2628 JOURNAL OF CLIMATE VOLUME 13 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 pre- vailed 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 sub- tropical 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. q 2000 American Meteorological Society Unauthenticated | Downloaded 09/27/21 02:38 AM UTC 15 JULY 2000 NICHOLSON ET AL. 2629 TABLE 1. Contribution of individual months to mean annual rainfall for 28 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 188±208N 2 6 15 41 23 5 2 84 168±188N 1 7 23 40 22 5 0 255 148±168N 2 8 24 38 22 5 0 498 128±148N 5 12 23 32 20 6 1 843 108±128N 8 12 19 25 20 9 2 1368 88±108N 9 12 17 18 18 10 3 1466 68±88N 11 16 12 10 14 12 5 1564 48±68N 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 128 to 188N 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 188 to 208N, July rainfall be- The analysis for West Africa is based on a region- comes less important. In the area from 88 to 128N 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 88N. In the area from 48 to 88N (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 Unauthenticated | Downloaded 09/27/21 02:38 AM UTC 2630 JOURNAL OF CLIMATE VOLUME 13 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 48 to 68N to 84 mm in the band from 188 to 208N. Both the standard deviation and the mean at individ- ual stations are calculated from the entire length of re- cord, 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).