An Analysis of Recent Rainfall Conditions in West Africa, Including the Rainy Seasons of the 1997 El Ninäo and the 1998 La Ninäa Years

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An Analysis of Recent Rainfall Conditions in West Africa, Including the Rainy Seasons of the 1997 El NinÄo and the 1998 La NinÄa Years

S. E. NICHOLSON Department of Meteorology, The Florida State University, Tallahassee, Florida

B. SOME AND B. KONE Centre ReÂgional AGRHYMET, Niamey, Niger

(Manuscript received 24 March 1999, in ®nal form 10 September 1999)

ABSTRACT This article examines recent rainfall conditions throughout the Sahel and in other parts of West Africa in detail and presents an overview of changes in rainfall on timescales of decades for Africa as a whole. In West Africa, there has been a pattern of continued aridity since the late 1960s that is most persistent in the more western regions. Some recovery occurred in the easternmost sectors during the 1990s, with rainfall in some years being near or just above the long-term mean. Dry conditions continued during 1997, but that year was not unusually dry compared to others of the last two decades. Hence, it appears that the 1997 El NinÄo did not have a large impact in the region. A preliminary analysis suggests that in 1998 rainfall was still below the long- term mean in most of the Sahel, but the central Sahel of Niger experienced localized ¯ooding due to high rainfall in September. Throughout the region, the wettest years of the last decades were 1978, 1988, 1994, and possibly 1998, but conditions in even these years exceeded the long-term mean in only a few sectors. A long-term change in rainfall has occurred in the semiarid and subhumid zones of West Africa. Rainfall during the last 30 yr (1968± 97) has been on average some 15% to 40% lower than during the period 1931±60. A similar but smaller change has occurred in semiarid and subhumid regions of southern Africa.

1. Introduction by the ®rst author in numerous publications (e.g., Nich- olson 1979, 1983, 1985, 1993, 1994; Nicholson and In 1994 the West African Sahel experienced one of Palao 1993; Nicholson et al. 1996). In addition to up- the wettest years since the early 1960s (LeCompte et dating material from these papers, this article also pre- al. 1994; Nicholson et al. 1996). With that year came sents new material concerning interdecadal variability some optimism that the dry conditions, which had prevailed for nearly three decades, had ended. However, in Africa as a whole and intraseasonal ¯uctuations over rainfall barely exceeded the long-term mean. In this West Africa. Included is new detail on individual article we examine post-1994 conditions to determine months and the seasonal cycle, transects showing shifts whether the dry conditions have persisted and compare in rainfall patterns, and a map showing changes in the recent conditions with those of earlier decades. This 30-yr climatological ``means'' for all of Africa. update includes new material on conditions prevailing Also included is new material on the eastern Sahel/ continent wide during the 1990s decade and on West Soudan zones and on the equatorial latitudes of West African rainfall in the 1997 season. Preliminary infor- and central Africa. In the mid-1980s it became increas- mation is presented on the 1998 season, noted for ¯ood- ingly dif®cult to obtain data for countries such as Chad, ing in some sectors of West Africa. These years are of Ghana, Nigeria, and the Sudan. For that reason, most interest in the context of the recent El NinÄo and La NinÄa of the recently published papers on West Africa have events. dealt with the western Sahel/Soudan zones. Our most The interannual variability of rainfall over Africa, recent update (Nicholson et al. 1996), for example, was especially in the Sahel, has been chronicled previously limited to the western Sahel/Soudan and to the subtropical latitudes and excluded the equatorial latitudes of West or central Africa. We have recently obtained good datasets from countries in these regions. With these Corresponding author address: Dr. Sharon Nicholson, Dept. of data, we present a new update on rainfall ¯uctuations Meteorology, The Florida State University, Tallahassee, FL 32306- 3034. along the Guinea Coast and in large regions of equatorial E-mail: [email protected] Africa farther east.

᭧ 2000 American Meteorological Society

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TABLE 1. Contribution of individual months to mean annual rainfall for 2Њ latitudinal sectors in the analysis domain of Fig. 3. Numbers are percentages and represent the ratio of monthly mean to annual mean rainfall averaged for stations in the sector. Latitude May Jun Jul Aug Sep Oct Nov Mean 18Њ±20ЊN 2 6 15 41 23 5 2 84 16Њ±18ЊN 1 7 23 40 22 5 0 255 14Њ±16ЊN 2 8 24 38 22 5 0 498 12Њ±14ЊN 5 12 23 32 20 6 1 843 10Њ±12ЊN 8 12 19 25 20 9 2 1368 8Њ±10ЊN 9 12 17 18 18 10 3 1466 6Њ±8ЊN 11 16 12 10 14 12 5 1564 4Њ±6ЊN 16 24 9 3 6 9 8 1810

of the Indian Ocean and El NinÄo±Southern Oscillation (ENSO) is higher in the east. Also, storms linked to Mediterranean synoptic systems occasionally in¯uence the east and offer drought conditions prevailing in the equivalent latitudes farther west. The western sector includes the semiarid Sahelo±Sa- hara (regions 9, 10, and 11), Sahel (regions 13, 14, 15), and Soudan (regions 18, 19, 20) and two more humid zones farther south, Soudano±Guinean (regions 23, 24, 25) and the Guinea Coast (regions 28, 29). Farther east, an analysis is carried out for the eastern Sahelo±Sahara (region 12), the eastern Sahel (regions 16 and 17), and the eastern Soudan/Soudano±Guinean zone (regions 21 and 26). Table 1 shows the contribution of individual months to the mean annual rainfall as a function of latitude. From approximately 12Њ to 18ЊN August is clearly the FIG. 1. (a) Map of station network and rainfall regions of West wettest month and July and September contribute about Africa used to construct multiregion time series of rainfall variability. (b) Map of station network and 90 homogeneous rainfall regions for equally to the annual rainfall. This is the case for the the continent as a whole. Sahelo±Saharan, Sahel, and Soudan zones, with August representing 32%±40% of the annual mean and July and September each representing about 20%±24%. Farther 2. Data and methodology north, in the area from 18Њ to 20ЊN, July rainfall be- The analysis for West Africa is based on a region- comes less important. In the area from 8Њ to 12ЊN alization (Fig. 1a) used in Nicholson and Palao (1993) (roughly the Soudano±Guinean zone), rainfall is more and Nicholson (1994). The scheme is slightly modi®ed evenly distributed throughout these three months, the from earlier studies (e.g., Nicholson 1985), especially contributions ranging from 17% to 25%. The maximum in the eastern part of the Sahel/Soudan zones. The orig- is still in August to the north, but shifts to September inal regionalization was based primarily on vegetation at around 8ЊN. In the area from 4Њ to 8ЊN (the Guinea zonation, following the practice of many climatological Coast), there is a double maximum in the annual cycle, studies ensuing after the severe drought in the early with the wettest month being June and a secondary max- 1970s. In the intervening years, research has shown that imum occurring in September or October. this is less than adequate in some regions. The revised The maps and graphs in section 2 are generally pre- regionalization shown in Fig. 1 uses regional boundaries sented in the form of percent departures from normal adjusted on the basis of climatology and station/region or in units of standard deviations. The former statistic correlations. is easier to interpret, but the latter is more appropriate In contrast to many of our earlier studies, eastern and for the computation of regional averages from stations western regions are considered separately. This distinc- with diverse climatologies. The signi®cance of rainfall tion is made because the eastern region appears to be anomalies, when expressed in either manner, is depen- dominated by somewhat different dynamic controls dent on the mean. That is, a large anomaly may not be (Bhatt 1989; Hastenrath 1990; Moron and Ward 1998), particularly meaningful for very dry stations. For this and the two regions therefore show somewhat different reason, mean annual rainfall for each latitudinal band patterns of interannual variability (Nicholson 1980; is presented in Table 1 in order to facilitate interpretation Nicholson and Palao 1993). For example, the in¯uence of the various analyses. These range from 1810 mm in

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FIG. 2. The number of stations in the archive used in previous rainfall studies (e.g., in Nicholson et al. 1996) in each year from 1980 to 1998, compared with the number in the archive used in this current study. the band from 4Њ to 6ЊN to 84 mm in the band from 18Њ to 20ЊN. Both the standard deviation and the mean at individual stations are calculated from the entire length of record, in order to encompass the strong low-frequency variance in many regions. Although the record length varies, in nearly all cases the records are long enough FIG. 3. Rainfall ¯uctuations in West Africa (1901±98) expressed to produce stable and comparable statistics. Of approx- as a regionally averaged standardized departure (departure from the imately 1400 stations in the overall dataset, only about long-term mean divided by the standard departures). Location of the 250 have a record length less than 40±50 yr. Most of ®ve regions is indicated in Fig. 1. the stations commenced well before 1925 and coverage was extremely good in the 1950s and 1960s, after which 302 stations in the 19 regions shown, but since 1980 time the station networks declined. By 1980 data could only 230 have reported with an availability of 80% or be obtained for only about 1000 of the original stations. greater. Most of the stations with a greater proportion Of these, roughly 800 have a record length of 60 yr or of missing data are from the eastern Sahel and Soudan/ longer. Soudano±Guinean zones or from the western Sahelo± As of the mid-1980s, the acquisition of African rain- Sahara. fall data was hindered by political and economic insta- In this paper, we ®rst examine the long-term vari- bilities and, on occasion, government policies. For that ability in these regions since 1901, using regionally av- reason, in previous updates by the ®rst author (e.g., eraged time series for the multiregion zones shown in Nicholson et al. 1996), few stations were available in Fig. 1a. Rainfall departures are expressed, following the more southern zones or in the eastern Sahel/Soudan previous studies (e.g., Nicholson 1985), as a standard- zones. Recent data acquisitions from numerous West ized annual departure from the long-term mean (i.e., the and central African countries allow us to consider these mean over the entire length of record). Thus, a value regions in more detail in this study. In particular, the of ϩ1(Ϫ1) indicates an average positive (negative) newly acquired data have permitted updating records departure of one standard deviation from the mean. So for Ghana, Sierra Leone, Nigeria, the Sudan, Chad, Ga- calculated regionally averaged departures are also pre- bon, Cameroon, the Congo, Zaire, Uganda, and the Cen- sented in map form to illustrate interdecadal variability tral African Republic. using the 90 regions shown in Fig. 1b. This has dramatically improved station coverage for Then the absolute magnitude of the rainfall variability the post-1980 time period. Figure 2 shows the number is examined by calculating multidecadal means for the recent dry decades and for previous decades. The results of stations available in each year since 1980. Numbers are presented in map form and as longitudinal transects are given both for the previous archive, upon which based on the individual stations in Fig. 1b. Finally, the earlier updates are based, and the updated archive used spatial patterns of anomalies in 1997 and 1998 are de- here. With the new additions to the archive, the number scribed. of stations in the early 1990s increased from about 600 to 800 and the number in the mid-1990s increased from roughly 200 to 600. Most of the increase is on account 3. Results of stations in the aforementioned countries. a. Interannual variability The station network for West Africa, the focus of this Figure 3 presents time series of standardized rainfall paper, is illustrated in Fig. 1a. In our archive there are departures for the period 1901±98 for the western zones.

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FIG. 4. Rainfall ¯uctuations in the Sahelo±Saharan region (1901± FIG. 5. Rainfall ¯uctuations along the Guinea Coast (1901±98) from 98) from Jun to Sep, expressed as a regionally averaged standardized Jun to Sep, expressed as a regionally averaged standardized departure departure (departure from the long-term mean divided by the standard (departure from the long-term mean divided by the standard depar- departures). tures).

These time series con®rm that dry conditions have con- tendency for relatively dry conditions prevailing since tinued throughout West Africa. Everywhere, relatively the 1970s in June and, to a lesser extent, September and dry conditions have prevailed since the late 1960s, and relatively wet conditions prevailing since that time in since 1968 rainfall has exceeded the long-term mean in July and August. Because this region has a double max- only a few sporadic cases. One such case was in 1994, imum in the seasonal cycle with June and September when rainfall slightly exceeded the mean in four of the or October being the wettest months, Fig. 5 suggests ®ve zones. In most of the region the next wettest years that during that last two to three decades there has been of the last 20 yr were 1978 and 1988, but rainfall was a change in the seasonal cycle. This question is further generally below the long-term mean in these years. It examined in section 3b. is interesting to note that numerous sources (e.g., Sir- Figure 6 depicts the situation in the more eastern coulon 1976; Kates et al. 1981) indicated that severe zones. The trends in interannual variability are similar droughts occurred in the Sahel in the 1910s and the to those in the western regions. The early and mid-1980s 1940s. Figure 3 makes it clear that these were actually were exceedingly dry, especially 1984, and relatively minor events compared to the recent prolonged dry spell. The departures for individual months (not shown) in- dicate that these basic trends were evident in most sectors from June to September. The largest and most persistent anomalies occurred in August and September, a conclusion reached by several previous studies (e.g., Dennett et al. 1985; Nicholson and Palao 1993). The individual months are considered in more detail in section 3b. The northern and southern extremes of the analysis sector are two exceptions to the general pattern of more arid conditions in all months. In the Sahelo±Sahara the trends in June and July (Fig. 4) do not strongly resemble the trends in annual rainfall. These months showed strong positive departures in some years of the recent decades and often had negative departures in the 1930s± 1960s. It should be noted, however, that the region re- FIG. 6. Rainfall ¯uctuations in eastern Sahel/Soudan (1901±97) ceives very little rainfall during June and considerably expressed as a regionally averaged standardized departure (departure less in July than in August. The more interesting ex- from the long-term mean divided by the standard departures). Lo- ception is the Guinea Coast (Fig. 5). There is a clear cation of the regions is indicated in Fig. 1.

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FIG. 7. (a) Mean rainfall for the decades 1950±59, 1960±69, 1970±79, and 1980±89 (from Nicholson 1993) and (b) for the 8-yr period 1990±97. Rainfall is expressed as a percent departure from the long-term mean, with station data averaged over 1Њ squares; positive values are shaded. dry conditions have continually prevailed except in b. Decadal-scale variability some individual years. However, there appears to have been some recovery in the 1990s, with near-normal con- It is clear from the time series in Figs. 3 and 6 that ditions in many years. The wettest years were 1994, as relatively wet conditions prevailed from the 1920s or farther west, and 1988. A previous satellite analysis earlier through the 1950s. Rainfall was generally near (Nicholson et al. 1996) indicated relatively wet condi- the long-term mean throughout the 1960s and the region tions in these sectors during those years; this analysis has been quite dry since the 1970s. These trends are con®rms that. highlighted in Fig. 7, which shows rainfall departures

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TABLE 2. For the ®ve western zones, the number of years since 1968 that fall within the 30 driest years on record. Sahelo±Sahara Sahel Soudan Soudano±Guinea Guinea Coast 19 20 22 22 11

1950s, rainfall in the Sahel/Soudan regions was 0.6 to 0.9 standard deviations above the long-term mean. In most of southern Africa, rainfall was roughly 0.3 to 0.4 standard deviations above normal. The negative departures in the equatorial regions were much smaller. By comparison, rainfall anomalies in the 1960s were relatively small, especially in the Sahel/Soudan, and were greatest in equatorial latitudes. The negative anomalies of the 1970s in West Africa were comparable in magnitude to the positive anomalies of the 1950s. Positive anomalies in this decade in southern Africa were equally strong. In the 1980s, rainfall in much of West Africa averaged nearly one standard deviation below the long- term mean. Anomalies in southern Africa were roughly FIG.7.(Continued) half as great, but notably stronger than in the dry 1960s. Only in the eastern equatorial region (particularly the Rift Valley highlands) are positive anomalies apparent for the decades 1950±59, 1960±69, 1970±79, and for the 1980s. 1980±89, as well as the mean for the 8-yr period 1990± It thus appears that the rainfall regime prior to the 97. Rainfall is expressed as a percent departure from 1970s was considerably different than the regime pre- the long-term mean, with station data averaged over 1Њ vailing in recent decades. Table 2 shows for each of the squares; positive values are shaded. western sectors the number of years since 1968 that In the 1950s, rainfall was well above the long-term would fall among the driest 30 yr on record. Except for mean virtually throughout the area north of 10ЊN, by at the Sahelo±Sahara, the records cover 97 yr, or essen- least 10%±20% near this latitude but well over 20% tially the entire century. In the Sahel, Soudan, and Sou- farther north. In most of the equatorial region, rainfall dano±Guinean zones, 20 or 22 yr since 1968 fall within was below this mean value but usually by less than 10%. the driest 30 yr. In the Sahelo±Sahara, where the record Positive anomalies on the order of 20% or more pre- length is only 77 yr, 19 yr since 1968 fall within the vailed in southern Africa. In the 1960s, rainfall was driest 30 yr. However, for the Guinea Coast, only 11 yr above the long-term mean throughout most of the con- fall among the driest 30 yr. tinent, but a marked reversal took place in southern Figure 9 further illustrates the change in rainfall con- Africa. There negative anomalies on the order of 10% ditions in recent decades. In this map, the difference to 20% prevailed in the 1960s. Small negative anomalies between mean rainfall for the 30-yr period 1931±60 and also appeared in the northern fringes of the central and for the protracted, 30-yr dry episode of 1968±97 is ex- eastern Sahel and in small sectors in the low latitudes. pressed as a percent of the 1931±60 mean. The ®rst The maps for the 1970s and 1980s show de®cits vir- period was selected because it was considered to be the tually everywhere in northern Africa, with decadal climatological ``normal'' for a long period that included means being generally 20% or more below the mean. times of intense agricultural and economic development Strong positive departures occurred throughout southern in West Africa. The difference map is objectively con- Africa in the 1970s. Rainfall was below average over toured, but a comparison was made with a map of station most of the continent in the 1980s, with the negative departures to ensure that the map contours are reason- anomalies exceeding 20% in the arid and semiarid zones able in areas with a lower density of stations. It shows of the Sahel/Soudan and southern Africa. In the 1990s, that the change between the two 30-yr periods is roughly the picture was marginally better, but rainfall was still 15% in the southern margin of the Soudan and around well below the long-term mean nearly everywhere ex- 40% in the northern Sahel. The 40% contour includes cept in small sectors of central equatorial Africa (i.e., areas north of the Sahel as well, but in the area from in parts of eastern Nigeria and parts of Cameroon and 22Њ to 28ЊN mean annual rainfall is generally less than Gabon). 20 mm. Thus, a 40% change represents a very small Figure 8 shows these same four decades, but presents amount of rainfall in that sector. regional averages expressed in units of standard devi- The difference map giving station data (not shown) ations. The rainfall regions are shown in Fig. 1b. In the indicates that throughout most of West Africa, mean

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FIG. 8. Mean regionally averaged rainfall for the decades 1950±59, 1960±69, 1970±79, and 1980±89. Rainfall is expressed in units of standard deviations from the long-term mean, as in Nicholson (1983) and subsequent studies. The regionalization is shown in Fig. 1b. annual rainfall was 100±200 mm lower during 1968± lighted in Fig. 10, which shows the meridional rainfall 97 than during the 1931±60 period. In the westernmost gradient near 0Њ and 5ЊW over West Africa. The reduc- sectors this difference was even larger, generally on the tion in annual rainfall is evident at all latitudes from 6Њ order of 200±500 mm and even attaining around 1000 to 20ЊN. Interestingly, these changes are associated with mm in some sectors. Hence, when drought struck in the a southward shift of the rainfall isohyets of only about late 1960s and early 1970s, its effects were particularly 1Њ of latitude. Figure 11 illustrates the meridional gra- severe in the region because agricultural and economic dient in the months of June through September. As sug- activity was based on the more humid conditions of gested earlier a change is evident in all months, but earlier decades. it is much more pronounced in August and September The contrast between the two 30-yr periods is high- than in June and July. Figure 11 also shows the shift to

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FIG. 9. Difference (in %) between mean annual rainfall during 1931±60 and 1968±97. wetter conditions in July and August in the latitudes of the Guinea Coast at 0ЊW. This shift is not apparent at 5ЊW, but it does occur throughout much of the equatorial region farther east. During the 1980s this shift was apparent to roughly 14ЊW (Fig. 12). The percent change in each of these months and for the year as a whole is indicated in Table 3, based on an average of all stations in the western ®ve zones. In the northernmost zones, the greatest percent change occurred in August. Mean August rainfall during the 1968±97 period was 55%, 37%, and 26% below the 1931±60 average in the Sahelo±Sahara, Sahel, and Sou- dan zones, respectively. However, the September mean FIG. 10. Mean annual rainfall (mm) as a function of latitude at (a) was also reduced by roughly 25% in each of these zones. 0ЊW and (b) 5ЊW for 1931±60 and 1968±97. The most stable mean was July, having changed by only 9% in the Sahelo±Sahara and 16% in the Soudan zone. Changes in June ranged from 20% to 34% in these maximum around 9ЊN to an August maximum during zones. In these regions annual rainfall was reduced by the 1968±97 period. However, this may re¯ect a shift 5%±35%. to an August maximum at some stations in individual The pattern switches markedly in the Soudano±Gui- years of the period. nean zone. There annual rainfall was 5% greater during the more recent period. Relatively little change occurred c. Spatial distribution of rainfall anomalies during in June through August (Ϫ1% to 8%) and in September the 1997 El NinÄo year and 1998 La NinÄa year rainfall increased in the more recent 30-yr period. This region represents a transition to the Guinea Coast, where A number of studies have evaluated the in¯uence of mean annual rainfall increased by 10% during the last the El Nino±Southern Oscillation phenomenon on rain- 30 yr. July and August means increased by 14% and fall in sub-Saharan West Africa (see review in Moron 43%, respectively. Surprisingly, these changes did not and Ward 1998). Some (e.g., Nicholson and Entekhabi markedly alter the seasonal cycle. The only readily ap- 1986; Ropelewski and Halpert 1987, 1989; Nicholson parent changes (Fig. 13) are a substantial decrease in and Kim 1997) suggest that the in¯uence is minimal. the intensity of the ``little dry season'' of July and Au- Others (e.g., Semazzi et al. 1988; Wolter 1989; Hasten- gust in the Guinea Coast and a shift from a September rath 1990; Bhatt 1989; Moron and Ward 1998) suggest

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FIG. 11. Mean rainfall (mm) as a function of latitude at (a) 0ЊW and (b) 5ЊW for 1931±60 and 1968± 97, for the months of Jun±Sep. that ENSO tends to reduce rainfall in the Sahel/Soudan through October. In May and June, rainfall was above zone during the summer rainy season of the El NinÄo normal in much of the region. In May, positive anomalies year, with the in¯uence increasing eastward across the were mainly in the area west of 10ЊE and strongest anom- zone, while La NinÄa tends to increase rainfall. These alies occurred in the far western Sahel. In June positive opposing views probably arise from the fact that the El anomalies prevailed throughout the area south of circa NinÄo/La NinÄa signal in the region is quite complex. 12ЊN, but rainfall was below the mean in much of the There appears to be greater in¯uence on the higher fre- Sahel. In July negative anomalies prevailed throughout quency, interannual ¯uctuations, rather than decadal- most of the analysis sector. Strongest negative anomalies scale anomalies. The in¯uence also appears to have been were in the far western Sahel. Abnormally dry conditions greater during the dry episode of the last 30 yr than continued throughout August and September, but the neg- earlier in the century (Semazzi et al. 1988; Ward 1998). ative anomalies exceeded one standard deviation in only In view of these various studies, the patterns of rain- a few areas. In contrast, positive anomalies prevailed fall over West Africa during the years 1997 and 1998 throughout most of the analysis sector in October. For are of interest. From these studies, it might be antici- the year as a whole there is a substantial region of extreme pated that 1997, an El NinÄo year, would be a relatively negative anomalies over the western Sahel and another dry year and that 1998, a La NinÄa year, would be a very far to the east. In most of the region, however, relatively wet one. The following sections examine rain- negative anomalies are less than one standard deviation. fall patterns in these two years. Positive anomalies in this same range are evident in a few areas within the latitudinal band of 5Њ±10ЊN. Thus, 1997 does not particularly ®t the expected pat- 1) RAINFALL IN 1997 tern of an ENSO event. For one, the strongest anomalies Figure 14 presents the 1997 annual rainfall departures are in the west. More importantly, Fig. 3 shows that and the departures for the individual months of May 1997 was not particularly wet compared to recent non±

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FIG. 11. (Continued)

El NinÄo years. It was the wettest of the last 30 yr in the of a good rainy season. Reports from various West Af- Sahelo±Sahara and the eighth wettest of the last 30 yr rican countries, as provided to the authors and as pub- in the western Sahel. Only in the western Soudano± lished in the AGRHYMET (1998) bulletin, provide Guinean zone was 1997 unusually dry, and rainfall was some information on the situation (Fig. 15). actually slightly above average in the eastern part of The only months in which positive rainfall anomalies that zone. prevailed throughout most of the region were June and September. During the months of July and August, rain- 2) RAINFALL IN 1998 fall was below the mean in most of the Sahel. In June, the positive anomalies were predominantly in the central Earlier in this year, some sources (e.g., FEWS 1998) Sahel, with western regions remaining relatively dry. In reported ¯ooding in parts of the Sahel, bringing hope September, strong positive anomalies were appparent throughout most of the Sahel, but rainfall was below TABLE 3. Percent change in mean rainfall from 1931±60 to 1968± the mean in most of the Guinea Coast region farther 97, expressed as a percent of the 1931±60 mean. Values are indicated south. In October, the pattern was one of predominantly for the ®ve zones depicted in Fig. 3 and for the months of Jun±Sep and the year as a whole. A negative value indicates drier conditions negative anomalies, but generally less than one standard in the more recent period. deviaiton. The annual pattern suggests that the high rainfall and Sahelo± Soudano± Sahara Sahel Soudan Guinea Guinea Coast ¯ooding were relatively localized. For the year as a whole, rainfall was below the mean in most areas west Jun Ϫ34 Ϫ24 Ϫ20 Ϫ8 Ϫ5 Jul Ϫ9 Ϫ29 Ϫ16 1 14 of about 5ЊW and along the Guinea Coast. In several Aug Ϫ55 Ϫ37 Ϫ26 Ϫ3 43 areas, the negative anomalies exceeded one standard Sep Ϫ25 Ϫ26 Ϫ26 Ϫ16 4 deviation. The strong positive anomalies were con®ned Year Ϫ35 Ϫ13 Ϫ5 5 10 mainly to two areas, one centered around Niamey, Niger

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FIG. 13. The seasonal cycle during 1931±60 (solid line) and 1968± 97 (dashed line) in three latitudinal zones (4Њ±6Њ,6Њ±8Њ, and 8Њ±10ЊN). The seasonal cycle is expressed as the ratio of the monthly mean to the annual mean for each month of the year (in %) and averaged over all stations in the zone extending from the west coast to 15ЊE.

mean. In the Sahel, Soudan and Soudano±Guinean zones, 1998 was only slightly below the mean and it was not markedly wetter than 1997 for the zones as a whole. The largest annual anomaly was for the Guinea Coast, where 1998 was one of the driest years since the beginning of the century.

4. Summary and conclusions This study has demonstrated that there has been a pattern of continued aridity since the late 1960s throughout North Africa south of the Sahara. This pattern has been most persistent in the more western regions. Some recovery occurred in the easternmost sectors during the 1990s, with rainfall in some years being near or just above the long-term mean. Throughout the region, the wettest years of the last decades were 1978, 1988, and 1994, but conditions in even these years exceeded the long-term mean in only a few sectors. Drier conditions continued to prevail in 1997, but the year was not exceedingly dry compared to other recent years. Thus, the ENSO event of that year did not have FIG. 12. Mean Aug rainfall (a) for the 1980±89 decade and (b) for a strong in¯uence in the Sahel. During 1998 preliminary the 8-yr period 1990±97. Rainfall is expressed as a percent departure from the long-term mean, with station data averaged over 1Њ squares; data suggests that the rainy season was relatively good positive values are shaded. in the central Sahel, primarily in Niger. This is consistent with studies suggesting an association between La NinÄa and high rainfall in the Sahel/Soudan. However, else- (near 15ЊN and 2ЊE) and another to the south and east where in the Sahel relatively dry conditions prevailed. in northern Nigeria. In Niger, intense rains occurred in September but these A comparison with earlier years is seen in the time were somewhat localized. Rainfall exceeded the mean series in Fig. 3, showing the standardized annual de- in the other months also in much of the country, re- partures for the ®ve western zones. In all but the Sahelo± sulting in a relatively good season. Sahara, the annual rainfall was below the long-term A long-term change in rainfall has occurred in the

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FIG. 14. (Top) Map of annual rainfall departures (percentage of standard departure) for 1997. (Bottom) Monthly rainfall departures (percentage of standard departure) for May±Oct of 1997. A bold line encircles areas with available data. semiarid and subhumid zones. Rainfall during the last during the 1970s and 1980s. Decadal means were gen- 30 yr (1968±97) has been on average some 15%±40% erally 20% or more below the long-term mean through- lower than during the period 1931±60. Virtually out much of the region in the 1970s and most of the throughout the area north of 10ЊN, rainfall in the 1950s region in the 1980s. In the 1990s, the picture was mar- was well above the long-term mean, generally by at least ginally better, but rainfall was still well below the long- 10%±20%. In the 1960s, rainfall exceeded the long-term term mean in most areas. mean throughout most of the analysis region. In con- The changes were most pronounced in August. Mean trast, rainfall was below average virtually everywhere August rainfall during the 1968±97 period was 55%, 37%, and 26% below the 1931±60 average in the Sah- elo±Sahara, Sahel, and Soudan zones, respectively. The September mean was reduced by roughly 25% in these zones. Considerably smaller changes occurred in June and July. A somewhat different pattern occurred in the wetter zones farther south. In the Soudano±Guinean zone, June through August rainfall changed little, but September rainfall increased during the last 30 yr. Far- ther south, along the Guinea Coast, there has been a marked increase in July and August rainfall during the

TABLE 4. September rainfall (mm) at stations in Niger, compared to the long-term mean. Station Rainfall Mean Agadez 78 13 Bilma 10 3 Birni N Konni 98 95 Gaya 178 158 Goure 125 52 Maine Soroa 153 56 Maradi 118 90 Nguigmi 109 18 Niamey 208 88 FIG. 15. (Top) Map of seasonal (May±Sep) rainfall departures (per- Tahou 115 57 centage of standard departure) for 1998. (Bottom) Monthly rainfall Tillaberry 248 77 departures (percentage of standard departure) for Jun±Sep of 1998. Zinder 86 63 A bold line encircles areas with available data.

Unauthenticated | Downloaded 09/27/21 02:38 AM UTC 2640 JOURNAL OF CLIMATE VOLUME 13 last 30 yr, and thus a decrease in the intensity of the variability in the European and African sectors. Weather, 53, little dry season along the Guinea Coast. This increase 287±295. Nicholson, S. E., 1979: Revised rainfall series for the West African is consistent with the noted southward shift of the rain- subtropics. Mon. Wea. Rev., 107, 620±623. fall isohyets of about 1Њ of latitude during the July and , 1980: The nature of rainfall ¯uctuations in subtropical West August period. Africa. Mon. Wea. Rev., 108, 473±487. , 1983: Sub-Saharan rainfall in the years 1976±80: Evidence of continued drought. Mon. Wea. Rev., 111, 1646±1654. Acknowledgments. We would like to acknowledge the , 1985: Sub-Saharan rainfall 1981±1984. J. Climate Appl. Me- important contribution of AGRHYMET and the various teor., 24, 1388±1391. meteorological services from which we obtained rainfall , 1993: An overview of African rainfall ¯uctuations of the last data. Data were provided by the services of nearly all decade. J. Climate, 6, 1463±1466. countries in Africa. Support for the project was provided , 1994: Recent rainfall ¯uctuations in Africa and their relationship to past conditions over the continent. Holocene, 4, 121± by NSF Grant NIBA-ATM-9521761 from the Climatic 131. Dynamics section. We thank D. Klotter for a critical , and D. Entekhabi, 1986: The quasi-periodic behavior of rainfall reading of the manuscript. variability in Africa and its relationship to the southern oscillation. Arch. Meteor. Geophys. BioKlimatol. Series A, 34, 311± 348. REFERENCES , and I. Palao, 1993: A re-evaluation of rainfall variability in the Sahel. Part I: Characteristics of rainfall ¯uctuations. Int. J. Cli- AGRHYMET, 1998: Mensuel Agrhymet. Vol. 8, May±September. matol., 13, 371±389. [Available from Centre ReÂgional AGRHYMET, B.P. 1011, Nia- , and J. Kim, 1997: The relationship of the El NinÄo±Southern mey, Niger.] Oscillation to African rainfall. Int. J. Climatol., 17, 117±135. Bhatt, U., 1989: Circulation regimes of rainfall anomalies in the Af- , M. B. Ba, and J. Y. Kim, 1996: Rainfall in the Sahel during rica±South Asian monsoon belt. J. Climate, 2, 1133±1144. 1994. J. Climate, 9, 1673±1676. Dennett, M. D., J. Elston, and J. R. Rodgers, 1985: A reappraisal of Ropelewski, C. F., and M. S. Halpert, 1987: Global and regional scale rainfall trends in the Sahel. J. Climatol., 5, 353±361. precipitation associated with El NinÄo/Southern Oscillation. Mon. FEWS, 1998: FEWS Bulletin. AFR/98-9, September, 6 pp. [Available Wea. Rev., 115, 985±996. from FEWS Project, 1611 N. Kent Street, Suite 1002, Arlington, , and , 1989: Precipitation patterns associated with the high VA 22209.] index phase of the Southern Oscillation. J. Climate., 2, 268± Hastenrath, S., 1990: The relationship of highly re¯ective clouds to 284. tropical climate anomalies. J. Climate, 3, 353±365. Semazzi, F. H. M., V. Mehta, and Y. C. Sud, 1988: An investigation Kates, R. W., and Coauthors, 1981: Drought impact in the Sahelian± of the relationship between sub-Saharan rainfall and global sea Sudanic zone of West Africa: A comparative analysis of 1910± surface temperatures. Atmos.±Ocean, 26, 118±138. 15 and 1968±74. Background Paper No. 2, Center for Technol- Sircoulon, J., 1976: Les donneÂes hydropluviomeÂtriques de la seÂch- ogy, Environment, and Development, Clark University, Worces- eresse reÂcente en Afrique intertrpicale. Comparaison avec les ter, 92 pp. [Available from Center for Technology, Environment, seÂcheresses ``1913'' et ``1940.'' Cah. O.R.S.T.O.M., SeÂr. Hydrol., and Development, Clark University, 950 Main St., Worcester, Vol. 13, No. 2, 174 pp. MA 01610.] Ward, N. M., 1998: Diagnosis and short-lead time prediction of sum- LeCompte, D., R. Tinker, J. Dionne, M. Halpert, and W. Thiao, 1994: mer rainfall in tropical North Africa at interannual and multi- Wettest rainy season in 30 years across African Sahel. Special decadal timescales. J. Climate, 11, 3167±3191. Climate Summary 94/2, NOAA, Washington, DC, 5 pp. Wolter, K., 1989: Modes of tropical circulation, southern oscillation, Moron, V., and M. N. Ward, 1998: ENSO teleconnections with climate and Sahel rainfall anomalies. J. Climate, 2, 149±172.

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