The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation

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The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part I: The Boreal Spring

SHARON E. NICHOLSON Department of Earth, Ocean and Atmospheric Science, The Florida State University, Tallahassee, Florida

AMIN K. DEZFULI Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland

(Manuscript received 9 November 2011, in ﬁnal form 23 August 2012)

ABSTRACT

This paper examines the factors governing rainfall variability in western equatorial Africa (WEA) during the April–June rainy season. In three of the five regions examined some degree of large-scale forcing is indicated, particularly in the region along the Atlantic coast. Interannual variability in this coastal sector also demonstrates a strong link to changes in local sea surface temperatures (SSTs) and the South Atlantic subtropical high. To examine potential causal mechanisms, various atmospheric parameters are evaluated for wet and dry composites. The results suggest that the intensity of the zonal circulation in the global tropics is a crucial control on rainfall variability over WEA. A La Nin˜ a (El Nin˜ o)–like signal in both SSTs and zonal circulation over the Pacific is apparent in association with the wet (dry) conditions in the western sector. However, remote forcing from the Pacific modulates the circulation over Africa indirectly by way of synchronous changes in the entire Indian or Atlantic Ocean. Anomalies in the local zonal winds are similar in all three regions: the wet (dry) composite is associated with an intensification (weakening) of the upper-tropospheric easterlies and low-level westerlies, but a weakening (intensification) of the midlevel easterlies. This work also suggests that, in most cases, the relationship between local SSTs and rainfall reflects a common remote forcing by the large-scale atmosphere–ocean system. This forcing is manifested via changes in the zonal circulation. Thus, the statistical associations between rainfall and SSTs do not indicate direct forcing by local SSTs. One point of evidence for this conclusion is the stronger association with atmospheric parameters than with SSTs.

1. Introduction 2009), atmospheric circulation (Nicholson and Grist 2003), and wave activity (Nguyen and Duvel 2008). The Equatorial Africa comprises roughly half of the goal of the current study is to expand our knowledge of equatorial landmass, yet relatively little is known about the region’s rainfall regime and, in particular, to identify the region’s meteorology. Most of the meteorological the factors governing its interannual variability. research on the region has focused on East Africa. The An outstanding characteristic of this region is the few studies of the more western sectors encompass extreme spatial heterogeneity of interannual variability. rainfall climatology and interannual variability (Hirst This stands in sharp contrast to most other regions of and Hastenrath 1983a,b; McCollum et al. 2000; Todd Africa (Nicholson and Palao 1993; Nicholson 1996; and Washington 2004; Balas et al. 2007; Samba et al. Hastenrath et al. 2011; Pohl and Camberlin 2006), in- 2007; Lienou et al. 2008; Yin and Gruber 2010; Dezfuli cluding the eastern equatorial region. It might be as- 2011), mesoscale convective systems (Laing and Fritsch sumed that the heterogeneity is a result of the complex 1993; Laing et al. 2008; Zipser et al. 2006; Jackson et al. topography of the region. However, topography is equally complex in eastern equatorial partsofAfrica,yetthatregion exhibits a coherent interannual signal throughout a sector Corresponding author address: Amin K. Dezfuli, Department of 8 8 8 8 Earth and Planetary Sciences, Johns Hopkins University, 3400 N. that extends from roughly 5 Nto10Sand28 to 43 E. Charles Street, 301 Olin Hall, Baltimore, MD 21218. This heterogeneity was noted by Nicholson (1986), E-mail: [email protected] who speculated that it might reﬂect the quality of the

DOI: 10.1175/JCLI-D-11-00653.1

Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/07/21 10:48 AM UTC 46 JOURNAL OF CLIMATE VOLUME 26 gauge data, rather than true spatial heterogeneity. Balas (2011b). During the boreal spring, the ITCZ’s position et al. (2007) performed more extensive quality control averages 158N, while peak rainfall occurs at about 38 to and reevaluated the regionalization used by Nicholson 58N. Thus the factors producing equatorial rainfall in (1986). That study confirmed the spatial heterogeneity the boreal spring are considerably more complex. A and showed also that the factors governing interannual separate zone of ascent lies to the south of the ITCZ variability varied markedly within the region and from and is linked with the rainfall maximum. We term this season to season. Dezfuli (2011a,b) extended that work the tropical rain belt. by performing a regionalization at the seasonal scale. Few studies have examined in detail the rainy season The current study is based on those results. of the boreal spring in western equatorial Africa. The The one commonality within equatorial Africa is first to do so was that of Balas et al. (2007). Defining the a bimodal rainy season, the typical ‘‘equatorial’’ regime season as March to May, they found that the association with peaks occurring during the transition seasons and between the interannual variability of rainfall and minima or dry seasons during the extreme seasons. The tropical SSTs was both regionally and seasonally spe- abovementioned studies and numerous works on East cific. The most pervasive associations are those of the Africa have clearly demonstrated strong contrasts be- Pacific ENSO and SSTs along the Benguela coast. tween the two rainy seasons in terms of their character During abnormally dry March–May seasons the El Nin˜ o and links to large-scale processes. Because of these pattern of Pacific SSTs is strongly developed. The study contrasts, our work will be presented in two parts, with also concluded that an opposition between the Atlantic the two rainy seasons considered separately. Part I focuses and Indian Oceans creates an east–west displacement of on April–June (AMJ); Part II (Dezfuli and Nicholson the convection. Changes in SSTs along the Benguela 2013, hereafter Part II) focuses on October–December coast also appear to be associated with such an east–west (OND) and on a comparison of interannual variability in displacement. the two seasons. The link to SSTs along the Benguela coast is partic- Section 2 of this article presents an overview of rele- ularly strong in March and April at stations right along vant past studies, including several dealing with eastern the coast from 58S to about 128S (Fig. 1). Areas that equatorial Africa. Section 3 describes data and meth- receive some 50 mm per month during cold-water years odology. It includes a brief summary of Dezfuli receive 100 to 300 mm in warm water years. The time (2011a,b) and Part II, wherein the precipitation regions scale of these variations is 5 to 6 years (Nicholson and used in this study are delineated. Section 4 presents the Entekhabi 1987), the peak time scale of both ENSO mean climatology of the region, including precipitation, and SST variation in the Atlantic and Indian Oceans the wind regime and atmospheric circulation during (Nicholson and Nyenzi 1990; Nicholson 1996). This time April–June. Section 5 presents the results, examining scale also accounts for the largest proportion of annual three regional case studies in detail. These focus on the variance throughout most of western equatorial Africa. sea surface temperatures and atmospheric circulation Jackson et al. (2009) examined convective activity and patterns associated with AMJ seasons that are anoma- found it to be much stronger during the boreal spring lously wet or dry. Section 6 examines several specific than the boreal autumn in most, but not all of the region. factors. Section 7 summarizes our main conclusions They attributed this to the influence of the midtropo- about the rainfall regime of this season and its contrasts spheric African easterly jet (AEJ) of the Southern with that in eastern equatorial Africa. Hemisphere (here called AEJ-S), which is strongly developed during the boreal autumn but weak or absent during the boreal spring. Convective activity, as in- 2. The equatorial rainy season of the boreal spring dicated by the contribution of mesoscale convective Most regions of equatorial Africa experience two systems (MCSs) and the frequency/intensity of light- rainy seasons during the course of the year. These occur ning, is also much stronger in western equatorial Africa during the transition seasons, but with somewhat vari- than in eastern equatorial Africa (Jackson et al. 2009; able timing. The boreal spring is the main rainy season in Mohr and Zipser 1996; Christian et al. 2003). eastern equatorial Africa, but it is the weaker of the two Convection and hence intraseasonal variability during seasons in western equatorial Africa. It is traditionally the boreal spring appears to be linked to wave pertur- considered to be associated with the northward equa- bations with a peak time scale of 5 to 6 days. These torial transit of the intertropical convergence zone appear to be convectively coupled Kelvin waves, some (ITCZ) and the mean ascent associated with it. How- of which originate in the equatorial Atlantic and prop- ever, over central Africa this zone remains well into the agate eastward (Nguyen and Duvel 2008). The waves Northern Hemisphere throughout the year Dezfuli do not trigger convective systems but enhance their

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(Camberlin and Philippon 2002). Two such factors include the Madden–Julian oscillation (MJO) and equatorial westerly winds over the continent. Changes in MJO amplitude can account for 44% of the March-to- May seasonal rainfall variance (Pohl and Camberlin 2006). The equatorial winds appear to be the most ro- bust link to rainfall during this season. Westerly wind anomalies are linked to rain events during this season (Camberlin and Wairoto 1997; Okoola 1998). Easterly anomalies at 700 mb characterize both dry years and dry spells within wet years (Okoola 1999a,b).

3. Data and methodology Figure 2 shows the western equatorial sector considered in this study. It includes the Zaire basin and the highlands surrounding it on all sides. The western ex- tension stretches along the Atlantic coast; the eastern extreme includes the western part of the Rift Valley highlands. We examine select aspects of rainfall variability for FIG. 1. March and April rainfall along the coast in warm water the April–June season in five regions within this geo- years and cold water years (from Nicholson and Entekhabi 1987). graphical area. These regions were delineated (Dezfuli 2011a,b) using a combination of rotated principal com- development into larger, organized convection, espe- ponent analysis and Ward’s clustering technique (e.g., cially over the Congo basin. Gong and Richman 1995; Unal et al. 2003; Rao and Much more research on the boreal spring rainy season Srinivas 2006). Each is homogeneous with respect to the of equatorial Africa has been carried out in eastern interannual variability of rainfall. equatorial regions. The results are reviewed here be- The regionalization was carried out using Tropical cause they may have some significance in interpreting Rainfall Measuring Mission (TRMM) 3B42 pre- our results for the more western regions. Compared to cipitation estimates, which have a 0.25830.258 spatial the boreal autumn, the spatial coherence of rainfall resolution. These are available online from the National anomalies is weak (Nicholson 1996; Camberlin and Aeronautics and Space Administration (NASA; http:// Philippon 2002). The temporal coherence within the mirador.gsfc.nasa.gov). TRMM was used instead of season is likewise weak and the links to atmospheric gauge data to delineate regions because of the irregular factors are markedly different in each month of the spatial distribution of gauges in the region and the sparse season (Beltrando 1990; Ambenje 1990; Macodras et al. network in some sectors. A comparison with region- 1989). May, in particular, stands out from the other alizations based on gauge data is presented in Dezfuli months (Camberlin and Philippon 2002). (2011b). The causes of interannual variability of the boreal The regions bear some relationship to topography. rainy season in eastern equatorial Africa are still largely Region 1, the easternmost, lies over the western high- unknown (Pohl and Camberlin 2006). The total amount lands of the Rift Valley. Region 2, the westernmost re- of rainfall is dependent on a combination of several gion, lies roughly between the Atlantic coast and the unrelated factors, such as number of rainfall events and highlands of Cameroon. Region 3 includes the highlands their intensity and the onset and cessation of the season. of Cameroon and the northern portion of the Zaire Strong relationships to large-scale atmospheric or ocean basin. Region 4 includes mainly the highlands of Central anomalies are not apparent (Camberlin and Philippon African Republic. Region 5 includes mainly the remaining 2002). As an example, the influence of ENSO is weak, areas of the Zaire basin and the northern slopes of the absent, or confined to local areas such as coastal Tan- central African plateau. zania (Nicholson and Kim 1997; Indeje et al. 2000; Mutai Gauge data were utilized to produce time series for and Ward 2000; Kijazi and Reason 2005). The lack of the regions shown in Fig. 2. The archive assembled by large-scale control suggests that interannual variability the first author includes 141 stations in the study area but is linked to internal ‘‘chaotic’’ factors in the atmosphere those with more than 10% missing data were eliminated

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FIG. 2. (left) Homogeneous rainfall regions for the April–June (AMJ) season, superimposed upon a map of topographic relief. Asterisks indicate stations utilized. The typical seasonal cycle for each region is also indicated. (right) Mean rainfall (mm) during the AMJ season for the period 1948 to 1988. from the analysis. Most analyses in the study are limited there is some tendency for the wettest seasons to occur to the period 1948 to 1988 because most stations are in the 1950s and 1960s and the driest to occur in the available throughout this period, allowing for a rela- 1970s and 1980s (Table 1). For example, at least one of tively homogeneous station network. the years 1955 or 1966 is among the wettest years in For each region, a standardized rainfall anomaly se- regions 2 to 5. ries for the three-month season is calculated, following The time series in Fig. 3 are correlated with sea sur- Nicholson (1986). These time series (Fig. 3) are utilized face temperatures and sea level pressure in the global for linear correlation and for identifying years that are tropics. The NOAA Extended Reconstructed SST anomalously wet or dry during the AMJ season. In each (ERSST) V3 product, which is available at a 28328 case, the four wettest and four driest AMJ seasons are resolution, is utilized (Smith et al. 2008). Three regions identified (see Table 1) and used to construct composites that show particularly strong links to SSTs are then ex- of surface and atmospheric parameters. The period amined in detail, using composites of the four wettest 1948–88 is chosen for the interannual analysis because it and the four driest seasons. This includes composites of represents the most reliable continuous period of ob- SST anomalies and various atmospheric parameters, servations. There are individual years outside this period such as winds, divergence, and vertical motion. A com- with adequate data for some of the regions. If the rainfall parison is made with the mean climatology, described in anomalies of those years are stronger than the extremes section 4. of the period 1948–88, we will include them in the Surface pressure and other atmospheric variables are composites. Thus, for two regions the year 1989 was obtained from the National Centers for Environmental considered as one of the extreme dry cases. A two-tailed Prediction (NCEP)–National Center for Atmospheric t test was used to test the significance of correlation Research (NCAR) reanalysis (Kalnay et al. 1996; coefficients and a bootstrapping procedure was used to Kistler et al. 2001). These are monthly data available on test the significance of the composite differences (Terray a 2.5832.58 grid. For both datasets, anomalies are cal- et al. 2003). The latter was chosen because the normality culated with respect to the 1948–88 mean. Various assumption of the composites may not be satisfied. shortcomings of the NCEP–NCAR reanalysis product Notably, in all but region 1 there is a modest reduction have been discussed by several authors (e.g., Stickler in rainfall starting in the mid to late 1960s. This shift is and Bro¨ nnimann 2011). However, these have been used roughly commensurate with the onset of drought con- by the current authors in numerous published works and ditions in the West African Sahel. The change is most also validated over West Africa in the analysis by Grist strongly apparent in regions 2 and 3. In contrast, region 1 and Nicholson (2001) via comparison with pilot balloon remained relatively dry until 1962, after which time data. The results with NCEP were consistent with those positive anomalies were much more pronounced and from pibals, although they appeared to exaggerate some frequent than negative anomalies. Except for region 1, of the wet–dry contrasts. Moreover, the circulation

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TABLE 1. The four wettest and driest years for each region.

Region Four wettest years Four driest years 1 1963 1974 1979 1962 1984 1989 1960 1969 2 1963 1966 1952 1955 1958 1981 1982 1983 3 1966 1978 1954 1948 1983 1972 1987 1971 4 1966 1964 1973 1955 1971 1986 1951 1989 5 1956 1955 1954 1976 1984 1970 1958 1973

summer. Region 1, the easternmost region, has more rainfall in the boreal spring, similar to eastern equatorial Africa. In region 2, the westernmost region, the second rainy season is the more intense. Regions 3 and 4 show a single maximum in the boreal summer and autumn. The seasonal cycle of region 5 represents a transition between the equatorial and subtropical rainfall regimes: a single dry season in the austral winter, but slight maxima in the two transition seasons. Mean rainfall for the AMJ season is shown in Fig. 2. Within the study area of western equatorial Africa 2 rainfall exceeds 25 mm month 1 in a zone that extends from roughly 138 to 148Nto148S. It exceeds 100 mm 2 month 1 from roughly 108 to 58N. Maximum rainfall occurs along the coast of eastern Nigeria and Cameroon, just north of the equator. The pattern for May (not shown) is much like that of the seasonal mean. For April (June) the latitudinal stretch receiving 25 mm or more is larger (smaller) and the pattern is shifted southward (northward) several degrees of latitude. The region is strongly influenced by its proximity to the Atlantic Ocean. Aridity near the coast is enhanced by the presence of the St. Helena subtropical high (Fig. 4), which is most intense in the austral winter and has its core at roughly 308S during the AMJ season. The high creates winds parallel to the coast, with upwelling in FIG. 3. Regionally averaged rainfall for the AMJ season, 1948 to the latitudes 158 to 308S (Fig. 4). The upwelling is weak 1988, for regions 1 to 5. The data are presented as standardized during the AMJ season. Over the Atlantic the temper- anomalies from the mean for the period (anomaly divided by the 8 standard deviation). ature maximum extends from roughly 5 N to the Guinea coast, and a weak ‘‘cold tongue’’ (e.g., Okumura and Xie 2004; Grodsky and Carton 2003) appears right at the equator. features noted in NCEP data over Africa are extremely Figure 5 shows the mean vector winds at four levels. In consistent with independent estimates of rainfall (e.g., the mid and upper troposphere easterly winds are vertical motion fields are strongly correlated in magni- clearly dominant. The well-known African easterly jet tude and space with rainfall). Furthermore, the results that dominates the meteorology of the Sahel region is are internally consistent and consistent among several well developed at 600 hPa. Its core lies over the equa- diverse analyses. torial Atlantic and its mean speeds over the season ex- 2 ceed 12 m s 1. A weakly developed tropical easterly jet (TEJ) is evident at 200 hPa, but its speeds over Africa 4. Mean climatology are lower than those of the AEJ. The weak TEJ may Figure 2 shows the mean seasonal cycle for the five suggest an incomplete zonal circulation cell along the regions. Regions 1 and 2 exhibit the typical bimodal equatorial Atlantic (Hastenrath 2001). The mean speed 2 equatorial rainfall regime, with a minimum in the boreal of the TEJ is about 8 to 10 m s 1 over equatorial Africa

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FIG. 4. Mean (top) sea surface temperatures (8C) and (bottom) sea level pressure (SLP; hPa) during AMJ for 1948 to 1988.

2 and 10 to 12 m s 1 over the western Indian Ocean. In Atlantic (r 5 0.48). Areas of significant negative the lower troposphere at 925 and 850 hPa, southerly correlation include the western tropical Indian Ocean flow prevails in the western equatorial region. At (r 520.39) and the eastern equatorial Pacific and 925 hPa the southerly flow is weak throughout most of western coast of South America (r 520.39). the region. These winds start out as southeasterlies in SST anomalies for wet and dry composites (Fig. 7) the Southern Hemisphere and take on a southwesterly show a general reversal of the anomaly sign between the direction after they cross the equator. The westerly wet and dry cases, with anomalies in the eastern Pacific component becomes well developed near the surface. respectively resembling typical La Nin˜ a/El Nin˜ o patterns. The SST anomalies are much greater in the dry case than in the wet case. Dry conditions along the At- 5. Results lantic coast are also associated with generally positive Figure 6 shows the simultaneous correlation between anomalies in the Indian Ocean, but with negative AMJ rainfall and SSTs. Only three of the five regions anomalies in the equatorial Atlantic and along the appear to show significant correlations; these are dis- Benguela coast of the Atlantic. The well-known Atlantic cussed in detail in the following sections. However, SST dipole—positive (negative) anomalies north (south) large-scale patterns in the correlations suggest that in of the equator—associated with abnormally high rainfall four of the five regions, high rainfall is associated with in the Sahel (e.g., Lamb and Peppler 1992; Joyce et al. a preponderance of lower than normal SSTs over the 2004) is also well developed in the dry composite. tropical and subtropical oceans and negative correla- The association with sea level pressure helps to ex- tions with sea level pressure over most of the tropics (not plain the SST anomalies over the Atlantic. Rainfall in shown). Only region 1, the easternmost region that in- the Atlantic coastal sector and sea level pressure are cludes the western Rift Valley highlands, does not fit this negatively correlated throughout the central and eastern pattern; a possible explanation is discussed in section 6. sectors of the equatorial Atlantic (Fig. 8), indicating a weaker (stronger) South Atlantic subtropical high a. Region 2: The Atlantic coastal region commensurate with wetter (drier) conditions. In the region stretching along the Atlantic coast high The weakening of the high influences Atlantic SSTs in rainfall is associated with above normal SSTs in the three ways. The reduced southerly and southeasterly equatorial and South Atlantic and in extratropical re- flow on its eastern flank reduces the advection of cold gions of the western Pacific. The correlations (Table 2) subpolar water and reduces upwelling. The relaxation of reach the 1% significance level in the Gulf of Guinea the equatorial trades brings warm water farther east- (r 5 0.39) and along the Benguela coast of the eastern ward along the equator. Such a condition is well known

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the Atlantic coastal region, is broader and more intense in the dry composite (Fig. 10). An even more striking contrast is apparent in the equatorial zone of ascent, just to the north. It is much broader and stronger in the wet composite. The zonal wind field corresponding to wet and dry composites is shown locally in the cross sections in Fig. 11 and globally via anomalies at 850 mb and 200 hPa in Fig. 12. These figures indicate that in the wet composite, compared to the dry, the westerlies over the equatorial At- lantic and the West African continent are more strongly developed in the lower troposphere, while the easterlies are stronger at 200 hPa, particularly over the Atlantic (Fig. 12). The correlation between rainfall and wind is particularly high near the surface in the equatorial At- lantic, where it reaches 0.59 (Table 2). The correlation with 200-hPa winds reaches 20.53 over much of the Atlantic, but it is also strong over much of the continent (Fig. 13). A very striking feature is the strong and well- developed zone of low-level westerlies from the surface to nearly 700 hPa in the wet composite (Fig. 11). The stronger westerlies would promote ascent by 1) enhancing the wind component perpendicular to the coast, thereby 2) enhancing the orographic impact of the high terrain, and 3) enhancing surface convergence over the coastal region. The enhanced westerlies might also FIG. 5. Mean wind vectors at four levels during AMJ for 1948 contribute to the ascent in the mid and upper tropo- to 1988. sphere. At the surface the flow is southwesterly. The intensification of this flow serves both to displace the ITCZ northward and intensify the convergence associ- to occur in the boreal summer (Xie and Carton 2004; ated with it. Consequently, the ascending motion asso- Carton and Huang 1994). Termed the ‘‘Atlantic Nin˜ o,’’ ciated with the ITCZ links into the tropical rain belt (Fig. this phenomenon is analogous to the Pacific El Nin˜ o. 10), contributing to its intensification. In the dry compos- The strong contrast in coastal SSTs in the wet and dry ite, these two zones of ascent are nearly independent. composites suggests that local SSTs play a role in mod- The vertical structure of the zonal winds probably also ulating rainfall in this coastal sector of Africa. However, contributes to the strong and wider column of ascent an analysis of vertical motion and specific humidity near the equator in the wet composite (Fig. 11). The suggests that the causal link to interannual variability is vertical shear in the mid and upper troposphere (be- not the local SSTs but the changes in pressure that tween 150 and 600 hPa) is much higher in the wet case: 2 2 produce them. The potential influence of SSTs includes 28ms 1 compared to 5 m s 1 in the dry case. Higher static stability changes, which influence vertical motion, shear also characterizes years in which the tropical rain and changes in atmospheric moisture. However, the wet belt is anomalously intense over West Africa (Grist and and dry composites show little contrast in specific hu- Nicholson 2001; Nicholson 2009a). midity (Fig. 9) and subsidence is actually greater near These local changes in atmospheric circulation appear the surface in the wet composite (Fig. 10), when SSTs to be adequate explanations for the interannual vari- are anomalously high along the coast. ability of rainfall in the coastal sector. However, the The contrasts in vertical motion between the wet and significant correlations with SSTs in the Pacific and In- dry composites can probably be attributed to the re- dian Oceans (Fig. 6) suggest that some large-scale ef- duced (enhanced) influence of the subtropical high in fects are also involved. The zonal wind at 200 hPa shows the wet (dry) years rather than local SST anomalies. For markedly stronger (weaker) easterlies [or weaker the troposphere as a whole, the zone of subsidence as- (stronger) westerlies] over the Atlantic (eastern Pacific) sociated with the subtropical high, which is directly over in the wet composite (Fig. 12). These changes in turn lead

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FIG. 6. Correlation between regional AMJ rainfall and concurrent sea surface temperatures for 1948–88. The 5% significance level is indicated by the dash-dotted line. Rainfall is also correlated with SST averages for the indicated boxes, with values given in Table 2. to reduced upper-level divergence over the eastern Pa- east–west zonal circulation over the entire tropics. In the cific and Indian Oceans and enhanced upper-level di- wet (dry) composite the Pacific cell is stronger (weaker) vergence over most of Africa and the eastern Atlantic. and the Atlantic and Indian Ocean cells are weaker This combination enhances vertical motion over Africa. (stronger) (Fig. 14). Such intensification (weakening) of The omega fields for the wet and dry composites the Pacific cell characterizes La Nin˜ a (El Nin˜ o) epi- further indicate large changes in the intensity of the sodes, consistent with the SST anomalies in the wet and

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TABLE 2. The correlation between rainfall and selected vari- the east–west SST gradient is reduced in the wet years ables. The values of r corresponding to the 0.05 and 0.01 signifi- and enhanced in the dry years. The result is a weakened cance levels are 0.30 and 0.39, respectively. Variables are averaged (enhanced) east–west zonal circulation cell over the over the indicated regions, which are also depicted in Figs. 6, 8, and 13. Atlantic in the wet (dry) years. Consequently, subsidence is reduced over the Atlantic coastal sector in the Label Region Variable Latitude Longitude r wet case, thus favoring increased rainfall. The reduced A 2 SST 08–188S88E–coast 0.48 subsidence, in turn, weakens the subtropical high in the B 2 SST 28N–28S268–68W 0.39 eastern Atlantic, which would further increase SSTs in 8 8 8 2 C 2 SST 4 N–4 S 120 W–coast 0.39 that region. D 2 SST 08–108S508–708E 20.39 E 3 SST 08–148S608–908E 20.46 b. Region 3: Cameroon highlands and northern 8 8 8 8 2 F 3 SST 4 N–4 S 120 –80 W 0.32 Zaire basin G 5 SST 148N–228S728–908E 20.50 H 2 SLP 08–208S108W–108E 20.57 Region 3, the Cameroon highlands and northern 8 8 8 8 I 2 U/1000 5 N–5 S40–10 W 0.59 Zaire basin, lies adjacent to region 2, the coastal Atlantic J 2 U/850 58N–58S408–108W 0.52 K 2 U/200 17.58–7.58N508–208W 20.58 sector, and rainfall time series for the two regions are L 2 U/200 08–88S308W–0820.55 well correlated (r 5 0.49). The two regions make an M 3 U/1000 17.58–12.58N08–308E 0.54 interesting comparison because the patterns of SSTs and N 3 U/850 17.58–12.58N08–308E 0.48 winds associated with the wet and dry composites are 8 8 8 8 2 O 3 U/200 20 –5 N45–15 W 0.49 very similar, but with a striking exception: the equatorial P 5 U/850 7.58–12.58S158W–108E 0.63 and South Atlantic SSTs. This contrast, apparent in every analysis, suggests that region 3 has a weaker as- dry composites. Thus, the global scale probably pro- sociation with the large scale. duces the local anomalies that enhance or reduce rain- Anomalously high (low) AMJ rainfall in both regions fall in coastal region 2. is associated with negative (positive) SST anomalies The changes that occur over the Atlantic in the wet over the Indian Ocean and eastern tropical Pacific. The and dry composites appear to result from a complex correlations are significant in the eastern equatorial feedback between the SSTs and atmospheric circulation. Pacific and the equatorial Indian Ocean (Fig. 6; Table 2). The SST patterns in the equatorial Atlantic are such that Notable anomalously dry conditions in both region 2

FIG. 7. Standardized SST anomalies for the wet composite and the dry composite of regions 2, 3, and 5. Anomalies are based on the period 1948–88.

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Overall, the analyses described above do not show any clear-cut reason for the occurrence of wet or dry extremes in region 3. It is likely that more local factors play a role. We hypothesize that the low-level westerlies, enhanced by a strong cross-equatorial pressure gradient, may be the key. The westerlies would enhance the orographic effects of the highlands over Cameroon. Rainfall is exceedingly high in parts of this region, 2 reaching 10 000 to 14 000 mm yr 1 at Debundscha and FIG. 8. Correlation between regional AMJ rainfall of region 2 other locations near Mt. Cameroon. The eastern extent and concurrent sea level pressure for the period 1948–88. The 5% of region 3 lies downstream from the highlands and significance level is indicated by the dash-dotted line. Rainfall is also correlated with SLP averages for the indicated box, with value might be influenced by rain systems developed over them. given in Table 2. Consistent with this hypothesis, omega fields averaged for the latitudes of 08 to 78N (not shown) indicate a local intensification of ascending motion over the highlands and and region 3 are associated with strong positive SST over Mt. Cameroon in particular. Some evidence of that is anomalies characteristic of El Nin˜ o and in the wet also seen in Fig. 14, where in the wet composite there is composites show negative La Nin˜ a–like anomalies in the a narrow but significant area of enhanced vertical motion eastern Pacific. In the equatorial and South Atlantic, the in the midtroposphere around 08 to 108E. association with SSTs is very different for the two regions: c. Region 5: Southern Zaire basin strongly positive for the Atlantic coastal sector (region 2) and weakly negative for region 3, located to the north. The interannual variability of rainfall in region 5, The links to zonal wind anomalies likewise indicates which is predominantly Southern Hemisphere sectors of similar global-scale patterns for the two regions (but the Zaire basin and the slopes of the highlands to the weaker associations for region 3) and marked contrast in south, does not appear to be closely linked to large-scale the equatorial and South Atlantic. In the wet (dry) factors. The only large area with significant correlation composites (Figs. 11 and 12) the upper-tropospheric between SSTs and rainfall (Fig. 6) is the central tropical easterlies and low-level westerlies are anomalously Indian Ocean (r 520.50). Notably, the overall spatial strong (weak) and midtropospheric easterlies are anom- pattern of correlation with SSTs is similar to that of re- alously weak (strong). However, the anomalies over the gion 3, the northern Zaire basin and Cameroon high- Atlantic associated with region 3 are weak and do not lands: wet (dry) conditions tend to be associated with reach the 5% significance level. negative (positive) SST anomalies in the global tropics Over the Atlantic and western equatorial Africa, the (Fig. 7). However, the association is stronger with dry wet and dry composites of region 3 show little contrast in conditions in region 3 and wet conditions in region 5. either upper-level divergence or vertical motion (Figs. The link to the zonal wind is relatively weak compared 12 and 14). These contrasts are strong for region 2: to regions 2 and 3 (Fig. 13). However, the opposition of stronger (weaker) divergence at 200 hPa in the wet (dry) zonal anomalies at 200 and 850 hPa in the wet-minus- composites and stronger (weaker) subsidence over the dry composites of Fig. 12 suggests that there is some continent. association between rainfall in the southern Zaire basin Surprisingly, the vertical motion over most of the re- and global-scale processes. The most direct association gion is marginally stronger in the dry composite, al- is with zonal winds over the eastern Atlantic around though the differences are not highly significant. This is 108S (i.e., in proximity to region 5). The negative (pos- apparent in the global vertical motion fields (Fig. 14) and itive) correlations with 200 hPa (850 hPa) zonal wind in the zonal cross sections of vertical motion (Fig. 10). suggest a weakening (intensification) of the zonal cir- The latter figure represents an average for the longitude culation over the south equatorial Atlantic in the wet span 108 to 208E. Because of the northwest to southeast (dry) composite (Fig. 13). The strongest correlation, orientation of region 3 and the counterintuitive result of 10.63, is with 850-hPa wind around 108S. The global weaker vertical motion in the wet composite, zonal cross omega fields (Fig. 14) confirm the changes in the zonal sections of vertical motion were also constructed for 108 circulation over the south equatorial Atlantic. Weaker to 158E and 158 to 208E (not shown). These confirm that changes in the zonal circulation over the Pacific and the contrast in vertical motion between wet and dry Indian Oceans are also apparent. composites is minimal for region 3, with marginally more Over the continent, the wet/dry contrasts in zonal intense vertical motion in the dry composite. wind at 200 and 850 hPa are weaker (Fig. 11). There the

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21 FIG. 9. Latitude–height cross sections of specific humidity (g kg ) for regions (top) 2, (middle) 3, and (bottom) 5 during AMJ. Specific humidity is zonally averaged over 108–158E, 108–208E, and 158–308E for regions 2, 3, and 5, respectively. The panel on the right shows the difference between the wet and dry composites; shading indicates areas where the difference is significant at 5% level.

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22 21 FIG. 10. Latitude–height cross sections of omega (10 Pa s ) for regions (top) 2, (middle) 3, and (bottom) 5 during AMJ. Omega is zonally averaged over 108–158E, 108–208E, and 158–308E, for regions 2, 3, and 5, respectively. Areas of rising motion are shaded. The panel on the right shows the difference between the wet and dry composites; shading indicates areas where the difference is signiﬁcant at 5% level.

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21 FIG. 11. Latitude–height cross sections of zonal winds (m s ) for regions 2 (top), 3 (middle) and 5 (bottom) during AMJ. Zonal wind is averaged over 108–158E, 108–208E, and 158–308E, for regions 2, 3, and 5, respectively. Areas of easterly winds are shaded. The panel on the right shows the difference between the wet and dry composites; shading indicates areas where the difference is signiﬁcant at 5% level.

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FIG. 12. (top) Wet-minus-dry composites of divergence 2 2 2 (10 6 s 1) at 200 hPa, and zonal wind (m s 1) at (middle) 200 hPa and (bottom) 850 hPa. The three-panel diagrams clockwise from top left represent regions 2, 3, and 5, respectively. The dashed line shows areas in which the difference between the wet and dry composites is signiﬁcant at the 5% level.

strongest contrast is apparent at 600 hPa (Figs. 11 and 15), this paradigm over West Africa. A major one is the the level of the African easterly jet of the Northern decoupling of the low-level convergence/rising motion Hemisphere (AEJ-N). In the wet (dry) composite the of the ITCZ and the broad region of ascent throughout AEJ-N is exceedingly weak (strong) over equatorial the troposphere that is associated with rainfall. The Africa. The correlation of 600-hPa zonal wind with latter she termed the ‘‘tropical rain belt.’’ The two fea- rainfall in region 5 reaches 0.5. tures are not decoupled over region 5. However, the Another notable difference between the wet and dry ‘‘rain belt’’ is much broader than the region of ascent composites for region 5 is the intensity and latitudinal at ;138N that is associated with the low-level ITCZ. extent of prevailing ascent (Fig. 10). In the wet com- We speculate that local topography contributes to this posite, ascending motion extends above most of region zone of ascent and that the combination of topography 5, across a latitude span of roughly 20 degrees. In the dry and the aforementioned wind anomalies play an im- composite the zone of ascent is displaced northward and portant role in interannual variability. The rainfall re- is marginally weaker. The global omega fields of Fig. 14 gime of the entire Zaire basin is dominated by an also show weaker ascent in the dry composite between orographic system with ascent over the surrounding 108 and 408E. The key to understanding the interannual highlands during the day and descent at night. The variability of April–June rainfall in the Zaire basin is an descending flow off the highlands converges into the explanation for this broad zone of ascent, but an expla- basin, resulting in abnormally strong mesoscale con- nation is not readily apparent from the foregoing analyses. vective systems (Jackson et al. 2009; Zipser et al. 2006). Traditionally the boreal spring rainfall in this region Figure 10 shows that the mean zone of ascending motion and the zone of rising motion associated with it are coincides with the expanse of the Zaire basin, where this assumed to be linked to the northward advance of the flow converges. Subsidence prevails over most of region ITCZ. Nicholson (2009b) showed the shortcomings of 5 (the southern basin), which roughly spans the latitudes

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FIG. 13. Correlation between AMJ rainfall and zonal wind at 200, 850, and 1000 hPa for regions 2 (top), 3 (middle), and 5 (bottom). Correlations are based on the period 1948–88 and only those exceeding the 5% signiﬁcance level are shown. Rainfall is also correlated with zonal wind averages for the indicated boxes, with values given in Table 2. of 28 to 108S. Region 5 lies on the slopes of the highlands only 11 days. Figure 17 shows the mean vertical motion to the south and receives rainfall from MCSs that form in the wettest and driest 6-day periods of April during these over the highlands and move downslope at night. four years. Maximum contrast occurs around 600 hPa, Jackson et al. (2009) showed that midlevel conver- where the mean vertical motion for the wet period is 3 to 4 gence over the Zaire basin appears to promote the de- times greater than during the dry period. This is consistent velopment of MCSs. The much weaker AEJ-N in the with the occurrence of strong MSCs in this region. wet composite for region 5 results in enhanced conver- It is also worth noting that May was almost completely gence at 600 hPa, suggesting enhanced MCS activity dry in the dry years but received substantial rainfall in the wet case. A similar mechanism was shown for during the wet years. Hence a longer rainy season the variability of summer rainfall over the central Sahel characterized the two wet years of Fig. 17. In the tradi- (Dezfuli and Nicholson 2011). The anomalously strong tional paradigm of the ITCZ bringing rains to this re- westerlies over the south equatorial Atlantic (Figs. 12 gion, this might be interpreted as an earlier northward and 13) enhance the trigger for uplift over the highlands, progression of the ITCZ in the dry years. Two facts a prerequisite for the MCS formation. contradict that interpretation. One is that the surface If a change in the number and/or intensity of MCS ITCZ (the zone of low-level vertical motion at ;138Nin activity plays a major role in the interannual variability Fig. 10) is well to the north of region 5 in both composites, of rainfall in region 5, the southern Zaire basin, this nor has it shifted northward in the dry composite. should be evident in the frequency distribution of in- Another factor that may play a role in this region is dividual rain events. Daily rainfall in wet and dry years, equatorially propagating Kelvin waves. During March examined from TRMM data, shows contrasts in rainfall and April these waves move eastward off the Atlantic intensity that are consistent with enhanced convective and interact with convective systems that are typically activity (Fig. 16). In two years with anomalously high triggered over the Rift Valley highlands (Nguyen and AMJ rainfall in region 5, 2002 and 2006, rainfall exceeds Duvel 2008). The presence of low-level westerlies 10 mm on 20 days (on 6 of which it exceeds 20 mm). It is enhances the propagation. Although the waves do not greater than 5 mm on a total of 29 days. In the two years trigger convection, they modify their development with anomalously low AMJ rainfall, 1998 and 2004, it into larger mesoscale convective systems. Pohl and exceeds 10 mm on only 2 days and exceeds 5 mm on Camberlin (2006) also note that the amplitude of the

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22 21 FIG. 14. Omega (10 Pa s ) for wet, dry, and wet-minus-dry composites. The three-panel diagrams clockwise from top left represent regions 2, 3, and 5, respectively. Omega of each region is meridionally averaged over its corresponding latitudes. The dashed lines in the bottom panel of each set of diagrams show areas in which the difference between the wet and dry composites is signiﬁcant at the 5% level.

Madden–Julian oscillation inﬂuences interannual vari- atmosphere. In only one case, the Atlantic coastal region 2, ability during this season. can a deﬁnitive causal explanation for rainfall variability be put forth. This is the only region in which remote forcing appears to be the major control. In the other regions, some 6. Discussion remote impact is apparent but the control appears to be The analyses carried out in this study identify associa- largely local and strongly modulated by orographic tions between rainfall in western equatorial Africa dur- effects. For these two cases, hypotheses concerning ing the April–June season and the tropical oceans and the controls on interannual variability are presented.

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21 26 21 FIG. 15. Zonal wind (m s , top) and divergence (10 s , bottom) at 600 hPa for the wet, dry, and wet-minus-dry composites of region 5.

For all three case studies, anomalies in the local zonal Nin˜ o–like SST anomalies and dry conditions. In both the winds are similar. In each of these regions, the wet wet and dry composites the SST anomalies in the At- composite is associated with anomalously strong low- lantic serve to enhance zonal SST gradients across the level westerlies and upper-tropospheric easterlies, but Atlantic. Consequently, the interannual variability is reduced easterly winds in the midtroposphere and a probably controlled by local factors that are modulated poleward displacement of the upper-tropospheric west- by the global tropical circulation. This is a region of high erlies in both hemispheres. The opposite pattern prevails terrain and intense convective activity. Our hypothesis is in the dry composites. These patterns are best developed that the low-level westerlies have a large impact by en- in region 2, along the Atlantic coast, and most weakly hancing the orographic effects in this region, where an- developed for region 5, the central Zaire basin. nual rainfall can reach 14 000 mm in some areas. The wet and dry extremes in region 2 can be readily Region 5, the southern Zaire basin, shows even explained in terms of large-scale SST, pressure, and weaker links to the large scale. A similar conclusion was upper-level wind anomalies that alter the east–west drawn by Pohl and Camberlin (2006), who examined the zonal circulation. Changes in these variables alter both March–May season in an area of East Africa that ex- the low-level wind field over the equatorial Atlantic tends into region 5. They did not find a link to large-scale and the vertical motion field over western equatorial processes. They did show that interannual variability is Africa. The impact is probably locally enhanced by changes clearly linked to a change in the intensity of individual in the intensity of the South Atlantic subtropical high rainfall events, consistent with our finding for region 5. and by the impact of the low-level westerlies as they Topography plays a large role in determining the in- meet the coastal terrain. Stronger westerlies enhance tensity of convection in this region (Jackson et al. 2009). orographic effects, low-level convergence, and vertical A change in the intensity of the Atlantic zonal circula- and horizontal shear. tion cell also contributes. Region 3 shows a weaker relationship to the large- In all three regions examined contrasts between wet scale, despite an apparent relationship between El and dry composites are apparent in the global zonal

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FIG. 16. Daily rainfall (mm) during AMJ for region 5 for the two wettest years (2002, 2006) and the two driest years (1998, 2004) of the period 1998–2010. The data are from TRMM 3B42. circulation cells (Fig. 14). The strongest anomalies arise atmospheric circulation over the whole of the tropical over equatorial Africa when the zonal circulation is Pacific and Indian Oceans are associated with the in- anomalous over both the Pacific and Indian Oceans. terannual variability of rainfall over western equatorial This is the case for Atlantic coastal region 2, with strong Africa. These represent coupled variability in the ocean wet/dry contrasts in vertical motion over Africa from and atmosphere over large sectors of the tropics. Vari- 308Wto308E. More moderate contrasts in the zonal cell ations in local cloudiness, local wind stress, or remote over Africa are apparent for regions 3 and 5. In the forcing of wind stress can all induce changes in SSTs former case, the Cameroon highlands and northern (Hirst and Hastenrath 1983a). Zaire basin, contrasts are strong in the Pacific cell but Thus, some of the associations that have been relatively weak in the Indian Ocean cell. The opposite is demonstrated between SSTs and rainfall, especially the true for region 5, the southern Zaire basin. more local links to the Gulf of Guinea or Benguela These results collectively suggest that the Pacific coast, can reflect the common forcing by the large-scale Ocean has little direct influence on western equatorial atmosphere, which at the same time impacts equatorial Africa. Its influence is manifested via changes in the rainfall via vertical motion fields and zonal winds. The zonal circulation over the Atlantic and Indian Ocean modified SST fields work in tandem to enhance the ef- sectors. This is consistent with the conclusions of nu- fects. This is similar to the situation described for the merous studies that have compared the influence of the link between East African rainfall during the boreal Indian and Pacific Oceans on rainfall in East Africa autumn and the Indian Ocean (e.g., Hastenrath et al. (e.g., Clark et al. 2003; Black et al. 2003; Behera et al. 2011). 2005; Ummenhofer et al. 2009; Hastenrath et al. 2011). The stronger association with atmospheric pro- However, those studies have examined the East African cesses is confirmed by the correlation between rain- ‘‘short rains’’ season of boreal autumn. Our results in- fall, SSTs, SLP, and zonal winds. For region 2, the dicate a similar situation in the boreal spring rainy sea- absolute values of the strongest correlations with SSTs son (i.e., AMJ). range from 0.39 to 0.48. Absolute values of the cor- A synthesis of our results suggests that, while some relation with zonal wind range from 0.52 to 0.59; regions show a strong association with local SSTs in the correlation with sea level pressure over the equatorial eastern Atlantic or western Indian Oceans, the associ- Atlantic is 20.57. For region 3 the range with SSTs is ation may not be a causal one. Large changes in the 20.32 to 20.46, but 0.48 to 0.54 for wind. For region 5

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22 21 FIG. 17. Omega (10 Pa s ) for the wettest and driest 6-day periods in April of region 5 during the four years shown in Fig. 16. Wet composite is for 12–17 Apr 2006, and dry composite is for 25–30 Apr 1998.

the correlation with low-level zonal wind over the At- 7. Summary and conclusions lantic reaches 0.63. A direct effect of local SSTs would be seen as This study has examined rainfall variability in five changes in specific humidity, moisture convergence, sectors of western equatorial Africa during the AMJ or static stability. The change in specific humidity season. Large-scale forcing was apparent for three of the between wet and dry composites for all three case regions, with wet (dry) conditions being associated with studies is negligible (Fig. 9). Moisture convergence positive (negative) SST anomalies in the tropical Pacific (not shown) does not show consistent contrast either. and Indian Oceans. The large-scale forcing is particu- Vertical motion (e.g., Fig. 10) suggests that the larly strong for region 2, stretching along the Atlantic changes in static stability are probably confined to the coast. In the remaining two, the southern Zaire basin layers just above the surface. Prior studies similarly and the Cameroon highlands/northern Zaire basin, the examined the links between rainfall and SSTs in this forcing appears to have a strong local component rep- region (Hirst and Hastenrath 1983a,b). Impacts of resenting a combination of topographic and mesoscale coastal SSTs on atmospheric moisture and stability effects. were demonstrated, but they appeared to have a clear A critical factor is the intensity of the east–west zonal association with rainfall variability only at stations right circulations over the three tropical ocean basins. In on the coast (Hirst and Hastenrath 1983a). In the in- some cases an ENSO-like signal in both SSTs and the terior (i.e., in the Zaire basin), the more important zonal circulation over the Pacific is evident in associa- factor appeared to be changes in the zonal circulation tion with the interannual variability of rainfall. How- over the Atlantic (Hirst and Hastenrath 1983b), con- ever, changes in the Pacific sector do not appear to sistent with our conclusions about the importance of impact Africa directly but instead modulate the zonal the zonal circulation. It is worth noting that since there circulation over Africa via influences on the Atlantic are some concerns about using NCEP–NCAR rean- and Indian Oceans. alysis data, we evaluated our hypotheses utilizing 40-yr Consistent changes in the zonal winds over Africa are European Centre for Medium-Range Weather Fore- apparent in association with wet and dry conditions in all casts (ECMWF) Re-Analysis (ERA-40) data (not shown). three regions evaluated in depth (i.e., the Atlantic The conclusions suggested by both datasets were overall coastal sector, Cameroon highlands/northern Zaire ba- consistent. sin, and southern Zaire basin). Wet (dry) composites

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