Possible causes of the Central Equatorial African long-term drought

CEA is the world's third most extensive region of deep convection and tropospheric heating and associated with this convection is a primary driver of the tropical general circulation (Todd and Washington 2004). This study focuses on the broad contiguous swath of land of CEA (10°S–8°N, 14°E–32°E; box in figure 1) covering the Congo Basin and surrounding areas. The chosen study region is slightly bigger than that of Zhou et al (2014) whose exclusive emphasis is the intact central Congo rainforest, in order to investigate the possibility that the Congo drought may reflect a large-scale pattern of rainfall changes over CEA as discussed above. The forested areas in West Africa are not included as these regions have experienced extensive deforestation and other human-induced land cover and land use change (Zhou et al 2014).

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CEA has the lowest gauge density in sub-Saharan Africa (Washington et al 2013) and a dramatic decline in the number of rain gauges, especially after 1980s (Asefi-Najafabady and Saatchi 2013, Zhou et al 2014). Maidment et al (2015) found distinct differences in the Central African rainfall trends in both sign and magnitude among multiple datasets. Lack of adequate observations presents a big challenge for understanding the rainfall variability and climatic changes in this region. To compensate for the lack of observed data, we combine both ground observations and satellite retrievals to represent rainfall quantities and characteristics.

Observational gridded monthly rainfall data sets from the Global Precipitation Climatology Centre (GPCC, Schneider et al 2014) at 1° × 1° resolution (1950–2014) and the Global Precipitation Climatology Project (GPCP, Adler et al 2003) at 2.5° × 2.5° resolution (1979–2014) are used in this study. The gauge-based GPCC data combines the Full Data Product (V7), based on quality-controlled data from 67 200 stations world-wide that feature record durations of ten years or longer, and the monitoring product, based on quality-controlled data from 7000 stations. The latest GPCP data provides the combined precipitation product derived from satellites and gauge measurements (together with other major improvements). We also use gauge observations archived from the recent rainfall records, such as for Congo-Brazzaville, Gabon, Nigeria, the Central African Republic and Cameroon over the period 1979 to 2014, which confirm the downward trend as shown in GPCC and GPCP (personal communication with one of the authors, S E Nicholson). To examine global SST variations, the updated monthly SST dataset is obtained from the UK Met office Hadley Centre's Global Sea Ice and SST (HadISST; Rayner et al 2003). The GPCC data are bilinearly interpolated into grid boxes (2.5° × 2.5°), consistent with the GPCP data resolution for ease of comparison on spatial features.

To examine the long-term variability of atmospheric circulation and related fields, we use two reanalysis products, the European Centre for Medium-Range Weather Forecast interim reanalysis (ERA-Interim) for the period 1979–2014 (Dee et al 2011) and the National Aeronautics and Space Administration (NASA) Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA2) for the period 1980–2014 (Molod et al 2015). Monthly mean variables of geopotential height (HGT), wind, and moisture flux at 850 hPa are used. With updated data assimilation systems, the ERA-Interim reanalysis represents the currently state-of-the-art reanalysis and is expected to have higher quality than other reanalyses (e.g., Mooney et al 2011, Bao and Zhang 2013, Lin et al 2014, Naud et al 2014). Similarly, MERRA2 is the latest reanalysis produced by NASA's Global Modeling and Assimilation Office using the Goddard Earth Observing System, version 5, data assimilation system (GEOS-5, Rienecker et al 2011) and makes many improvements and updates to the data assimilation GCM model and observing system (Molod et al 2015). We consider these two reanalysis products because of their inclusions of homogeneous satellite observations with global coverage into the data assimilation systems, which may help, in particular, to reduce uncertainties over regions, such as the CEA, with a paucity of data.

The seasonal cycle of rainfall in CEA is bimodal with two rainfall peaks in the transition seasons of September to November and March to May (Washington et al 2013). As the relationships between the CEA rainfall and SSTs are very complex and vary significantly by season (Balas et al 2007, Farnsworth et al 2011), here we focus only on the three-month period of April, May and June (AMJ), the first of the two peaks growing seasons over CEA, when the drying trend is most significant and its impacts on photosynthetic activity of vegetation are most pronounced (Zhou et al 2014).

Two methods are used to examine the variations in rainfall and associated changes in atmospheric circulation. The first method is to estimate a linear trend using least squares regression and the slope of the regression is defined as the trend per decade. Student t test is applied to quantify the statistical significance of the trend. The second method is to use composite analysis to quantify the changes between wet and dry periods. As the rainfall over our study region has decreased significantly since 1990s, we define two 15-year periods 1979–1993 and 2000–2014 from the ERA-Interim reanalysis as the dry and wet periods, respectively. For the MERRA2, the dry period (2000–2014) is compared against the wet period (1980–1993). Note that the wet and dry periods can be defined differently such as the choice of the last and first 10 or and 18 years for the composite analysis but similar results will be obtained. These two methods should bolster our confidence if consistent results are obtained.

Figure 1 shows the climatology of AMJ rainfall patterns from GPCC and GPCP. There is generally more rainfall over the Guinea coast, Congo Basin and Ethiopian highlands. Maximum rainfall occurs along the coast of Guinea exceeding 6 mm d⁻¹, just north of the equator. Within the study area of CEA rainfall exceeds 4 mm d⁻¹ from roughly 8°N to 5°S. Despite their differences in temporal coverage, GPCC and GPCP are mostly identical, with a spatial correlation of 0.99 across Africa (n = 896, p < 0.01), and thus provide consistent rainfall patterns over the study region. It is also worth noting that GPCC and GPCP are not independent from each other (Adler et al 2003), but both are considered because GPCP provides more reliable information over the Congo Basin where there are limited gauges numbers.

Figures 2(a) and (b) display the spatial patterns of rainfall trend from GPCC (1950–2014) and GPCP (1979–2014), respectively. Most of the study regions show a significant negative trend (p < 0.05). A decreasing trend in rainfall is also evident from the areal mean time series averaged over the entire study region (figure 2(c)). The two datasets show consistently strong negative anomalies falling since the 1990s. The regional mean rainfall declines significantly by −0.09 mm d⁻¹ decade^–1 (p < 0.01) for the period 1950–2014 from GPCC and by −0.21 mm d⁻¹ decade^–1 (p < 0.01) for the period 1979–2014 from GPCP. As the drying trend occurs from the late 1980s onward, we focus on the recent three decades in the remainder of the paper. The long-term drying seen over CEA seen from GPCC and GPCP is consistent with the findings presented in previous studies (Malhi and Wright 2004, Yin and Gruber 2010, Asefi-Najafabady and Saatchi 2013, Diem et al 2014, Zhou et al 2014).

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To identify possible large-scale mechanisms driving the drying trend, we examine its linkage with internal dynamical processes and atmospheric-ocean interactions. It is well known that West African climate is strongly influenced by the West African monsoon (WAM) circulation (e.g., Thorncroft et al 2011). The large-scale pressure gradient between the hot thermal Saharan low and the cold Atlantic subtropical high leads to the southerly WAM flow from the ocean towards the land in spring and summer (Nicholson 2013). From the view of atmospheric circulation in the lower troposphere at 850 hPa, the St. Helena subtropical high located roughly 30°S creates winds parallel to the west coast of Africa (figure 3). For East Africa, the southeast flow from Indian Ocean is seen through the coast into the equatorial Africa which is strongly influenced by the presence of the subtropical high centered over Southern Africa and the Indian Ocean. Both the ERA-Interim and MERRA2 reanalysis capture the intertropical convergence zone and the subtropical high. Overall the two reanalysis data exhibit similar features of atmospheric circulation (figure 3).

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The linear trend in the atmospheric circulation fields from the ERA-Interim reanalysis, including HGT and winds at 850 hPa are shown in figure 4. There are two anomalous high pressure centers across Africa during the recent decades. One is near the northwestern sector of the Congo Basin, and the other is centered over Southern Africa roughly around 15°S (figure 4(a)). The land-sea low-level pressure gradient is weakened between the African continent and the Atlantic Ocean, an indicator of a weaker monsoon flow (figure 4(c)) and a smaller meridional thermal gradient. This might induce a drying trend around the Central Africa. Comparing the dry versus wet period, an anomalous high pressure center and anti-cyclonic circulation in the lower troposphere are located in western Central Africa and near the Congo Basin (figure 4(e)). As discussed above, the AMJ drought over CEA is likely part of the large-scale atmospheric circulation changes (e.g., pressure and wind) induced by weaker monsoon circulation.

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Next we analyze the MERRA2 data to verify the above results. Figure 4(b) shows the trend analysis of HGT and winds at 850 hPa in the MERRA2 reanalysis. Eastern Central Africa is controlled by an anomalous high pressure center accompanied by anomalously easterlies around western Central Africa and the Cong Basin. The composite analysis indicates that an anomalous high pressure center and anti-cyclonic circulation in the lower troposphere are located in eastern Central Africa (figure 4(f)). The anomalous high pressure and associated east wind, which lead to descending motions and weaker monsoon circulation, reduce the rainfall over equatorial Africa, although the changes and locations of atmospheric circulation are somewhat different in the two reanalyses.

Rainfall over equatorial Africa is also largely influenced by the zonal (Walker) circulation (e.g., Nicholson and Grist 2003, Nicholson and Dezfuli 2013, Cook and Vizy 2016). The Walker circulation usually includes upward branches over the western Pacific warm pool (95°E–150°E), eastern Indian Ocean and equatorial Africa (e.g., the Congo Basin) and a downward branch over western boundary of Indian Ocean around 45°E–60°E (figures 5(a) and 6(a)). Figure 5(b) presents the AMJ linear trends of meridional mean (10°S–10°N) of vertical velocity (ω, 10⁻² Pa s^–1 decade^–1) from the ERA-Interim reanalysis. The convective activity in equatorial Africa has declined as the upward branch of the Walker circulation has increased over the Indo-Pacific Ocean (figure 5(e)). Previous observational studies have indicated that the Walker circulation has strengthened during the late 20th century (Chen et al 2002, Minobe 2004, Ma and Zhou 2016). Williams and Funk (2011) found a westward extension of the Walker circulation as a result of an extension of the western edge of the tropical warm pool in the Indian Ocean. The intensification and westward extension of tropical Walker circulation is clearly seen when comparing meridional mean of vertical velocity during the wet period (figure 5(c)) to that of the dry period (figure 5(d)). Increased vertical ascent over the western Pacific warm pool is accompanied by decreased ascent over equatorial Africa. Hence an increase in subsidence across the western edge of Indian Ocean and a decrease in convection over the Congo Basin lead to the reduction in moisture transport and rainfall across CEA.

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We similarly examine the zonal circulation cells using the MERRA2 analysis. The composite results are similar to those from the ERA-Interim reanalysis. The anomalies descend over the equatorial Africa when the Walker circulation strengthens over Indo-Pacific Oceans. Compared with the meridional mean of vertical velocity in 1980s (figure 6(c)), the intensification and westward extension of tropical Walker circulation and decreased ascent over equatorial Africa are evident in recent decades (figure 6(d)). Figure 6(b) displays the linear trend of AMJ meridional mean of vertical velocity. Increased subsidence across equatorial Africa and convection over warm pool over Pacific Ocean are also evident, resembling the results of the ERA-Interim reanalysis shown in figure 5(b). The main differences are the changes in zonal circulation cells over Indian Ocean (figures 5(e) and 6(e)).

The above results indicate that the long-term drying trend in AMJ over CEA is associated with the low-level atmospheric circulation changes (e.g., pressure and wind), including a weaker monsoon and the strengthening and westward extension of Indo-Pacific Walker circulation. Note that such changes in large-scale atmospheric circulation (figures 4(a) and (b)) are usually triggered by SSTs or other remote conditions/teleconnections (Dai 2011), which will be examined next.

SSTs in the Atlantic and Indian Oceans have large impacts on African rainfall through their influences on large-scale circulation (e.g., Hoerling et al 2006, Ting et al 2009). The pattern of diabatic heating caused by variations in SST in the equatorial regions exhibits strong departures from zonal symmetry. Such SST variations produce zonally asymmetric atmospheric circulation. The dominant cell is in the equatorial Pacific, i.e., the Walker circulation.

SSTs over the Indo-Pacific Oceans have increased 2–3 times faster than SSTs in the tropical central Pacific and convective activities have correspondingly increased over most of the warm pool and Indian Ocean in the 20th century (Williams and Funk 2011). The intensification and westward extension of tropical Walker circulation result partly from the rapid warming in the Indian Ocean and warm pool, which has effectively expanded the warm-pool westward and enhanced subsidence over equatorial Africa (Hoerling et al 2006, Williams and Funk 2011).

The AMJ SST trends from 1979 to 2014 suggest that western Pacific SSTs have increased faster than Indian Ocean SSTs, which may induce different responses in vertical velocity associated with the large-scale circulation over the Indian Ocean. It's important to note that SST trends exhibit an extensive warming in the western Pacific and a cooling in the central eastern Pacific (figure 7(a)). Previous studies have indicated that the El Niñ-like or La Niña-like patterns of the equatorial Pacific SST weaken or strengthen the zonal SST gradient and thus drive the weakening or strengthening of the tropical Pacific Walker circulation (e.g., Collins et al 2010, Solomon and Newman 2012, Ma and Zhou 2016). Figures 7(b) and (c) compare the SST anomaly differences between the wet and dry periods. The latter resembles the La Niña-like pattern, whereas the former is similar to an El Niño-like pattern. In response to the observed La Niña-like SST forcing, a strengthening and westward shift in tropical Pacific Walker circulation is apparent in the trends and composite differences (figure 7). This La Niña-like SST pattern is the dominant forcing to the enhanced and westward extended Walker circulation (Ma and Zhou 2016). On the other hand, the recent enhanced Atlantic warming may also modulate the CEA climate via changes in zonal circulation which needs further investigation (e.g., McGregor et al 2014). As discussed above, the drying trend over equatorial Africa results from the large-scale response of the atmosphere to tropical SST variations. The intensification and westward extension of the tropical Walker circulation lead to a reduction in the convective and rainfall over CEA.

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SSTs modulate the rainfall variability not only via their direct and indirect impacts on atmospheric circulation, but also via their effects on moisture transport, which can be examined by the changes in moisture flux. Decreased convection over the Congo Basin leads to reduced moisture transport and thus reduced rainfall. At 850 hPa, the moisture flux converges over the Congo Basin but diverges over West Africa and the horn of Africa in the ERA-Interim (figure 8(a)). The MERRA2 reanalysis exhibit some small differences in the moisture flux climatology over the Congo Basin (figure 8(c)). The strong contrast in AMJ moisture flux in the wet and dry composites suggests that the subsidence flow associated with the reduced low-level moisture transport causes the drying trend over equatorial Africa (figures 8(b) and (d)).

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