the physical mechanisms of a drying trend observed over tropical North Africa in the boreal summer during the 20th Century by analyzing datasets of several climate-model experiments forced with various combinations of natural and anthropogenic forcings. Increased anthropogenic aerosols thermodynamically induce a drying trend due to a tropospheric cooling and dynamically induce an additional drying trend due to an atmospheric local circulation change stirred up by the strong gradient of a sea surface temperature anomaly over the tropical Atlantic Ocean. Increased greenhouse gases, on the other hand, induce a drying trend through the large-scale dynamic effect, which is canceled out by the thermodynamically induced moistening trend due to tropospheric warming. Therefore, the drying trend observed over tropical North Africa during the 20th Century is strongly affected by the increased anthropogenic aerosols through both the dynamic and thermodynamic effects.
Received 4 March 2010; accepted 31 March 2010; published 11 May 2010.
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