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Possible Causes of the Central Equatorial African Long-Term Drought

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LETTER • OPEN ACCESS Related content - Amplified subtropical stationary waves in Possible causes of the Central Equatorial African boreal summer and their implications for regional water extremes long-term drought Jiacan Yuan, Wenhong Li and Yi Deng - Multi-scale drought and ocean–atmosphere variability in monsoon To cite this article: Wenjian Hua et al 2016 Environ. Res. Lett. 11 124002 Asia Manuel Hernandez, Caroline C Ummenhofer and Kevin J Anchukaitis - Uncertainty in recent near-surface wind speed trends: a global reanalysis View the article online for updates and enhancements. intercomparison Verónica Torralba, Francisco J Doblas- Reyes and Nube Gonzalez-Reviriego Recent citations - Leveraging social protection to advance climate-smart agriculture: An empirical analysis of the impacts of Malawi’s Social Action Fund (MASAF) on farmers’ adoption decisions and welfare outcomes Antonio Scognamillo and Nicholas J. 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Lett. 11 (2016) 124002 doi:10.1088/1748-9326/11/12/124002 LETTER Possible causes of the Central Equatorial African long-term drought OPEN ACCESS Wenjian Hua1,2,4, Liming Zhou1, Haishan Chen2, Sharon E Nicholson3, Ajay Raghavendra1 and Yan Jiang1 RECEIVED 1 Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY 12222, USA 22 July 2016 2 Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and REVISED Environment Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), 11 October 2016 Nanjing University of Information Science & Technology, Nanjing 210044, People’s Republic of China ACCEPTED FOR PUBLICATION 3 Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA 21 October 2016 4 Present address: Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, 1400 PUBLISHED Washington Avenue, Albany, NY 12222, USA. 22 November 2016 E-mail: [email protected] Original content from this Keywords: equatorial Africa, drought, sea surface temperature, rainfall work may be used under the terms of the Creative Commons Attribution 3.0 licence. Abstract Any further distribution of ( ) this work must maintain Previous studies found that Central Equatorial Africa CEA has experienced a long-term drying trend attribution to the over the past two decades. To further evaluate this finding, we investigate possible mechanisms for this author(s) and the title of the work, journal citation drought by analyzing multiple sources of observations and reanalysis data. We examine the atmospheric and DOI. circulation changes related to sea surface temperature (SST) variations that control the equatorial African rainfall. Our results indicate that the long-term drought during April, May and June over CEA may reflect the large-scale response of the atmosphere to tropical SST variations. Likely the drought results primarily from SST variations over Indo-Pacific associated with the enhanced and westward extended tropical Walker circulation. These are consistent with the weakened ascent over Central Africa that is associated with the reduced low-level moisture transport. The large-scale atmospheric circulation changes associated with a weaker West African monsoon also havesomecontribution.Theseresultsreinforcethe notion that tropical SSTs have large impacts on rainfall over equatorial Africa and highlight the need to further distinguish the contribution of SSTs changes (e.g., La Niña-like pattern and Indian Ocean warming) due to natural variability and anthropogenic forcing to the drought. 1. Introduction and Gruber 2010, Asefi-Najafabady and Saatchi 2013, Diem et al 2014), while similar multi-decadal/chronic Rainfall trend and variability across Africa have received droughts are absent over other tropical rainforests much attention during the past few decades (Giannini (e.g., Amazon and Southeast Asia). A recent study et al 2008,Maidmentet al 2015). For example, detected a widespread decline in forest photosynthetic many studies focus on the multi-decadal Sahel drought capacity over the Congo Basin and attributed this (e.g., Zeng 2003,Daiet al 2004) and the decline in the large-scale decline, at least partially, to this drying spring rains over Eastern Horn of Africa (Williams and trend (Zhou et al 2014). Tropical rainforests have the Funk 2011, Lyon and DeWitt 2012,Yanget al 2014).The potential to modulate climatic changes via feedbacks interannual and multi-decadal rainfall features over to the climate system. A continued decline of photo- Southern Africa have also been detected (Richard synthetic capacity and moisture content driven by a et al 2001,Hoerlinget al 2006). Central Equatorial Africa persistent drying trend could alter the composition (CEA), on the other hand, is the least studied region in and structure of the rainforests and thus impact biodi- Africa and is a region that has been the subject of much versity and carbon storage (Nepstad et al 2007, Enquist less investigation than other large-scale convective and Enquist 2011, Fauset et al 2012, Chambers and regions in the tropics (Washington et al 2013). Roberts 2014). Hence, understanding the nature and Equatorial Africa, where the second largest tropi- cause of this drought and assessing its impacts on cal rainforest on Earth is located, has experienced a the forests are of significant societal, economic and long-term drying trend (Malhi and Wright 2004, Yin environmental importance. © 2016 IOP Publishing Ltd Environ. Res. Lett. 11 (2016) 124002 Oceanic conditions, especially sea surface tem- pattern of rainfall changes over CEA as discussed peratures (SSTs), have been highlighted as the driver above. The forested areas in West Africa are not for rainfall variations over Africa and other regions included as these regions have experienced extensive (Giannini et al 2003, Lu and Delworth 2005, Hoerling deforestation and other human-induced land cover et al 2006, Shanahan et al 2009, Dai 2013). Previous and land use change (Zhou et al 2014). studies have attributed the African droughts to chan- ges in SST patterns in the Atlantic and Indian Ocean 2.2. Observational and reanalysis data ( ) Hagos and Cook 2008,Lu2009, Ting et al 2009 . The CEA has the lowest gauge density in sub-Saharan recent drought in the Eastern Horn of Africa is related Africa (Washington et al 2013) and a dramatic decline ( to the SST warming in the Indian Ocean Funk in the number of rain gauges, especially after 1980s ) et al 2008, Williams and Funk 2011 and the multi- (Asefi-Najafabady and Saatchi 2013, Zhou et al 2014). fi decadal SSTs variations in the tropical western Paci c Maidment et al (2015) found distinct differences in the ( ) ( ) Lyon and DeWitt 2012 . Hoerling et al 2006 pro- Central African rainfall trends in both sign and posed that the steady warming in the Indian Ocean magnitude among multiple datasets. Lack of adequate enhances convection over those warm waters and thus observations presents a big challenge for understand- drives drying via subsidence over Southern Africa. ing the rainfall variability and climatic changes in this They also concluded that the Sahel drought mainly region. To compensate for the lack of observed data, results from the inter-hemispheric Atlantic SST con- we combine both ground observations and satellite trast and the associated southward displacement of the retrievals to represent rainfall quantities and Intertropical Convergence Zone. Although the Indian characteristics. Ocean and Atlantic Ocean are the key regions respon- Observational gridded monthly rainfall data sets fi sible for rainfall variability over Africa, the Paci c SST from the Global Precipitation Climatology Centre fl forcing also has in uences via its impacts on atmo- (GPCC, Schneider et al 2014) at 1°×1° resolution ( spheric circulation Seager et al 2003, Lyon and (1950–2014) and the Global Precipitation Climatology ) DeWitt 2012, Yang et al 2014, Dong and Dai 2015 . Project (GPCP, Adler et al 2003) at 2.5°×2.5° resolu- Compared to elsewhere in Africa, we know rela- tion (1979–2014) are used in this study. The gauge- tively little about the long-term drought over the CEA based GPCC data combines the Full Data Product and its mechanisms due to lack of observations (V7), based on quality-controlled data from 67 200 (Nicholson and Grist 2003, Todd and Washing- stations world-wide that feature record durations of ton 2004). In addition, the linkages between the equa- ten years or longer, and the monitoring product, based torial African rainfall and SSTs in the tropical Atlantic, on quality-controlled data from 7000 stations. The lat- Pacific and Indian Oceans are extremely complex and est GPCP data provides the combined precipitation differ substantially by season (Balas et al 2007, Farns- product derived from satellites and gauge measure- worth et al 2011, Dezfuli et al 2015). Here we explore ments (together with other major improvements).We the nature and cause of this long-term drought over also use gauge observations archived from the recent
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