On the factors modulating the intensity of the tropical rainbelt over West Africa

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 29: (2009) Published online 9 July 2008 in Wiley InterScience ( On the factors modulating the intensity of the tropical rainbelt over West Africa Sharon E. Nicholson* Department of Meteorology, Florida State University, Tallahassee FL 32306, USA ABSTRACT: This article presents a case study of a wet and a dry year over West Africa. These 2 years, 1955 and 1983, are characterized by rainfall anomalies of one sign throughout West Africa, including the Sahel and Guinea Coast (the no-dipole cases defined in previous studies). The contrast in rainfall is related to a general weakening and intensification of the tropical rainbelt in the dry year (1983) and the wet year (1955), respectively. The study, limited to the month of August, examines the factors that modulate the intensity of the rainbelt. The more intense rainbelt of 1955 is associated with an anomalously strong tropical easterly jet (TEJ) and a slight northward displacement of the African easterly jet of the Northern Hemisphere (AEJ-N). The link between the TEJ and rainfall is a causal one, with the strong TEJ enhancing rainfall by enhancing upper-level divergence and sustaining a strong Hadley-type overturning over West Africa, with strong vertical motion. In 1983, a weak TEJ and vertical alignment of the AEJ-N and TEJ axes promote a weak rainbelt and ubiquitous drought. The factors that produce a rainfall dipole (i.e. an opposition in the sign of anomalies between the Sahel and the Guinea Coast) are notably different. The dipole results from meridional displacements of the circulation and rainbelt, rather than changes in rainbelt intensity. Although the strong TEJ is a prime factor, the development of the dipole requires the development of strong and vertically extensive equatorial westerlies. These are produced by strong surface pressure gradients over the Atlantic and West Africa. Copyright 2008 Royal Meteorological Society KEY WORDS tropical rainbelt; West Africa; tropical easterly jet; African easterly jet; Sahel; rainfall Received 10 July 2007; Revised 25 February 2008; Accepted 2 March Introduction The semi-arid Sahel zone of West Africa is well known for the episodic severe droughts that ravage the land and the large magnitude of variability on inter-decadal time scales. A voluminous archive of meteorological studies have demonstrated various characteristics of wet and dry years, in an attempt to identify the underlying causes of variability. The hypotheses fall into two basic categories: ocean/atmosphere dynamics and land surface processes. The current consensus is that the former are the source of the variability and the latter may modulate the dynamically forced variability. The dynamical hypotheses that have been proposed are quite diverse, but they tend to fall along the lines of remote marine forcing (e.g. Lamb, 1978a,b; Hastenrath, 1984, 1990; Folland et al., 1986, 1991; Palmer, 1986; Janicot, 1992; Lamb and Peppler, 1992; Rowell et al., 1992, 1995; Fontaine and Bigot, 1993; Fontaine and Janicot, 1996; Fontaine et al., 1998; Semazzi et al., 1988, 1996; Ward, 1998; Giannini et al., 2003) and circulation changes over the African continent (e.g. Kanamitsu and Krishnamurti, 1978; Newell and Kidson, 1984; Fontaine and Janicot, 1992; Fontaine et al., 1995; * Correspondence to: Sharon E. Nicholson, Department of Meteorology, Florida State University, Tallahassee FL 32306, USA. sen@met.fsu.edu Grist and Nicholson, 2001). The former generally relate rainfall variability to sea-surface temperatures (SSTs). Unfortunately the mechanism creating the link is typically a black box : a statistical or model connection is shown but no link to the circulation over Africa is explicitly determined. On the other hand, the studies of atmospheric dynamics provide a mechanism (i.e. a circulation change) but fail to identify the underlying causes. Only relatively recently have studies such as Trzaska et al. (1996); Camberlin et al. (2001); Rowell (2001), and Diedhiou and Mahfouf (1996) attempted to link the two approaches. We have developed a new conceptual framework (Nicholson and Grist, 2001; Nicholson, 2008) for studying rainfall variability over West Africa that likewise allows us to merge the surface forcing and the local atmospheric dynamics, using a combination of observational analysis and dynamic models. At the core of the conceptual model is the tropical rainbelt, i.e. the latitudinal zone of precipitation that is loosely linked to the intertropical convergence zone (ITCZ) during its north/south seasonal excursion over Africa. Seasonal rainfall in West Africa is a function of the intensity, width and latitude of the rainbelt. These characteristics determine the length of the rainy season, the amount of rainfall during the season, and its latitudinal distribution. Interannual variability can be interpreted in terms of two distinct modes: latitudinal displacements of the tropical rainbelt and changes in its Copyright 2008 Royal Meteorological Society

2 674 S. E. NICHOLSON Figure 1. Summary of new conceptual framework of rainfall variability sub-saharan West Africa, illustrated for August (based on Nicholson, 2008). The light and dark shading over Africa indicates negative and positive rainfall anomalies, respectively. These illustrate the four most common anomaly patterns over West Africa. Variability is a function of the intensity, width and location of the rainbelt. The schematic illustrates variations in these characteristics corresponding to each of the four patterns. Intensity is indicated by shading, with the horizontal lines representing latitudes where rainfall exceeds 100 mm per month. The light and dark shading between the lines indicates a rainbelt of normal or abnormally strong intensity, respectively. No shading indicates an anomalously weak rainbelt. The development of the dipole pattern, with contrasting anomalies in the Sahel and Guinea Coast, requires shifts in the latitude of the AEJ-N, also shown. No shift is evident in the no-dipole patterns. intensity. These are respectively linked to the two most common patterns of rainfall anomalies (Figure 1): the dipole in which anomalies of the opposite sign prevail in the Sahel and Guinea Coast and the no-dipole years with ubiquitous positive or negative anomalies throughout the region. Admittedly, this framework is largely descriptive. However, the tropical rainbelt integrates the effects of the dynamic processes influencing the production of rainfall in West Africa and as such the rainbelt can be used as an index to facilitate the identification of the underlying dynamic factors. These factors, such as convergence, vertical motion, latent heat release, dynamic stability and vorticity are in turn linked to atmospheric features at the synoptic, regional and planetary scales. The features of primary importance include the mid-tropospheric African easterly jet of the Northern Hemisphere (AEJ-N), which extends across West Africa; the upper-tropospheric tropical easterly jet (TEJ), a summer feature extending from Asia across to Africa; and the African easterly waves (AEWs) that propagate across West Africa. The broad goal of our work is to use a combined analysis of the rainbelt over West Africa and the associated patterns of atmospheric circulation to answer two key questions: what causes a shift in the position of this zone from year to year and what causes a change in the intensity and amount of rainfall associated with this zone? These issues are considered in a set of five recent papers. Nicholson (2008) describes and demonstrates the validity of the conceptual model. Nicholson et al. (2007, 2008) use numerical modelling to explain the role of the underlying dynamics and its relationship to wave activity over West Africa. Nicholson and Webster (2008) provide a physical explanation for latitudinal displacements of the rainbelt and the existence of the rainfall dipole. The current paper focuses on the question of what causes changes in the intensity of rainfall within the rainbelt, essentially the efficiency of the precipitation process. To do so, case studies of one wet and one dry nodipole year (1955 and 1983) are carried out. The analyses include vertical cross-sections and spatial fields of divergence, vertical motion, and vorticity and an examination of the major circulation features that determine these: the surface monsoon westerlies, the AEJ-N and the TEJ. Surface fields of temperature and moisture and atmospheric profiles of temperature and moisture are also examined. An overview of the conceptual framework is presented in Section 2, the data and methodology in Section 3. Section 4 presents results related to vertical motion fields and other aspects of the atmospheric circulation. Section 5 considers SSTs, static stability and moisture. The discussion in Section 6 summarizes the main contrasts between 1983 and 1955 and considers possible causes for the main one, the intensity of vertical motion. A comparison is also made with factors associated with the dipole years. Section 7 summarizes the role of the principal circulation features in determining the intensity of rainfall over West Africa and the implications of our results for seasonal forecasting and modelling. 2. The conceptual framework The conceptual framework we are presenting grew out of various analyses of the spatial patterns of rainfall anomalies over West Africa. One of the outstanding characteristics of the region is that seasonal precipitation shows a remarkable degree of spatial coherence. Consequently, two basic configurations of rainfall anomalies generally prevail: a dipole, with a node around 9 or 10 N, and ubiquitous negative or positive anomalies. Because the two patterns have a negative and a positive phase, the net result is the four modes shown in Figure 1. In our previous papers, these are labelled wet and dry according to the sign of rainfall anomalies in the Sahel and dipole versus no-dipole.

3 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 675 An evaluation of seasonal rainfall anomalies showed that during the period , one of these four modes prevailed in 54 of the 77 years (Nicholson, 2008). Of these, 29 were dipole years. These configurations were more prominent during the 30-year period : the dipole was well defined in 11 years and the uniform negative anomalies of the wet and dry modes were apparent in 12 years. Numerous studies have shown that years with and without the rainfall dipole differ with respect to both circulation anomalies over West Africa and SST-rainfall associations (e.g. Semazzi et al., 1988; Janicot, 1992; Fontaine and Bigot, 1993; Fontaine et al., 1995; Janicot et al., 1996; Ward, 1998; Nicholson and Grist, 2001). Nicholson and Grist (2001) further hypothesize that the dipole and dry patterns have fundamentally different causes. Accordingly, they associate the dipole with factors that influence the position of the tropical rainbelt, while ubiquitous negative or positive anomalies are associated with factors that influence its intensity (Figure 1). In this case, intensity refers to the magnitude of vertical motion and the magnitude of peak monthly rainfall associated with its core. In the no-dipole years the tropical rainbelt is abnormally weak or strong but near its usual latitudinal location. In the wet or dry dipole years it is displaced anomalously far to the north or south but no change in intensity is apparent. In this framework, the jet streams over West Africa play a major role in the interannual variability. The core Figure 2. Rainfall in 1983 and 1955 during the West African rainy season (June through September) (expressed in units of standard departure, based on means for the period ). Circles indicate rainfall stations; size of the circle indicates magnitude of the anomaly, with open circles indicating negative anomalies and shaded circles indicating positive anomalies. of the rainbelt lies between the axes of the AEJ-N and the TEJ (Nicholson, 2008). Thus the two jet streams determine not only the location of the rainbelt, but also its latitudinal span. The TEJ is also much stronger in both types of wet years than in the dry years and the low-level westerlies near the Guinea Coast are likewise stronger in the wet years. Our current study helps to explain this link and to compare the role played by these factors in the development of dipole and no-dipole years. 3. Methodology 3.1. Data The data sets used in this study consist of the author s African precipitation archive (Nicholson, 1986, 1993), Reynold s reconstructed SST data set (Smith et al., 1996), and the National Centers for Environmental Prediction (NCEP) Reanalysis Data Set (Kalnay et al., 1996; Kistler et al., 2001). A map of rainfall stations in the archive is shown in Figure 2; most records extend from around 1920 to For each station, long-term means are calculated as the average of all years in the data set. Thus, in most cases the mean represents approximately years. The NCEP data set includes daily and monthly values of various dynamical parameters. For most variables, the data set covers the years The use of NCEP Reanalysis Data for historical analyses has been questioned by some. In particular, it has been suggested that a discontinuity occurred around 1968 or 1970 (Poccard et al., 2000; Janicot et al., 2001; Chelliah and Bell, 2004; Kinter et al., 2004). This time period was coincidently a discontinuity in the Sahel rainfall record: the onset of the multi-decadal dry conditions (Nicholson, 1993). There is a consensus that NCEP estimates of wind fields are relatively reliable, but that there are difficulties in tropical divergent circulations and rainfall (Poccard et al., 2000; Kinter et al., 2004). It is nevertheless routinely used in long-term studies. In such a study, Grist and Nicholson (2001) used West African pibal and rawinsonde reports to verify conclusions based on NCEP. Also, in other analyses in which we utilized NCEP (e.g. Grist and Nicholson, 2001; Nicholson and Grist 2001, 2003), coherent and physically reasonable results emerged from a mixture of data sources. In this study, we use primarily what are termed A variables, those strongly influenced by observational data and hence most reliable (Kalnay et al., 1996). These include, for example, wind and pressure fields. Less reliable are the B variables, the derivation of which is about equally dependent on observations and modelling. We have reduced the possible error by calculating the B variables (divergence, vorticity, and velocity potential) directly from NCEP winds. Calculating these off-line allowed us to examine these variables with greater vertical resolution and to extend the analysis back to Rainfall is a C variable (largely model dependent) and NCEP rainfall is known to be unreliable over Africa

4 676 S. E. NICHOLSON Figure 3. Rainfall as a function of latitude over West Africa for 1955 and 1983 (averaged for 5 W 5 E). Rainfall is in mm per month and is given for June, July, August and September. (Poccard et al., 2000). This problem was avoided by the use of the author s gauge archive. The 150 mb level is used to represent the TEJ in most analyses. Over West Africa it is generally the level of the jet core. The AEJ-N is generally strongest at 650 mb. Depending on the availability of data, either the 700 or 600 mb level is used to represent the AEJ-N Overview of the approach To investigate the factors controlling interannual variability the approach taken in our recent work has been to focus on case studies (years or composites) representing each of the four patterns shown in Figure 1. The years 1950/1955 represent the wet dipole/no-dipole and 1984/1983 typifies the dry dipole/no-dipole. The selection was based on the magnitude of rainfall anomalies and the representativeness of the year. Nicholson (2008) showed that the wet and dry dipole years (1950 and 1984, respectively) were associated with a north/south displacement of the rainbelt and AEJ-N and that the wet/dry conditions of 1955/1983 were associated with an intensification/weakening of the rainbelt and the vertical motion in its core, but no latitudinal displacements. Extreme years rather than composites are used for two reasons. One is that some aspects of our work, such as the model simulation of waves (Nicholson et al., 2007, 2008), requires examination of individual years. The second is that extreme years underscore the most salient contrasts between the types of years examined. Later studies using multi-year composites can be used to confirm the initial findings. This has been done in the case of the dipole patterns (Nicholson and Webster, 2008). The current study focuses on the dynamic factors modulating the intensity rather than the position of the tropical rainbelt. It builds upon the Nicholson (2008) study by including a more detailed analysis of vertical motion and an examination of kinematic and thermodynamic aspects of the prevailing atmospheric circulation. It also includes a brief summary of relevant model simulations of wave activity. The basic methodology is to compare and contrast the years 1983 and In 1983, rainfall in the boreal summer was below the mean at nearly every station in West Africa (Figure 2). The exceptions are a few stations in the highlands of Sierra Leone and Guinea and stations east of Lake Chad. At most stations, the anomaly was roughly half a standard deviation. In 1955, rainfall was above the mean at nearly all stations south of the Sahara; at most stations the anomaly was at least one standard deviation. Figure 3 underscores the contrast in intensity. The latitudinal profile of rainfall is nearly identical for 1983 and 1955 in all 3 months, but there is considerably less rainfall at all latitudes during Also noteworthy is that the rainfall maximum does not move northward in August as it usually does. From July through September the core of the rainbelt lies at ca N. The analyses in this article emphasize the month of August. This is the wettest month in most of the region and the month that contributes the lion s share of the interannual variability (Dennett et al., 1985; Nicholson and Palao, 1993). Examination of other months showed relatively little variability within the season. However, this may be due to the relatively coarse spatial resolution of the NCEP data(2.5 of latitude and longitude). 4. Results: atmospheric dynamics 4.1. Vertical motion Figure 4 shows the vertical profile of mean vertical motion as a function of latitude for August of the years 1955 and The areas of strong ascent are highlighted and rainfall as a function of latitude is shown at the bottom. Profiles are given for three longitudes (10 W, 0 and 10 E) spanning the continent from near the Atlantic coast to just west of Lake Chad. In both 1983 and 1955, a core of strong vertical motion lies north of the equator and extends throughout the troposphere. At all three longitudes it is considerably stronger in 1955 than in 1983, but in both years the

5 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 677 Figure 4. Mean vertical motion (10 3 ms 1 ) in August at 10 W, 0 and 10 E. The vertical dashed lines indicate the axes of the AEJ-N and TEJ, with the more northern axis corresponding to the AEJ-N. Rainfall is also indicated at the bottom, the amount (in mm per month) being given by the right hand y-axis. Figure 5. August mean zonal winds (m s 1 ) at three levels for 1955 and Only easterly speeds are plotted for 150 and 600 mb and only westerly speeds are plotted for 850 mb. The tropical easterly jet and the African easterly jet are clearly apparent at 150 and 600 mb, respectively.

6 678 S. E. NICHOLSON core of the rising column lies near 10 N. As seen in Figure 3, the core of the rain belt also lies at this latitude. The greatest contrast is at 10 E. In 1955, vertical velocity reaches ms 1, but the maximum in the dry year is on the order of ms 1.The weakest contrast is at 10 W, with maximum vertical velocities of 6 and ms 1 in 1955 and 1983, respectively. The reasons for the longitudinal variation are not completely clear, but are probably related to the strong impact of the AEJ of the Southern Hemisphere in wet years in the eastern Sahel and to the prevailing topographic influence in the western Sahel. Two other points of contrast are evident. One is that the column of rising motion is considerably broader in 1955 than in 1983, especially in the two eastern most regions. The second is that in 1983 two regions of vertical motion are separated by strong subsidence. The northern-most region corresponds to the heat low over the Sahara. In the wet case, the subsidence is markedly reduced and the two coresmerge,especially at10 E. These configurations are typical of wet and dry years in the Sahel (Nicholson, 2008). Figure 4 also indicates the latitude of the AEJ-N and TEJ axes in August of 1983 and 1955 (dashed vertical lines). Clearly, the intense core of vertical motion is constrained between the axes of the two jets. When the AEJ-N and TEJ are nearly vertically aligned, as is the case in 1983, the region of ascent is markedly reduced. The latitudinal distribution of rainfall (thick line at the bottom, with amounts indicated on the right vertical axis) is also markedly different in the two cases, but in both, peak rainfall corresponds to the area between the two jet axes Atmospheric circulation Wind field Figure 5 shows the mean wind at three levels during August of 1955 and These levels correspond to the most prominent features of the circulation over West Africa: the TEJ (150 mb), the AEJ-N (700 mb), and the equatorial westerlies (850 mb). The most apparent contrasts between the 2 years are the intensity of the equatorial westerlies at 850 mb, the strength of the AEJ-N, and the development of the TEJ over West Africa. In 1955, the westerlies were well developed at 850 mb Figure 6. August mean meridional wind (m s 1 ) at 1000, 850, 700 and 150 mb for 1955 and 1983 and difference fields. Southerly winds are shaded.

7 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 679 and extended as high as 700 mb, but in 1983 westerly flow barely extended to 850 mb. The AEJ-N was further west and considerably weaker in 1983, compared to The core of the TEJ was confined to the Indian Ocean in 1983, but it extended well across West Africa in Consequently mean 150 mb winds over West Africa exceeded 30 m s 1 in 1955, but were generally less than 15 m s 1 in In 1983 the AEJ-N was also broader and somewhat further equatorward than in The meridional winds (Figure 6) are most strongly developed over the ocean surface and quite weak over the continent. The moist southwest monsoon over West Africa is seen to originate in the region of upwelling along the Benguela coast. At the surface, the most apparent contrast between 1983 and 1955 is the stronger southerly flow in the Gulf of Guinea in This presumably enhances rainfall along the Guinea Coast. At 850 mb, the most apparent contrasts are the weaker and protracted region of southerly flow and northerlies over the Gulf of Guinea in This suggests a very shallow and local cell of meridional overturning in the Gulf of Guinea in 1983 that would limit the transfer of moisture towards the continent and would weaken the coastal circulation cell that feeds into the tropical rainbelt circulation. The low-level meridional winds also suggest somewhat stronger surface convergence in the ITCZ in Strong contrast between the 2 years is again apparent in the mid- and upper troposphere. At 700 mb strong cores of northerly and southerly flow over the central Sahel and western equatorial region, respectively, suggest strong convergence into the AEJ-N over the Sahel in 1955, but in 1983 the meridional winds in these areas are markedly weaker and suggest convergence much further to the south east. At 150 mb a shift from southerly to northerly flow is evident near the TEJ core in both years, but the strength of these meridional winds is markedly greater in Velocity potential The velocity potential (Figure 7) shows predominantly Hadley-type overturning in 1955 and Walker-type overturning in Both can result in rising motion over West Africa. In general, the velocity potential is weak near the surface. In 1983, Walker overturning circulations prescribe ascent predominantly in the hyper-arid eastern Sahara, but subsidence over much of West Africa. In 1955, the Hadley overturning would place the rising motion over all of West Africa. Also notable is that the velocity potential gradients are much weaker in 1983 than 1955, indicating much weaker overturning. This is consistent with the stronger vertical motion in 1955 (Section 4) and with the wider rainbelt in 1955 (Section 3). Figure 7. Velocity potential (10 6 m 2 s 1 ) for August 1983 and 1955 at the and sigma levels (roughly 1000 and 150 mb, respectively).

8 680 S. E. NICHOLSON Divergence and vorticity The divergence field (Figure 8) confirms the patterns suggested by the meridional winds. At the surface two convergence zones are evident in both years. One, at roughly N, is associated with the ITCZ. It extends all the way across Africa and the Arabian Peninsula. The second runs along the Atlantic coast, merging with the ITCZ convergence in the west and extending southward along the Benguela coast to about 30 S. It is associated with onshore winds and frictional effects. Strong divergence is evident in the major regions of coastal upwelling (Canary and Benguela currents, off Somalia, in the eastern equatorial Atlantic). At the surface and in the lower troposphere (850 mb), the only notable contrast in divergence fields between the 2 years is stronger convergence in 1955 in the ITCZ over West Africa. However, the ITCZ lies at the same latitude in both years. At 700 mb, the AEJ-N level, the contrast was also weak but probably significant. In 1983, divergence prevailed throughout most of the Sahel and it was quite strong in the east. In 1955 the convergence was weaker and displaced northward, so that strong convergence prevailed into the northern Sahel around 17 N. Strong contrast was also apparent at 150 mb, the TEJ level. The divergence field was quite weak in 1983, but strong in The strong divergence at 150 mb Figure 8. Mean divergence (10 6 ms 1 ) in August at 925, 700 and 150 mb and difference fields for the years 1955 and 1983.

9 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 681 Figure 9. Mean relative vorticity (10 6 ms 1 ) in August at 925, 850 and 700 mb for the years 1955 and 1983 and the difference between the 2 years. corresponded to convergence at the AEJ-N level in The effect was to couple the upper atmosphere with the mid- and lower troposphere and to complete a Hadleytype circulation cell that promotes deep and intense vertical motion, as seen in Figure 4. This is the situation that promotes intense easterly wave activity over the Sahel (Nicholson et al., 2008). Figure 9 shows the relative vorticity patterns in the 2 years. In the lower troposphere, the shape of the fields is nearly identical, but in 1955 the relative vorticity was markedly stronger over the Sahel, especially the eastern Sahel. At the AEJ-N level a different picture emerges. Across the expanse of the continent from roughly 5 to 15 N strong cyclonic vorticity prevailed along the equatorward side of the AEJ-N in In 1983, a similar expanse extended along the jet axis, but the vorticity was markedly weaker. Also, the zone of cyclonic vorticity was constricted compared to 1955.Consequently, anticyclonic vorticity, extending along the poleward side of the AEJ- N, prevailed over the Sahel in African easterly waves Grist (2002); Grist et al. (2002), and Nicholson et al. (2008) examined contrasts in wave activity in wet and dry years. Observations show that in the wet years the waves tend to have longer periods, especially in late August and in September, and larger amplitude. This is consistent with model projections of faster growth rates in the wet years. Both models and observations suggest that wave activity contributes to variations in the intensity of the tropical rainbelt over West Africa. Nicholson et al. (2008) used a linearized general circulation model (GCM) to simulate wave growth and development in 4 years, two of which contrast in the intensity of the rainbelt (1983 and 1955) and two in which wet and dry conditions in the Sahel are related to latitudinal displacements (1950 and 1984). The vertical wind shear was very strong in the wet years and very weak in the dry years. The experiments were initial value experiments in which a barotropic vortex was placed at 17 N in a basic state corresponding to August of each year. The vortex was allowed to grow and propagate; the growth rate of developing waves, maximum wind speed, and day-to-day development were evaluated. Figure 10 shows the simulated growth rate as a function of wavelength for 1955 and 1983 (Nicholson et al., 2008). The model predicted markedly stronger wave activity in 1955, when the rainbelt was anomalously

10 682 S. E. NICHOLSON Figure 10. Growth rates as a function of wavelength (in km) in August of the wet year 1955 and the dry year 1983 (from Nicholson et al., 2008). intense, than in 1983, when it was anomalously weak. The model also predicted shorter wavelengths of peak growth/higher frequency waves in Notably, the simulations also showed faster growth rates for 1955 than for 1950; both were wet in the Sahel, but the rainbelt was not anomalously intense in The study showed other strong contrasts between wave activities in the 2 years. In both years the disturbance propagated westward of the initial vortex, but markedly further in At 625 mb (the level of the AEJ-N), the waves covered a much broader latitudinal span in 1955 than in The waves were also markedly stronger in 1955, well developed throughout the troposphere, and best developed in the mid-troposphere. They had a baroclinic structure below the level of the AEJ-N and a barotropic structure above. In 1983, the waves were weak, baroclinic, mostly limited to the lower troposphere and best developed near the surface. This was consistent with the vertical profile of wind shear in the 2 years. The observed waves for these 2 years are depicted via Hoevmueller diagrams of meridional winds from NCEP (Figure 11). A 62-point FFT band-pass filter was applied (Grist, 2002; Nicholson et al., 2007). The filter retains periodicities on the time scale of 2 6 days, the basic time scale of the AEWs (Burpee, 1972; Adefolalu, 1974). The observed waves show some but not all of the contrasts apparent in the simulated waves. The observed waves were markedly stronger in 1955 than in The periodicity also appears to have been roughly every 4 days, compared to 5 days in 1983, and planetary scale waves are apparent in the upper troposphere. There was also a greater tendency for the waves to extend throughout the troposphere in Results: surface forcing, moisture convergence, and thermodynamics 5.1. Surface pressure and temperature Figure 12 shows the sea-level pressure in August of 1983 and The difference is striking. Over the Sahara, pressure was several millibars lower in 1955 than in Also the St Helena High in the South Atlantic was weaker in 1955 than in 1983, and was displaced westward relative to The net result of these two changes was positive pressure anomalies throughout most of the continent Figure 11. Hoevmueller diagrams of filtered meridional wind at 0 W for August of 1983 and 1955 (averaged for N).

11 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 683 Figure 12. Mean sea-level pressure (mb) in August for 1983 and 1955 and the difference field. Figure 13. Mean August SSTs ( C) for the years 1983 and 1955 and the difference field. in 1983 and negative anomalies in The largest anomalies are over the arid and semi-arid subtropics of both hemispheres in 1955, but in the equatorial latitudes and Southern Hemisphere in The result is much stronger on-shore winds in the eastern Atlantic in 1955, a factor that may contribute to the wetter conditions in equatorial and West Africa in that year. The SST anomalies (Figure 13) show the well-known SST dipole (e.g. Lamb, 1978a,b; Lamb and Peppler, 1992, Ward, 1998), with warmer/colder SSTs in the subtropics and colder/warmer SSTs in the equatorial region in 1955/1983. This is consistent with the contrasts in the cross-equatorial pressure gradient in the 2 years. Interestingly, there is relatively little temperature contrast in the Gulf of Guinea, an area in which SSTs are generally linked to rainfall variability in the Sahel (e.g. Opoku- Ankomah and Cordery, 1994) Moisture convergence Figure 14 shows the moisture convergence at 1000 and 700 mb for August 1983 and Difference fields

12 684 S. E. NICHOLSON Figure 14. Moisture convergence in August 1983 and 1955 at 1000 and 700 mb and the difference field. Units are 10 7 s 1. are indicated on the right. The most striking features at 1000 mb are the lack of a coherent difference over West Africa and relatively small differences there compared to the eastern Sahel. West of 10 E, the differences are generally close to zero. The pattern shows some zonal asymmetry, with weak moisture divergence in the west and weak convergence in the east, except for a small sector around 12 N. The differences at 700 mb are even smaller, although there is a slight tendency for greater/weaker moisture divergence over the Sahel/Guinea Coast in This is consistent with the somewhat stronger AEJ-N in Thermodynamic variables Figure 15 shows the vertical profiles of temperature and relative humidity in August. Data are given for 1983, 1950 and The year 1950 is a wet year in the Sahel, but of the dipole type (dry south of 10 N). The most striking feature of the profiles is that there is markedly more contrast between the 2 wet years than between 1955 and 1983, wet and dry years respectively. This is particularly true for relative humidity. At all three latitudes shown, relative humidity is significantly greater in 1950 than in the other 2 years. At 15 and 10 N, temperatures are slightly lower in the lower troposphere in 1955 than in This difference, if real, could be a consequence of the surface conditions in 1955, more surface moisture and more productive vegetation growth. Since relative humidity does not indicate absolute moisture content, an analysis of mean specific humidity is shown in Figure 16. Interestingly, specific humidity is higher in 1983 than in 1955 over the Sahel and over the Atlantic. The region of high humidity in the lower troposphere is considerably broader in 1983 than in The upper atmosphere over West Africa contains more moisture in 1955, possibly a consequence of the stronger vertical motion (Figure 4) and increased updraft strength associated with wetter conditions. 6. Discussion In terms of rainfall, the latitudinal profiles over West Africa are virtually identical in 1983 and However, at most latitudes there is nearly twice as much rainfall in 1955 as in This is apparent in the 3 wettest months of the boreal summer, July, August and September. Besides rainfall, the most striking contrasts between the years are evident in the upper atmosphere, rather than at the surface. The contrasts in surface pressure, SSTs, moisture convergence and temperature and moisture in the lower troposphere are relatively small and unlikely to explain the two-fold difference in rainfall. Likewise, near surface contrasts in vorticity, divergence, and vertical shear are insubstantial Contrasts between 1983 and 1955 in the month of August The most striking contrast is in the intensity of vertical motion within the rainbelt. During August the mean ascent in the core of the rainbelt reached ms 1 in 1955 but only 2 to ms 1 in The

13 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 685 Figure 15. Vertical profiles of air temperature ( C) and relative humidity (%). Figure 16. Vertical profile of specific humidity (g kg 1 ) as a function of latitude in August of the 1955 and 1983 and the difference field. latitudinal extent of the ascent was also markedly greater in 1955 than in The shallow surface regions of ascent linked to coastal circulation cell near 5 N andto the ITCZ over the Sahara were also more intense in All three regions of ascent were strongly coupled. In 1983, the ITCZ and the tropical rainbelt were decoupled, so that a clear zone of subsidence prevailed over the Sahelian latitudes. Other notable contrasts between the 2 years are the intensity of the TEJ and low-level westerlies, the vertical shear, and the vorticity, divergence and velocity potential at upper levels over West Africa. The TEJ was very

14 686 S. E. NICHOLSON strong over West Africa during August of 1955, with speeds generally between 15 and 30 m s 1 (Figure 5). In August of 1983, TEJ speeds were generally below 15 m s 1 and its core was well to the east of the continent. The equatorial westerlies were much more developed in 1955, extending to 700 mb. Strong cyclonic vorticity prevailed over the Sahel in 1955 but not in The velocity potential indicates weak, Walker-type overturning in 1983 but strong Hadley-type overturning in Kanamitsu and Krishnamurti (1978) noted a similar contrast between the normal year 1967 and the dry year The upper level divergence field shows anomalies that are consistent with this. In 1983, the divergence is weak at 150 mb, the TEJ level, and divergence also prevails throughout most of the Sahel at 700 mb, the AEJ-N level. In 1955 strong divergence at 150 mb overlies strong convergence at the AEJ-N level over the Sahel. The effect is to couple the upper atmosphere with the mid- and lower troposphere and to complete the Hadley-type circulation cell that promotes deep and intense vertical motion, as seen in Figure 4. This situation promotes intense easterly wave activity over the Sahel (Nicholson et al., 2008). Redelsperger et al. (2002) suggest that latent heat released by convection can enhance upper-level shear, and therefore enhance the TEJ. This evokes a question as to whether the stronger TEJ in 1955 is a factor in or a product of the more intense rainfall. Several arguments can be made to support the former position. For one, the stronger jet in 1955 is consistent with the large-scale temperature gradients at 150 and 200 mb in the 2 years (Figure 17). Temperature gradients are stronger in 1955 in the higher latitudes. The difference in patterns suggest an influence both from the mid-latitudes of the Southern Hemisphere and from the Indian monsoon. Even more relevant is that the upper-troposphere was warmer in 1983, when convection was weak, than in 1955, when convection was strong. Other relevant observations are that the intense TEJ had developed prior to the rainy season (Grist and Nicholson, 2001) and the TEJ was anomalously strong during the year Although 1950 was relatively wet in the Sahel, the cause was a northward displacement of the rainbelt. The intensity of convection was the same as in the dry year 1984, when the TEJ was exceedingly weak over West Africa (Nicholson, 2008). These facts conclusively demonstrate that the stronger TEJ in 1955 was not a product of the increased rainfall Factors enhancing vertical motion The enhanced vertical motion is presumably the key to the greater intensity of rainfall in Thus, a fundamental question is what enhances the vertical motion. Several of the conditions described above could play a role. The most important may be the strong Hadley-type overturning that begins with strong convergence at the AEJ-N level in the mid-troposphere, and extends to the upper-troposphere, where a strong TEJ is associated with strong divergent outflow. This situation, which would enhance the ascent within the tropical rain belt, did not exist in The stronger surface convergence into the surface ITCZ in 1955 and the coupling of the circulation cell over the Saharan heat low with that of the tropical rainbelt were probably additional factors. Both would contribute to the intensification of the ascent within the rainbelt. The coupling also serves to replace the subsidence over the Sahel with ascent. The presence of strong equatorial westerlies was probably another important factor. These served to displace the AEJ-N slightly northward in This displacement was presumably the reason for the coupling between the surface ITCZ and tropical rainbelt in Because the rainbelt is generally bounded by the axes of the AEJ-N and TEJ, the northward displacement of the AEJ-N served to broaden the region of ascent and the rainbelt associated with it Other factors contributing to intensification of the tropical rainbelt The enhanced vertical motion in 1955 was certainly one factor contributing to the more intense rainbelt. Other probable factors were dynamic instability and wave activity. AEWs organize convection. Although higher rainfall is not unequivocally linked to stronger and more frequent waves, a number of studies (e.g. Grist, 2002; Grist et al., 2002) have in fact produced evidence of this. The waves were stronger in 1955 than in 1983 and were more frequent. Clearly, the contrasts in the basic states of August 1983 and 1955 contributed to the stronger wave activity in the latter year. The presence of strong equatorial westerlies, the slight northward displacement of the AEJ-N, and the very strong TEJ in 1955 enhanced both vertical and horizontal shear (Nicholson et al., 2008). The combined baroclinic barotropic dynamic instabilities associated with the shear promote wave development. It is notable that 1955 was a La Niña year and 1983 was an El Niño year. This suggests the possibility that El Figure 17. Mean 150 mb temperature ( C) in August of 1983 and 1955.

15 FACTORS OF THE INTENSITY OF THE TROPICAL RAINBELT OVER WEST AFRICA 687 Figure 18. Speed of the westerlies (m s 1 ) at 850 mb versus (a) surface pressure gradient (10 3 mb km 1 ) (from Nicholson and Webster, 2008) and (b) Sahel rainfall. Data are for August of the years Wind speed and pressure data are averaged for 5 W 5 E. The open circles represent the years and the solid circles represent the years Niño-Southern oscillation (ENSO) and Pacific SSTs play a role in the no-dipole years. ENSO s influence in the Sahel is primarily in the high-frequency variability that is superimposed on the longer term trends (Ward, 1998), but it does tend to reduce rainfall there. Its impact is reasonably strong along the Guinea Coast, but during the boreal summer its impact is limited to a small increase in rainfall during La Niña years (Nicholson and Kim, 1997; Nicholson and Selato, 2000). On the other hand, typical El Niño wind anomalies during this season include a weakening of the TEJ (Figure 10, Arkin, 1982). This is consistent with the weakening of the TEJ during both the dry no-dipole years and dry dipole years. Thus, ENSO may play some role. However, there is no consistent association with dry or wet years Comparison with factors producing the rainfall dipole The latitudinal profiles of rainfall and the vertical motion fields confirm that the contrast between the wet conditions of 1955 and the dry conditions of 1983 in West Africa was associated with differences in the intensity of the tropical rainbelt, with little latitudinal displacement of this belt. Rainfall at nearly all latitudes was roughly twice as great in 1955 as in This was true for all 3 months of peak rainfall, July, August and September. This is distinctly different from the dipole mode evaluated by Nicholson (2008). In that case, wet or dry conditions in the Sahel were associated with a northward/southward displacement of the rainbelt but no change in its intensity. The displacement results in anomalies of the opposite sign north and south of 10 N (i.e. in the Sahel and Guinea Coast), hence the term dipole. The main factors linked to the development of the dipole are also the strength of the TEJ and the presence of strong equatorial westerlies, but the latter plays a much greater role. An elevated westerly jet core is evident in the wet dipole cases, with westerlies extending well into the mid-troposphere (Nicholson, 2008). Nicholson and Webster (2008) demonstrate that the likely mechanism is inertial instability that develops as a consequence of strong surface pressure and SST anomalies over the Atlantic. The strength of the westerlies (Figure 18) is strongly correlated with the surface pressure gradient (r = 0.84). It is also strongly correlated with Sahel rainfall (r = 0.75). The prerequisites for inertial instability were present in August of 1950 and other wet dipole years, but not in the dry dipole years. In 1955 the surface pressure gradient (Figure 12) was notably weaker (3.5 vs mb km 1 ). The core speeds of the low-level westerlies in the 2 years were 17 and 10 m s 1, respectively (Figure 18). In the two dry years (1983 and 1984), the surface pressure gradient was 2.9 and 2.1, respectively and the maximum westerly speeds were 4 and 3 m s 1. The SST anomaly patterns in the wet and dry no-dipole years (Figure 13) showed some semblance to those of the wet and dry dipole years (Nicholson and Webster, 2008). In both cases, the wet-minus-dry anomalies exhibited the Atlantic SST dipole, with a warm/cold subtropical Atlantic and a cold/warm equatorial Atlantic in the wet/dry years. However, the pattern was much better developed in the dipole years. The SST differences between wet and dry no-dipole years were generally C, compared to C between the wet and dry dipole years. Also, the anomalies were strongest in the equatorial Atlantic/Gulf of Guinea in the dipole cases, but in the subtropical and south Atlantic in the no-dipole cases. 7. Summary and conclusions Our results suggest that the primary factor in the enhancement of rainfall during 1955 was enhanced vertical motion within the tropical rainbelt. The enhanced vertical motion was associated with a combination of planetary and regional scale factors. The most important appear to be the enhanced Hadley-type overturning and the extension of the TEJ core over West Africa during The development of strong equatorial westerlies played a secondary role. Critical factors are the alignment of the axes of the AEJ-N and TEJ and the strength of the TEJ. The principal

16 688 S. E. NICHOLSON role of the TEJ is to determine the strength of upperlevel divergence. Overlying the convergence associated with the mid-level AEJ-N, this produces the Hadley-type overturning. Because the axis of the TEJ, at 6 8 N, is relatively stationary from year to year the latitude of the AEJ-N determines the axis separation and the latitudinal extent of the rainbelt. The role of the westerlies involves their influence on the latitude of the AEJ-N and both vertical and horizontal shear. The most favourable configuration for intensification of the rainbelt is a strong TEJ with its axis several degrees equatorward of the AEJ-N. This also enhances the vertical shear. The vertical shear, which was particularly large in August of 1955 (Nicholson et al., 2008), may have been a factor in the development of extremely strong easterly waves in The wave activity was also a factor in the enhanced rainbelt. The association between a stronger TEJ and enhanced rainfall over West Africa is not a result of upper-level heating via latent heat release. Confirmation of this is that the upper troposphere is actually warmer in the dry year. Several other observations also support this position. The stronger TEJ appears to be related to extra-tropical factors. Particularly notable is the enhanced temperature gradient in the higher latitudes of the Southern Hemisphere. An influence from the Indian monsoon is also possible. Numerous studies suggest that fluctuations in the TEJ over India are related to the influence of convection (Raman and Ramanathan, 1964; Krishnamurti et al., 1985; Chen and Yen, 1991; Webster and Yang, 1992). This would likely have an impact on its strength over Africa as well. Determination of the factors influencing the TEJ s strength over West Africa is a critical issue in understanding rainfall variability in the Sahel. An important conclusion from this study is that the years 1983 and 1955 differed mainly with respect to features of the upper atmosphere. Surface SST and pressure fields showed much less contrast than for the wet and dry dipole years. This suggests that dynamical aspects of the upper level circulation are the primary control on the intensity of the tropical rainbelt, but that conditions over the Atlantic also play some role. Moisture and moisture transport do not appear to be major factors. This supports conclusions of earlier studies on the role of moisture (e.g. Lamb, 1983; Long et al., 2000). In contrast, surface temperature and pressure fields play a major role in determining the development of the dipole patterns of rainfall anomalies via the relationship between surface pressure gradient and strength of the equatorial westerlies. Hence, conditions over the Atlantic are critical factors in the latitudinal displacements of the tropical rainbelt that produce the dipoles. The practical implications of this study involve aspects of modelling and forecasting. The strong link between rainfall variability over West Africa and the TEJ and AEJ-N axes, the TEJ strength, and the equatorial westerlies suggests that prediction of these variables may provide a means to seasonal forecasting of Sahel rainfall. Likewise, model validation should examine these features, as well as rainfall. Acknowledgements This work was supported by NSF grant ATM The author is much indebted to the programming and scientific assistance of Douglas Klotter of Florida State University. She would also like to acknowledge the assistance of Jeff Baum, who produced the Hoevmueller analysis. Thanks also to the two anonymous reviewers for their critical reading of the manuscript and very useful suggestions for revisions. 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