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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 20, issue 9 | Copyright
Hydrol. Earth Syst. Sci., 20, 3789-3798, 2016
https://doi.org/10.5194/hess-20-3789-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 13 Sep 2016

Research article | 13 Sep 2016

Synchrony of trend shifts in Sahel boreal summer rainfall and global oceanic evaporation, 1950–2012

Alima Diawara1, Yoshihiro Tachibana1, Kazuhiro Oshima2, Hatsumi Nishikawa1, and Yuta Ando1 Alima Diawara et al.
  • 1Weather and Climate Dynamics Division, Mie University, Tsu, Japan
  • 2Institute of Arctic Climate and Environment Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan

Abstract. Between 1950 and 2012, boreal summer (rainy season) rainfall in the Sahel changed from a multi-decadal decreasing trend to an increasing trend (positive trend shift) in the mid-1980s. We found that this trend shift was synchronous with similar trend shifts in global oceanic evaporation and in land precipitation on all continents except the Americas. The trend shift in oceanic evaporation occurred mainly in the Southern Hemisphere (SH) and the subtropical oceans of the Northern Hemisphere (NH). Because increased oceanic evaporation strengthens the atmospheric moisture transport toward land areas, the synchrony of oceanic evaporation and land precipitation is reasonable. Surface scalar winds over the SH oceans also displayed a positive trend shift. Sea surface temperature (SST) displayed a trend shift in the mid-1980s that was negative (increasing, then decreasing) in the SH and positive in the NH. Although SST had opposite trend shifts in both hemispheres, the trend shift in evaporation was positive in both hemispheres. We infer that because strong winds promote evaporative cooling, the trend shift in SH winds strengthened the trend shifts in both SST and evaporation in the SH. Because high SST promotes evaporation, the trend shift in NH SST strengthened the NH trend shift in evaporation. Thus differing oceanic roles in the SH and NH generated the positive trend shift in evaporation; however, the details of moisture transport toward the Sahel are still unclear, or perhaps there is no single determining influence.

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The western African Sahel region has been one of the most important research areas for studying climatic variability due to its fragile climate conditions. Between 1950 and 2012, summer rainfall in the Sahel changed from a multi-decadal decreasing trend to an increasing trend (positive trend shift) in the mid-1980s. We found that this trend shift was synchronous with similar trend shifts in global oceanic evaporation and in land precipitation on all continents except the Americas.
The western African Sahel region has been one of the most important research areas for studying...
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