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Weakened Seasonality of the African Rainforest Precipitation in Boreal Wang et al. Geosci. Lett. (2021) 8:22 https://doi.org/10.1186/s40562-021-00192-w RESEARCH LETTER Open Access Weakened seasonality of the African rainforest precipitation in boreal winter and spring driven by tropical SST variabilities Xin‑Yue Wang1,2, Jiang Zhu1,2, Meijiao Xin1,2, Chentao Song1,2, Yadi Li1,2, Yi Zhou1,2 and Xichen Li1,3* Abstract Precipitation in the equatorial African rainforest plays an important role in both the regional hydrological cycle and the global climate variability. Previous studies mostly focus on the trends of drought in recent decades or long‑time scales. Using two observational datasets, we reveal a remarkable weakening of the seasonal precipitation cycle over this region from 1979 to 2015, with precipitation signifcantly increased in the boreal winter dry season (~ 0.13 mm/ day/decade) and decreased in the boreal spring wet season (~ 0.21 mm/day/decade), which account for ~ 14% (the precipitation changes from 1979 to 2015) of their respective climatological means. We further use a state‑of‑the‑ art atmospheric model to isolate the impact of sea surface temperature change from diferent ocean basins on the precipitation changes in the dry and wet seasons. Results show that the strengthening precipitation in the dry season is mainly driven by the Atlantic warming, whereas the weakening precipitation in the wet season can be primarily attributed to the Indian Ocean. Warming Atlantic intensifes the zonal circulation over the African rainforest, strength‑ ening moisture convergence and intensifying precipitation in the boreal winter dry season. Warming Indian Ocean contributes more to reducing the zonal circulation and suppressing the convection in the boreal spring wet season, leading to an opposite efect on precipitation. This result has important implication on local ecology as well as global climate system. Keywords: Seasonality of precipitation, African rainforest, Walker circulation, Convergence of moisture Introduction the global tropical circulation (Hart et al. 2019; Webster African rainforest, located in the Congo Basin, is the 1973), which make great contributions to the global cli- world’s second-largest tropical forest region (Malhi mate system. In addition, deep convection over the Afri- et al. 2013; Mayaux et al. 2013; Fig. 1a). Rainforests play can rainforest is larger than any other region in the global important roles in hydrology and carbon storage due to tropics, dominating the global tropical rainfall distribu- their rich biodiversity and high ecological function (Bac- tion in the transition seasons (Washington et al. 2013). cini et al. 2012; Saatchi et al. 2011). African rainforest is However, long-term drying trend occurred over the one of three key convective regions, and the other two equatorial western and central Africa (Asef-Najafabady are Maritime Continent and Amazon basin (Washing- and Saatchi 2013; Diem et al. 2014; Hua et al. 2016; Malhi ton et al. 2013; Todd and Washington 2004). Deep con- and Wright 2004; Yin and Gruber 2010), resulting in a vection and diabatic heating over three regions drive declining of Congo rainforest greenness and altering the forest structure (Chambers and Roberts 2014; Zhou et al. 2014). Terefore, it is crucial to understand the change of *Correspondence: [email protected] climate in African rainforest regions, in particular pre- 1 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China cipitation in diferent seasons. Full list of author information is available at the end of the article © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Wang et al. Geosci. Lett. (2021) 8:22 Page 2 of 12 Fig. 1 African rainforests and monthly mean precipitation. a The distribution of tropical African rainforest, based on 2011–2015 MCD12C1 datasets. In this study, we focus on the domain (7°E‑32°E, 4°S‑5°N, the box), covering the most of African rainforest. b The spatial averaged (the box in a) climatology of monthly precipitation, based on 1979–2015 GPCP and CMAP datasets Rainfall in equatorial Africa is afected by the shift of (Jackson et al. 2009; Nicholson 2009; Pokam et al. 2012). the intertropical convergence zone (ITCZ, Nicholson Under the impact of these complex factors, the seasonal 2018; Sultan and Janicot 2000) and is related to West cycle of equatorial African precipitation is characterized African monsoon (Nicholson 2009; Redelsperger et al. by a bimodal feature, with two rainy seasons (March, 2006; Torncroft et al. 2011) and local zonal circulation April, May, MAM; September, October, November, SON) (Leduc‐Leballeur et al. 2013; Nicholson and Dezfuli 2013; and two dry seasons (June, July, August, JJA; December, Williams and Funk 2011). An upward branch of Walker January, February, DJF) (Creese and Washington 2018; circulation over the Congo Basin and a downward branch Washington et al. 2013; Fig. 1b). over the Gulf of Guinea play important roles in the pro- Tere is a seasonal diference in driving the variation of cess of precipitation over the West Africa (Cook and Vizy regional precipitation by sea surface temperature (SST) 2016). Deep convection caused by ascending motion forcing (Balas et al. 2007; Farnsworth et al. 2011). On over the Congo Basin is positively associated with pre- intraseasonal timescales, the equatorial SST cooling plays cipitation in West Africa (Cook and Vizy 2016) while a role in the precipitation along the African coast during subsidence makes a contribution to maintain moisture boreal spring (Leduc‐Leballeur et al. 2013). On interan- transport into this region (Dezfuli et al. 2015). Some nual timescales, Atlantic SST governs the movement of regional factors, such as topography, mid-level jets, and the ITCZ and further infuence boreal summer precipi- mesoscale convective systems all infuence precipitation tation in equatorial Africa (Nicholson 2009). Te Indian Wang et al. Geosci. Lett. (2021) 8:22 Page 3 of 12 Ocean is more related to rainfall in late summer/early recent long-term trends in the dry and wet seasons in fall, whereas Pacifc is important for central Africa in “Results” section. We found that the seasonality of pre- MAM (Balas et al. 2007; DiNezio et al. 2013; Farnsworth cipitation over the African rain forest is weakening over et al. 2011). However, ENSO is less directly connected to the past ~ four decades, i.e., precipitation has signifcantly African rainforest precipitation (Malhi and Wright 2004). increased in the boreal winter dry season and decreased Long-term drought in West and Central Africa is mainly in the boreal spring wet season. Further, we investigate linked to the change of Atlantic circulation (Shanahan the impacts of tropical SSTs on the observed precipita- et al. 2009) and SST (Camberlin et al. 2001; Diem et al. tion changes using atmospheric model simulations in 2014), and warming Indian Ocean also makes an impor- “Results” section. Te change of specifc humidity, mois- tant contribution (Diem et al. 2014; Hoerling et al. 2006; ture fux and Walker circulation forced by SST over the Hua et al. 2016; Richard et al. 2001). In boreal winter/ African rainforest region are presented in “Results” sec- spring season, tropical North Atlantic may force anoma- tion. Finally, conclusions and discussions are given in last lous circulation and increased precipitation in central section. equatorial Africa (Todd and Washington 2004). Addi- tionally, Pacifc SST can also modulate precipitation over Methods eastern and southern Africa by atmospheric bridge (Dez- In this study, we use MODIS land cover map data- fuli et al. 2013; Hua et al. 2016; Maidment et al. 2015). set (MCD12C1, Friedl et al. 2010) to describe the dis- Previous studies indicated that long-term dry trend tribution of the African rainforest. To examine the caused the shrinking of Congo rainforest (Zhou et al. precipitation change, we used two observed precipitation 2014). It is also reported that the dry season (JJA) length datasets, the Global Precipitation Climatology Project has been increased by 6.4–10.4 days per decade in the (GPCP, Hufman et al. 2009) and the CPC Merged Analy- period of 1988–2013 over the Congo rainforest (Jiang sis of Precipitation (CMAP, Xie and Arkin 1997). Tese et al. 2019), which is mainly relates to an earlier dry sea- datasets are based on gauge stations and satellite obser- son onset and a delayed dry season end. Te decreased vations, and are widely used as reliable precipitation precipitation during AMJ (April, May, June) result in measurements (Yin et al. 2004). Te pattern of tropical the decline of soil moisture and further cause the longer SST in recent decades has an important impact on pre- boreal summer (JJA) dry season over the Congo rainfor- cipitation change. To investigate observed SST changes est (Jiang et al. 2019). In East Africa, the precipitation in in diferent basins, we use the Hadley Centre Sea Ice and MAM (defned as “long rains”) decreased due to abrupt Sea Surface Temperature dataset (HadISST, Rayner et al. change of tropical Pacifc SST (Lyon and DeWitt 2012). 2003). Te same dataset was also used to drive the atmos- In addition, anthropogenic forcing has infuenced the pheric models.
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