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Recent summer precipitation trends in the Greater Horn of Africa and the emerging role of Indian Ocean sea surface temperature |
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| Authors: | A Park Williams Chris Funk Joel Michaelsen Sara A Rauscher Iain Robertson Tommy H G Wils Marcin Koprowski Zewdu Eshetu Neil J Loader |
| Institution: | 1. Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM, USA 2. Geography Department, University of California, Santa Barbara, CA, USA 3. Earth Resources Observation and Science (EROS), U.S. Geological Survey, Sioux Falls, SD, USA 8. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM, USA 4. Department of Geography, College of Science, Swansea University, Swansea, UK 5. Department of Geography, Rotterdam University, Rotterdam, The Netherlands 6. Laboratory of Dendrochronology, Institute of Ecology and Environment Protection, Nicolaus Copernicus University, Toruń, Poland 7. Forestry Research Centre, Ethiopian Institute of Agricultural Research, Addis Ababa, Ethiopia |
| Abstract: | We utilize a variety of climate datasets to examine impacts of two mechanisms on precipitation in the Greater Horn of Africa (GHA) during northern-hemisphere summer. First, surface-pressure gradients draw moist air toward the GHA from the tropical Atlantic Ocean and Congo Basin. Variability of the strength of these gradients strongly influences GHA precipitation totals and accounts for important phenomena such as the 1960s–1980s rainfall decline and devastating 1984 drought. Following the 1980s, precipitation variability became increasingly influenced by the southern tropical Indian Ocean (STIO) region. Within this region, increases in sea-surface temperature, evaporation, and precipitation are linked with increased exports of dry mid-tropospheric air from the STIO region toward the GHA. Convergence of dry air above the GHA reduces local convection and precipitation. It also produces a clockwise circulation response near the ground that reduces moisture transports from the Congo Basin. Because precipitation originating in the Congo Basin has a unique isotopic signature, records of moisture transports from the Congo Basin may be preserved in the isotopic composition of annual tree rings in the Ethiopian Highlands. A negative trend in tree-ring oxygen-18 during the past half century suggests a decline in the proportion of precipitation originating from the Congo Basin. This trend may not be part of a natural cycle that will soon rebound because climate models characterize Indian Ocean warming as a principal signature of greenhouse-gas induced climate change. We therefore expect surface warming in the STIO region to continue to negatively impact GHA precipitation during northern-hemisphere summer. |
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