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A detailed look at the recent drought situation in the Greater Horn of Africa
Journal of Arid Environments 103 (2014) 71e79
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Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
A detailed look at the recent drought situation in the Greater Horn of Africa Sharon E. Nicholson Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2013 Received in revised form 17 December 2013 Accepted 19 December 2013 Available online 12 February 2014
News reports have described a crisis situation in several countries in the Greater Horn of eastern Africa. Reportedly, drought has prevailed during much of the period 2008e2011, resulting in extreme food shortages and massive migration. This article presents the first detailed meteorological analysis of the situation. The analysis shows that during the years 2008 through 2010 and the first half of 2011 rainfall was 30%e75% below normal over the Horn of Africa, northern Kenya and most of the Sudan. This followed intense drought in 2005/2006. The drought situation became increasingly severe during late 2010 and early 2011, with rainfall being at least 50%e75% below normal in near roughly half of the droughtstricken region. The drought ended in during the last half of 2011, giving way to flood conditions that may have created a more critical situation. During 2012 the situation was mixed, with good adequate rainfall during the summer rainy season but strong deficits during the equatorial rainy seasons of the boreal summer and autumn. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Climate change Climate variability East Africa Rainfall
1. Introduction The international press reported in 2011 that a series of meteorological calamities, both drought and floods, had taken a severe toll in eastern Africa. The primary countries affected include Kenya, Somalia, Uganda, Ethiopia, and the Sudan. The World Food Program estimated that 10 million people faced severe food shortages and tens of thousands in Somalia died of malnutrition, a situation brought about both by drought and warfare (Tran, 2011). Information on the situation has come mainly from relief agencies and a detailed meteorological analysis of the situation is lacking. In this article we take a step toward that goal by evaluating the severity and spatial extent of the drought situation during in recent years. 2. Data and analyses The analysis of recent rainfall conditions over the Greater Horn of Africa is hindered by the difficulty in obtaining relevant data. The problem is particularly acute for Ethiopia, a region of complex terrain where the elevation varies from below sea level to 4620 m. Although the National Meteorological Service of Ethiopia operates over 500 stations, only the 15 stations transmitted via the Global Telecommunications System (GTS) are routinely available to the
E-mail address: [email protected]. 0140-1963/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaridenv.2013.12.003
research community (Dinku et al., 2007). That network is too sparse to get a good spatial estimate of precipitation in. Numerous satellite products are available but most do not perform well over Africa. Again that is particularly true for regions of complex terrain, such as Ethiopia (Dinku et al., 2011, 2008; Diro et al., 2008). The most reliable product appears to be TRMM 3B43 (Dinku et al., 2007), which combines TRMM data with other satellite products and gauge estimates (Nicholson et al., 2003). Here TRMM 3B43 data are utilized to examine rainfall conditions over eastern Africa during 1998e2012 on a monthly time scale and at a spatial resolution of .25 .25 . Maps of monthly rainfall anomalies were constructed by calculating departures from the long-term mean (i.e., 1998e2012) for each grid cell. TRMM 3B43 was not available after June 2011, so various other satellite and gauge products were tested as potential substitutes. Only GPCC, a gauge-only product (Rudolf et al., 2005), showed strong agreement with TRMM over the 15 years for which the latter is available. Fig. 2 shows an example of the comparison for central Kenya. Within eastern Africa lies the transition between the subtropical latitudes with maximum rainfall in summer and the equatorial region with two rainy seasons (March-to-May and Octobere November) and two dry seasons. Maps of monthly mean rainfall (Fig. 3) show that much of the region, particularly the north, receives rainfall during all three seasons. Most of Ethiopia, Kenya and Uganda receive significant rainfall during the boreal winter months as well. There is a sharp transition between the areas with peak
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Fig. 1. Map of eastern Africa, showing geographical boundaries and the month of maximum rainfall.
rainfall in summer and those with peak rainfall during one of the two transition seasons (Fig. 1). These will henceforth be referred to as the summer rainfall region and the equatorial rainfall region. Precipitation time series for these two regions were also constructed. In order to compare different months and stations with vastly different amounts of rainfall, the rainfall in each grid cell was expressed as a departure from the mean for that grid cell and month. All grid cells within a region were averaged together to produce a single time series for that region. 3. Results 3.1. Monthly rainfall anomalies Maps of rainfall anomalies were created for each month from January 2008 to December 2011. These cover Ethiopia, Eritrea, Djibouti, Somalia, Kenya and parts of Uganda, Tanzania, and the Sudan. A cursory examination showed erratic anomalies that were seldom consistent throughout a season. Consequently an overview of the situation requires a look at monthly anomalies. Those for all but the driest months are presented in Figs. 4e7.
During 2008 (Fig. 4) anomalies were predominantly negative throughout the region in March and May and in much of the region in April. However, rainfall was more than 50 mm below the monthly mean only in the southwestern Ethiopian highlands, and in parts of southwestern Kenya. Dryness became more intense in the boreal summer months in the summer rainfall region, with rainfall being 50e150 mm below the monthly mean in many areas. Rainfall was below normal in most of the region in October and December, but above normal in November. In 2009 (Fig. 5) dry conditions again prevailed throughout the region during the March-to-May season and in many areas in Julyto-September. Rainfall was 100e150 mm below normal in August, the month with the most extreme anomalies. Rainfall was above normal throughout the region in October and December, but below normal conditions occurred in November, with rainfall being 50e 150 mm below the monthly mean in most of Kenya and parts of Somalia. Abnormally dry conditions prevailed in April and May of 2010 (Fig. 6). During the summer rainy season, conditions were similar to 2009, except that the anomalies were extreme in both July and August. The boreal spring season of 2011 was exceedingly dry over much of the Horn of Africa (Fig. 7), especially during April. However, heavy rains occurred in the western equatorial region (Uganda, western Tanzania, Rwanda, Burunda, eastern Democratic Republic of the Congo) in March. Conditions were near normal in May. News reports indicated a swift change to flood conditions toward the end of the year. In arid regions of Kenya and Somalia, anomalies in October were on the order of 100 to over 200 mm, while large deficits were apparent in regions further north and west. The situation in Kenya and Somalia was comparable to the extreme flood situation of 1961, one of the highest on record (Nicholson, 1996). High rainfall continued through November in this region. Consistent with this, an analysis of the Normalized Difference Vegetation Index (NDVI) indicates abnormally high “greenness” over the region (Fig. 8). During 2012 conditions were extremely inconsistent from month to month and within the region. Deficits on the order of 50 mme200 mm prevailed in equatorial regions in January through March, but the remainder of the border spring season was good throughout most of the region. The summer rainy season was also very good, particularly during September. Dry conditions followed during the boreal autumn, particularly in October (Fig. 9).
3.2. Rainfall conditions since 1998 Figs. 10 and 11 show the time series of monthly anomalies averaged for the summer rainfall region and the equatorial rainfall region (see Fig. 1). These figures also indicate the percent of grid cells in each region with rainfall below the monthly mean and with
Fig. 2. Comparison of rainfall estimates (mm per month) over central Kenya (36.5Ee39.5E, 0.5Ne3.5N) from TRMM 3B43 and GPCC.
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Fig. 3. Monthly mean rainfall (mm) over the Horn of Africa, based on TRMM data for the years 1998e2011.
rainfall less than 50% of the monthly mean. This illustrates the severity and spatial extent of the drought situation. In the summer rainfall region of northern Ethiopia, Eritrea, Djibouti and southern Sudan, rainfall reached the 1998e2011 mean in only one month of the period February 2007eNovember 2009 (Fig. 10). Rainfall was typically 20%e70% below the mean. This followed a period of severe drought in 2005 and early 2006. Drought conditions again prevailed in late 2010 and the first half of 2011. That drought was even more severe, with rainfall on the order of 60%e80% below the mean. In the driest months negative anomalies prevailed over some 80%e95% of the region, and rainfall totaled less than 50% of the mean in 50%e80% of the region. Drought conditions were not as frequent or persistent in the equatorial rainfall region of Kenya, Uganda, Somalia and southern Ethiopia (Fig. 11). The most extreme episodes occurred from December 2008 to September 2009 and from July 2010 to June 2011. Rainfall was on the order of 50%e90% below the mean during both periods and during the driest months rainfall was below the mean in 80%e100% of the grid cells. A similar situation occurred during the earlier, aforementioned drought of 2005. In contrast, extreme flood situations occurred during the boreal autumn of
both 2006 and 2011. Further descriptions of the flood years can be found in Hastenrath et al. (2007, 2010) and Kijazi and Reason (2009). What is noteworthy about the more recent droughts, including that of 2005, is that they extended over two or three rainy seasons. In the summer rainfall region, rainfall was below the mean from February 2007 through November 2009, with the exception of one month. This includes all three rainy seasons in three consecutive years. The drought of 2005/2006 extended over the boreal summer and autumn rainy seasons and the subsequent rainy season of the boreal spring. Prior to that time, rainfall was below the mean in at most six consecutive months. In the equatorial rainfall region, sub-normal rainfall was also more persistent than in earlier droughts, but the situation was not as severe as in the summer rainfall region. Rainfall was below the long-term mean during all but 2 of the 32 months from February 2008 to September 2010. This region also experienced drought during 2005/2006. The crisis situation evoked by the droughts was exacerbated by abrupt shifts to flood conditions. These fluctuations between extremes were markedly more pronounced during the period 2005e2012 than in the earlier years of the analysis.
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Fig. 4. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2008. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
Fig. 5. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2009. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
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Fig. 6. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2010. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
Fig. 7. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2011. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
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Fig. 8. NDVI departures from the long-term mean for October-to-December, 2011.
Figs. 10 and 11 also demonstrate that drought frequency increased over the interval 1998e2011. Prolonged periods of below average rainfall over most of the equatorial rainfall region occurred in only two years during the seven from 1998 to 2004, but in four years of the following seven. In the summer rainfall region, such conditions occurred in only three of the seven years from 1998 to 2004, but in five of the next seven years. This is not, however, a sign of climate change because droughts equally persistent and severe occurred early in the nineteenth century and in the 1880s (Nicholson et al., 2012). The analysis further shows that the major drought episodes tend to affect both the summer and equatorial rainfall regions. The
correlation between the two departure series is .55. On the seasonal time scale, the correlation between the two regions is .76 for July-to-September (JAS) and .75 for October-to-December (OND). Those two seasons are also well correlated: .79 for the summer rainfall region and .66 for the equatorial rainfall region. On the other hand, the March-to-May season shows no significant correlation with the other two rainy seasons. The correlation between JAS and OND is surprising in view of the very different factors that have been identified as causes of variability in the three rainy seasons (Camberlin and Okoola, 2003; Diro et al., 2011; Hastenrath, 2007; Hastenrath et al., 2011, 2010; Mutai and Ward, 2000).
Fig. 9. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2012. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
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Fig. 10. Top: Time series of monthly departures (percent above of below the mean) for the summer rainfall region shown in Fig. 1. Bottom: percent of grid boxes with rainfall below the mean (dotted line) and percent of grid boxes with rainfall at least 50% below the mean (solid line).
3.3. Severity and spatial extent of recent drought Some indication of the spatial extent of drought can be gleaned from the percent of area experiencing abnormal conditions. During the most extreme drought conditions in the summer rainfall region
(2000, 2001, 2005/2006, 2010/11) 90%e100% of the grid cells had deficits of at least 50%. During less severe droughts such extreme deficits were evident at some 50%e60% of the grid cells. For the equatorial rainfall region, the percent of grid cells with deficits of at least 50% is similar. Thus, the droughts are widespread.
Fig. 11. Top: Time series of monthly departures (percent above of below the mean) for the equatorial rainfall region shown in Fig. 1. Bottom: percent of grid boxes with rainfall below the mean (dotted line) and percent of grid boxes with rainfall at least 50% below the mean (solid line).
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Fig. 12. a. Map of mean annual rainfall averaged over the period January 2008eJune 2011 and expressed as a percent above or below the annual mean. b. Map of mean annual rainfall averaged over the period June 2005eJune 2011 and expressed as a percent above or below the annual mean. The period April 2006eJanuary 2007 is omitted from the calculation, because this brief interval of heavy rains masks the severity of the long-term drought situation over the six-year period.
Fig. 12 depicts the severity as well as the spatial extent of recent drought conditions. Fig. 12a shows the average annual deficit over the lengthy drought interval from January 2008 to June 2011; Fig. 12b shows this for the longer interval June 2005 to June 2011. The latter calculation demonstrates the longer-term drought severity and identifies the regions most strongly impacted. The
period April 2006eJanuary 2007 has been removed from the calculation, because the heavy rains of this very brief period mask the severity of the prevailing drought situation during the six-year period. During the three and one-half years commencing in January 2008, rainfall was at least 30% below normal over nearly all of Ethiopia, Eritrea and Djibouti, most of Somalia and the Sudan, and much of Kenya. The areas most strongly affected include the central highlands of Ethiopia, the extreme eastern tip of Ethiopia, and northern Somalia. In these regions rainfall was generally 40%e75% below normal. Fig. 12b, commencing in June 2005, shows much more extreme conditions over the same areas most strongly post2008, with rainfall being at least 40% below normal over nearly half of Ethiopia. The situation was particularly severe during the twelve months of July 2010eJune 2011 (Fig. 13), hence over three consecutive rainy seasons. Rainfall was at least 50% below normal over nearly all of Somalia, northern Kenya and southern Ethiopia. In much of the region rainfall was at least 75% below normal, making this one of the driest years on record.
4. Summary and conclusions
Fig. 13. Map of rainfall during the period July 2010eJune 2011, expressed as a percent above or below the annual mean.
Drought has occurred frequently over eastern Africa during the last decade or so. However, both the frequency of drought episodes and the duration of conditions with below-average rainfall have been greater during the last seven years than in the seven prior to that. “Drought”, i.e., widespread conditions of well below average rainfall, occurred during 1998, 2000, 2005/06, 2007, 2008, 2009 and 2011. The last four episodes have been particularly long and severe. Notably, all of these episodes affected both the summer and equatorial rainfall regions. The year 2010 was also abnormally dry in the summer rainfall region. The regions most strong affected include the central highlands of Ethiopia, the extreme eastern tip of Ethiopia, and northern Somalia. The drought conditions were interspersed with floods in 2006/2007, 2009, and 2010 (Hastenrath et al., 2010, 2007; Kijazi and Reason, 2009).
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A notable feature of these recent droughts is the persistence through all three rainy seasons. This may indicate a common factor operating in all three seasons. However, prior work has indicated very different factors in interannual variability in each of these seasons. This suggests that any prediction of drought conditions should address each season separately. The countries of eastern Africa have been hard hit by extreme conditions almost continuously since 2005. While drought has prevailed, this interval has also included extreme flood conditions that likewise have devastating consequences. Clearly, the severity and persistence of drought along with the abrupt change between extremes is at the heart of the crisis in eastern Africa. Acknowledgments This work was supported by NSF Grant No. 1160750. Partial support was also received from NSF Grant No. 1158984. References Camberlin, P., Okoola, R.E., 2003. The onset and cessation of the long rains in eastern Africa and their annual variability. Theor. Appl. Climatol. 75, 43e 54. Dinku, T., Ceccato, P., Grover-Kopec, E., Lemma, M., Connor, S.J., Ropelewski, C.F., 2007. Validation of satellite rainfall products over East Africa’s complex topography. Int. J. Rem. Sens. 28, 1503e1526. Dinku, T., Chidzambwa, S., Ceccato, P., Connor, S.J., Ropelewski, C.F., 2008. Validation of high-resolution satellite rainfall products over complex terrain. Int. J. Rem. Sens. 29, 4097e4110.
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Dinku, T., Ceccato, P., Connor, S.J., 2011. Challenges of satellite rainfall estimation over mountainous and arid parts of east Africa. Int. J. Rem. Sens. 32, 5965e 5979. Diro, G.T., Grimes, D.I.F., Black, E., O’Neill, A., Pardo-Iguzquiza, E., 2008. Evaluation of reanalysis rainfall estimates over Ethiopia. Int. J. Climatol. 29, 67e78. Diro, G.T., Grimes, D.I.F., Black, E., 2011. Teleconnections between Ethiopian summer rainfall and sea surface temperature. part I e observation and modelling. Clim. Dyn. 37, 103e119. Hastenrath, S., 2007. Circulation mechanisms of climate anomalies in East Africa and the equatorial Indian Ocean. Dyn. Atmos. Oceans 43, 25e35. Hastenrath, S., Polzin, D., Mutai, C., 2007. Diagnosing the 2005 drought in equatorial East Africa. J. Clim. 20, 4628e4637. Hastenrath, S., Polzin, D., Mutai, C., 2010. Diagnosing the droughts and floods in equatorial East Africa, during boreal autumn 2005-08. J. Clim. 23, 813e817. Hastenrath, S., Polzin, D., Mutai, C., 2011. Circulation mechanisms of Kenya rainfall anomalies. J. Clim. 24, 404e412. Kijazi, A.L., Reason, C.J.C., 2009. Analysis of the 2006 floods over northern Tanzania. Int. J. Climatol. 29, 955e970. Mutai, C.C., Ward, M.N., 2000. East African rainfall and the tropical circulation/ convection on intraseasonal to interannual timescales. J. Clim. 13, 3915e 3939. Nicholson, S.E., 1996. A review of climate dynamics and climate variability in eastern Africa. In: Johnson, T.C., Odada, E. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach, Amsterdam, pp. 25e56. Nicholson, S.E., et al., 2003. Validation of TRMM and other rainfall estimates with a high-density gauge data set for West Africa. Part II. Validation of TRMM rainfall products. J. Appl. Meteorol. 42, 1355e1368. Nicholson, S.E., Dezfuli, A.K., Klotter, D.A., 2012. A two-century precipitation dataset for the continent of Africa. Bull. Am. Meteorol. Soc. 93, 1219e1231. Rudolf, B., Beck, C., Grieser, J., Schneider, U., 2005. Global Precipitation Analysis Products. Global Precipitation Climatology Centre (GPCC), DWD, Internet publication, pp. 1e8. Tran, M., July 12 2011. Explainer. The east Africa food crisis. Guardian.
Contents lists available at ScienceDirect
Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
A detailed look at the recent drought situation in the Greater Horn of Africa Sharon E. Nicholson Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2013 Received in revised form 17 December 2013 Accepted 19 December 2013 Available online 12 February 2014
News reports have described a crisis situation in several countries in the Greater Horn of eastern Africa. Reportedly, drought has prevailed during much of the period 2008e2011, resulting in extreme food shortages and massive migration. This article presents the first detailed meteorological analysis of the situation. The analysis shows that during the years 2008 through 2010 and the first half of 2011 rainfall was 30%e75% below normal over the Horn of Africa, northern Kenya and most of the Sudan. This followed intense drought in 2005/2006. The drought situation became increasingly severe during late 2010 and early 2011, with rainfall being at least 50%e75% below normal in near roughly half of the droughtstricken region. The drought ended in during the last half of 2011, giving way to flood conditions that may have created a more critical situation. During 2012 the situation was mixed, with good adequate rainfall during the summer rainy season but strong deficits during the equatorial rainy seasons of the boreal summer and autumn. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Climate change Climate variability East Africa Rainfall
1. Introduction The international press reported in 2011 that a series of meteorological calamities, both drought and floods, had taken a severe toll in eastern Africa. The primary countries affected include Kenya, Somalia, Uganda, Ethiopia, and the Sudan. The World Food Program estimated that 10 million people faced severe food shortages and tens of thousands in Somalia died of malnutrition, a situation brought about both by drought and warfare (Tran, 2011). Information on the situation has come mainly from relief agencies and a detailed meteorological analysis of the situation is lacking. In this article we take a step toward that goal by evaluating the severity and spatial extent of the drought situation during in recent years. 2. Data and analyses The analysis of recent rainfall conditions over the Greater Horn of Africa is hindered by the difficulty in obtaining relevant data. The problem is particularly acute for Ethiopia, a region of complex terrain where the elevation varies from below sea level to 4620 m. Although the National Meteorological Service of Ethiopia operates over 500 stations, only the 15 stations transmitted via the Global Telecommunications System (GTS) are routinely available to the
E-mail address: [email protected]. 0140-1963/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaridenv.2013.12.003
research community (Dinku et al., 2007). That network is too sparse to get a good spatial estimate of precipitation in. Numerous satellite products are available but most do not perform well over Africa. Again that is particularly true for regions of complex terrain, such as Ethiopia (Dinku et al., 2011, 2008; Diro et al., 2008). The most reliable product appears to be TRMM 3B43 (Dinku et al., 2007), which combines TRMM data with other satellite products and gauge estimates (Nicholson et al., 2003). Here TRMM 3B43 data are utilized to examine rainfall conditions over eastern Africa during 1998e2012 on a monthly time scale and at a spatial resolution of .25 .25 . Maps of monthly rainfall anomalies were constructed by calculating departures from the long-term mean (i.e., 1998e2012) for each grid cell. TRMM 3B43 was not available after June 2011, so various other satellite and gauge products were tested as potential substitutes. Only GPCC, a gauge-only product (Rudolf et al., 2005), showed strong agreement with TRMM over the 15 years for which the latter is available. Fig. 2 shows an example of the comparison for central Kenya. Within eastern Africa lies the transition between the subtropical latitudes with maximum rainfall in summer and the equatorial region with two rainy seasons (March-to-May and Octobere November) and two dry seasons. Maps of monthly mean rainfall (Fig. 3) show that much of the region, particularly the north, receives rainfall during all three seasons. Most of Ethiopia, Kenya and Uganda receive significant rainfall during the boreal winter months as well. There is a sharp transition between the areas with peak
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Fig. 1. Map of eastern Africa, showing geographical boundaries and the month of maximum rainfall.
rainfall in summer and those with peak rainfall during one of the two transition seasons (Fig. 1). These will henceforth be referred to as the summer rainfall region and the equatorial rainfall region. Precipitation time series for these two regions were also constructed. In order to compare different months and stations with vastly different amounts of rainfall, the rainfall in each grid cell was expressed as a departure from the mean for that grid cell and month. All grid cells within a region were averaged together to produce a single time series for that region. 3. Results 3.1. Monthly rainfall anomalies Maps of rainfall anomalies were created for each month from January 2008 to December 2011. These cover Ethiopia, Eritrea, Djibouti, Somalia, Kenya and parts of Uganda, Tanzania, and the Sudan. A cursory examination showed erratic anomalies that were seldom consistent throughout a season. Consequently an overview of the situation requires a look at monthly anomalies. Those for all but the driest months are presented in Figs. 4e7.
During 2008 (Fig. 4) anomalies were predominantly negative throughout the region in March and May and in much of the region in April. However, rainfall was more than 50 mm below the monthly mean only in the southwestern Ethiopian highlands, and in parts of southwestern Kenya. Dryness became more intense in the boreal summer months in the summer rainfall region, with rainfall being 50e150 mm below the monthly mean in many areas. Rainfall was below normal in most of the region in October and December, but above normal in November. In 2009 (Fig. 5) dry conditions again prevailed throughout the region during the March-to-May season and in many areas in Julyto-September. Rainfall was 100e150 mm below normal in August, the month with the most extreme anomalies. Rainfall was above normal throughout the region in October and December, but below normal conditions occurred in November, with rainfall being 50e 150 mm below the monthly mean in most of Kenya and parts of Somalia. Abnormally dry conditions prevailed in April and May of 2010 (Fig. 6). During the summer rainy season, conditions were similar to 2009, except that the anomalies were extreme in both July and August. The boreal spring season of 2011 was exceedingly dry over much of the Horn of Africa (Fig. 7), especially during April. However, heavy rains occurred in the western equatorial region (Uganda, western Tanzania, Rwanda, Burunda, eastern Democratic Republic of the Congo) in March. Conditions were near normal in May. News reports indicated a swift change to flood conditions toward the end of the year. In arid regions of Kenya and Somalia, anomalies in October were on the order of 100 to over 200 mm, while large deficits were apparent in regions further north and west. The situation in Kenya and Somalia was comparable to the extreme flood situation of 1961, one of the highest on record (Nicholson, 1996). High rainfall continued through November in this region. Consistent with this, an analysis of the Normalized Difference Vegetation Index (NDVI) indicates abnormally high “greenness” over the region (Fig. 8). During 2012 conditions were extremely inconsistent from month to month and within the region. Deficits on the order of 50 mme200 mm prevailed in equatorial regions in January through March, but the remainder of the border spring season was good throughout most of the region. The summer rainy season was also very good, particularly during September. Dry conditions followed during the boreal autumn, particularly in October (Fig. 9).
3.2. Rainfall conditions since 1998 Figs. 10 and 11 show the time series of monthly anomalies averaged for the summer rainfall region and the equatorial rainfall region (see Fig. 1). These figures also indicate the percent of grid cells in each region with rainfall below the monthly mean and with
Fig. 2. Comparison of rainfall estimates (mm per month) over central Kenya (36.5Ee39.5E, 0.5Ne3.5N) from TRMM 3B43 and GPCC.
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Fig. 3. Monthly mean rainfall (mm) over the Horn of Africa, based on TRMM data for the years 1998e2011.
rainfall less than 50% of the monthly mean. This illustrates the severity and spatial extent of the drought situation. In the summer rainfall region of northern Ethiopia, Eritrea, Djibouti and southern Sudan, rainfall reached the 1998e2011 mean in only one month of the period February 2007eNovember 2009 (Fig. 10). Rainfall was typically 20%e70% below the mean. This followed a period of severe drought in 2005 and early 2006. Drought conditions again prevailed in late 2010 and the first half of 2011. That drought was even more severe, with rainfall on the order of 60%e80% below the mean. In the driest months negative anomalies prevailed over some 80%e95% of the region, and rainfall totaled less than 50% of the mean in 50%e80% of the region. Drought conditions were not as frequent or persistent in the equatorial rainfall region of Kenya, Uganda, Somalia and southern Ethiopia (Fig. 11). The most extreme episodes occurred from December 2008 to September 2009 and from July 2010 to June 2011. Rainfall was on the order of 50%e90% below the mean during both periods and during the driest months rainfall was below the mean in 80%e100% of the grid cells. A similar situation occurred during the earlier, aforementioned drought of 2005. In contrast, extreme flood situations occurred during the boreal autumn of
both 2006 and 2011. Further descriptions of the flood years can be found in Hastenrath et al. (2007, 2010) and Kijazi and Reason (2009). What is noteworthy about the more recent droughts, including that of 2005, is that they extended over two or three rainy seasons. In the summer rainfall region, rainfall was below the mean from February 2007 through November 2009, with the exception of one month. This includes all three rainy seasons in three consecutive years. The drought of 2005/2006 extended over the boreal summer and autumn rainy seasons and the subsequent rainy season of the boreal spring. Prior to that time, rainfall was below the mean in at most six consecutive months. In the equatorial rainfall region, sub-normal rainfall was also more persistent than in earlier droughts, but the situation was not as severe as in the summer rainfall region. Rainfall was below the long-term mean during all but 2 of the 32 months from February 2008 to September 2010. This region also experienced drought during 2005/2006. The crisis situation evoked by the droughts was exacerbated by abrupt shifts to flood conditions. These fluctuations between extremes were markedly more pronounced during the period 2005e2012 than in the earlier years of the analysis.
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Fig. 4. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2008. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
Fig. 5. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2009. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
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Fig. 6. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2010. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
Fig. 7. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2011. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
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Fig. 8. NDVI departures from the long-term mean for October-to-December, 2011.
Figs. 10 and 11 also demonstrate that drought frequency increased over the interval 1998e2011. Prolonged periods of below average rainfall over most of the equatorial rainfall region occurred in only two years during the seven from 1998 to 2004, but in four years of the following seven. In the summer rainfall region, such conditions occurred in only three of the seven years from 1998 to 2004, but in five of the next seven years. This is not, however, a sign of climate change because droughts equally persistent and severe occurred early in the nineteenth century and in the 1880s (Nicholson et al., 2012). The analysis further shows that the major drought episodes tend to affect both the summer and equatorial rainfall regions. The
correlation between the two departure series is .55. On the seasonal time scale, the correlation between the two regions is .76 for July-to-September (JAS) and .75 for October-to-December (OND). Those two seasons are also well correlated: .79 for the summer rainfall region and .66 for the equatorial rainfall region. On the other hand, the March-to-May season shows no significant correlation with the other two rainy seasons. The correlation between JAS and OND is surprising in view of the very different factors that have been identified as causes of variability in the three rainy seasons (Camberlin and Okoola, 2003; Diro et al., 2011; Hastenrath, 2007; Hastenrath et al., 2011, 2010; Mutai and Ward, 2000).
Fig. 9. Monthly rainfall anomalies in mm for the three rainy seasons of eastern Africa in 2012. Top: boreal spring, March-to-May. Center: boreal summer, July-to-September. Bottom: boreal autumn, October-to-December.
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Fig. 10. Top: Time series of monthly departures (percent above of below the mean) for the summer rainfall region shown in Fig. 1. Bottom: percent of grid boxes with rainfall below the mean (dotted line) and percent of grid boxes with rainfall at least 50% below the mean (solid line).
3.3. Severity and spatial extent of recent drought Some indication of the spatial extent of drought can be gleaned from the percent of area experiencing abnormal conditions. During the most extreme drought conditions in the summer rainfall region
(2000, 2001, 2005/2006, 2010/11) 90%e100% of the grid cells had deficits of at least 50%. During less severe droughts such extreme deficits were evident at some 50%e60% of the grid cells. For the equatorial rainfall region, the percent of grid cells with deficits of at least 50% is similar. Thus, the droughts are widespread.
Fig. 11. Top: Time series of monthly departures (percent above of below the mean) for the equatorial rainfall region shown in Fig. 1. Bottom: percent of grid boxes with rainfall below the mean (dotted line) and percent of grid boxes with rainfall at least 50% below the mean (solid line).
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Fig. 12. a. Map of mean annual rainfall averaged over the period January 2008eJune 2011 and expressed as a percent above or below the annual mean. b. Map of mean annual rainfall averaged over the period June 2005eJune 2011 and expressed as a percent above or below the annual mean. The period April 2006eJanuary 2007 is omitted from the calculation, because this brief interval of heavy rains masks the severity of the long-term drought situation over the six-year period.
Fig. 12 depicts the severity as well as the spatial extent of recent drought conditions. Fig. 12a shows the average annual deficit over the lengthy drought interval from January 2008 to June 2011; Fig. 12b shows this for the longer interval June 2005 to June 2011. The latter calculation demonstrates the longer-term drought severity and identifies the regions most strongly impacted. The
period April 2006eJanuary 2007 has been removed from the calculation, because the heavy rains of this very brief period mask the severity of the prevailing drought situation during the six-year period. During the three and one-half years commencing in January 2008, rainfall was at least 30% below normal over nearly all of Ethiopia, Eritrea and Djibouti, most of Somalia and the Sudan, and much of Kenya. The areas most strongly affected include the central highlands of Ethiopia, the extreme eastern tip of Ethiopia, and northern Somalia. In these regions rainfall was generally 40%e75% below normal. Fig. 12b, commencing in June 2005, shows much more extreme conditions over the same areas most strongly post2008, with rainfall being at least 40% below normal over nearly half of Ethiopia. The situation was particularly severe during the twelve months of July 2010eJune 2011 (Fig. 13), hence over three consecutive rainy seasons. Rainfall was at least 50% below normal over nearly all of Somalia, northern Kenya and southern Ethiopia. In much of the region rainfall was at least 75% below normal, making this one of the driest years on record.
4. Summary and conclusions
Fig. 13. Map of rainfall during the period July 2010eJune 2011, expressed as a percent above or below the annual mean.
Drought has occurred frequently over eastern Africa during the last decade or so. However, both the frequency of drought episodes and the duration of conditions with below-average rainfall have been greater during the last seven years than in the seven prior to that. “Drought”, i.e., widespread conditions of well below average rainfall, occurred during 1998, 2000, 2005/06, 2007, 2008, 2009 and 2011. The last four episodes have been particularly long and severe. Notably, all of these episodes affected both the summer and equatorial rainfall regions. The year 2010 was also abnormally dry in the summer rainfall region. The regions most strong affected include the central highlands of Ethiopia, the extreme eastern tip of Ethiopia, and northern Somalia. The drought conditions were interspersed with floods in 2006/2007, 2009, and 2010 (Hastenrath et al., 2010, 2007; Kijazi and Reason, 2009).
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A notable feature of these recent droughts is the persistence through all three rainy seasons. This may indicate a common factor operating in all three seasons. However, prior work has indicated very different factors in interannual variability in each of these seasons. This suggests that any prediction of drought conditions should address each season separately. The countries of eastern Africa have been hard hit by extreme conditions almost continuously since 2005. While drought has prevailed, this interval has also included extreme flood conditions that likewise have devastating consequences. Clearly, the severity and persistence of drought along with the abrupt change between extremes is at the heart of the crisis in eastern Africa. Acknowledgments This work was supported by NSF Grant No. 1160750. Partial support was also received from NSF Grant No. 1158984. References Camberlin, P., Okoola, R.E., 2003. The onset and cessation of the long rains in eastern Africa and their annual variability. Theor. Appl. Climatol. 75, 43e 54. Dinku, T., Ceccato, P., Grover-Kopec, E., Lemma, M., Connor, S.J., Ropelewski, C.F., 2007. Validation of satellite rainfall products over East Africa’s complex topography. Int. J. Rem. Sens. 28, 1503e1526. Dinku, T., Chidzambwa, S., Ceccato, P., Connor, S.J., Ropelewski, C.F., 2008. Validation of high-resolution satellite rainfall products over complex terrain. Int. J. Rem. Sens. 29, 4097e4110.
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Dinku, T., Ceccato, P., Connor, S.J., 2011. Challenges of satellite rainfall estimation over mountainous and arid parts of east Africa. Int. J. Rem. Sens. 32, 5965e 5979. Diro, G.T., Grimes, D.I.F., Black, E., O’Neill, A., Pardo-Iguzquiza, E., 2008. Evaluation of reanalysis rainfall estimates over Ethiopia. Int. J. Climatol. 29, 67e78. Diro, G.T., Grimes, D.I.F., Black, E., 2011. Teleconnections between Ethiopian summer rainfall and sea surface temperature. part I e observation and modelling. Clim. Dyn. 37, 103e119. Hastenrath, S., 2007. Circulation mechanisms of climate anomalies in East Africa and the equatorial Indian Ocean. Dyn. Atmos. Oceans 43, 25e35. Hastenrath, S., Polzin, D., Mutai, C., 2007. Diagnosing the 2005 drought in equatorial East Africa. J. Clim. 20, 4628e4637. Hastenrath, S., Polzin, D., Mutai, C., 2010. Diagnosing the droughts and floods in equatorial East Africa, during boreal autumn 2005-08. J. Clim. 23, 813e817. Hastenrath, S., Polzin, D., Mutai, C., 2011. Circulation mechanisms of Kenya rainfall anomalies. J. Clim. 24, 404e412. Kijazi, A.L., Reason, C.J.C., 2009. Analysis of the 2006 floods over northern Tanzania. Int. J. Climatol. 29, 955e970. Mutai, C.C., Ward, M.N., 2000. East African rainfall and the tropical circulation/ convection on intraseasonal to interannual timescales. J. Clim. 13, 3915e 3939. Nicholson, S.E., 1996. A review of climate dynamics and climate variability in eastern Africa. In: Johnson, T.C., Odada, E. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach, Amsterdam, pp. 25e56. Nicholson, S.E., et al., 2003. Validation of TRMM and other rainfall estimates with a high-density gauge data set for West Africa. Part II. Validation of TRMM rainfall products. J. Appl. Meteorol. 42, 1355e1368. Nicholson, S.E., Dezfuli, A.K., Klotter, D.A., 2012. A two-century precipitation dataset for the continent of Africa. Bull. Am. Meteorol. Soc. 93, 1219e1231. Rudolf, B., Beck, C., Grieser, J., Schneider, U., 2005. Global Precipitation Analysis Products. Global Precipitation Climatology Centre (GPCC), DWD, Internet publication, pp. 1e8. Tran, M., July 12 2011. Explainer. The east Africa food crisis. Guardian.