The Itcz and the Seasonal Cycle Over Equatorial Africa

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THE ITCZ AND THE SEASONAL CYCLE OVER EQUATORIAL AFRICA

Sharon E. Nicholson

This article challenges the assumption that the seasonal cycle of rainfall in equatorial Africa is controlled by the seasonal excursion of the ITCZ and calls for additional research on the seasonal cycle.

he intertropical convergence zone (ITCZ) has 2000; Nesbitt and Zipser 2003) and globally propa- long been assumed to be a major control on gating convection-triggering waves (e.g., Mekonnen T tropical rainfall over both oceans and land. Over et al. 2008; Janiga and Thorncroft 2016) and the role Africa, the cycle of the rainfall seasons is generally of inertial instability in generating convection (e.g., associated with its north–south displacement as this Tomas and Webster 1997). In view of this evolution zone “follows” the sun. The assumed link to the tropi- of our understanding of tropical rainfall, it seems cal rainy seasons stems back to a time when tropical meaningful to also reexamine the ITCZ concept. rainfall was thought to be produced primarily by Here, this question is raised specifically concerning local thunderstorms, with the ascent in the ITCZ equatorial Africa, where rainy seasons occur twice promoting their development when thermodynamic annually—in the boreal spring and autumn. conditions were favorable. This assumption of purely This article commences with an overview of localized convection has long been overturned, various published definitions of the ITCZ. This is with numerous studies showing the importance of followed by a historical look at the development of the mesoscale convective systems (e.g., Nesbitt et al. image of the ITCZ over Africa, as well as controver- sies surrounding it. The development of the current scenario for West Africa is then described. The avail-

AFFILIATION: Nicholson—Department of Earth, Ocean, and ability of reanalysis datasets permits the evaluation Atmospheric Science, The Florida State University, Tallahassee, of this scenario. Here, the issue is examined for the Florida two equatorial rainy seasons, focusing on April and CORRESPONDING AUTHOR: Sharon E. Nicholson, November. [email protected] The abstract for this article can be found in this issue, following the DEFINITIONS OF THE ITCZ. The ambiguity table of contents. of the ITCZ concept is made clear with a brief sur- DOI:10.1175/BAMS-D-16-0287.1 vey of definitions in the literature. The usages differ In final form 15 August 2017 with respect to the variable chosen to define this ©2018 American Meteorological Society For information regarding reuse of this content and general copyright zone. In the Encyclopedia of World Climatology, Yan information, consult the AMS Copyright Policy. (2005) states that the ITCZ is “an east-west oriented low-pressure region near the equator where surface

AMERICAN METEOROLOGICAL SOCIETY FEBRUARY 2018 | 337 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC northeasterly and southeasterly trade winds meets”; the term ITCZ in discussing rainfall over Africa. For that is, the emphasis is on pressure. According to example, in describing the rainfall maximum, Ross Miller (1996) in the Encyclopedia of Weather and and Krishnamurti (2007) prefer the term equatorial Climate, it is “a region near the equator where the rain belt. Zhang et al. (2006) use the term rain band trade winds converge.” The American Meteorological and Nicholson (2009) substitutes the term tropi- Society’s (AMS) Glossary of Meteorology (Glickman cal rain belt. The latter term is used in this article. 2000) provides a similar definition: “The axis, or Clearly, over most of the tropical landmasses, and a portion thereof, of the broad trade-wind current over Africa in particular, the use of the term ITCZ of the tropics” and the “dividing line between the should be avoided except in coastal areas where the southeast trades and the northeast trades.” A second trade winds are influential. definition there equates it with the meteorological equator. Holton et al. (1971) define the ITCZ as the HISTORICAL DEVELOPMENT OF THE “loci of cloud clusters associated with westward- ITCZ CONCEPT. A search of the early meteoro- propagating tropical wave disturbances,” a definition logical literature uncovers just as much confusion and shared by Lockwood (1974). ambiguity about the ITCZ and its historical origin. Perhaps as a result of this ambiguity in definition, Nieuwolt (1977), in his book on tropical meteorol- the tracking of the ITCZ has variously been based on ogy, suggests that the concept goes back to Hadley’s the pressure minimum, surface wind convergence (1735) model. Hadley, however, did not mention the (Grodsky et al. 2003), the rainfall maximum (Sultan convergence, but it is implicit in his model of the and Janicot 2000; Philander et al. 1996), the vorticity vertical cells. The concept had definitely come into maximum (Magnusdottir and Wang 2008), the mini- vogue by the 1920s and 1930s, when meteorologists mum in outgoing longwave radiation (Gu and Zhang attempted to apply midlatitude frontal concepts of the 2002), or the cloudiness maximum (Waliser and Bergen school to the tropics (Barry and Chorley 1992; Gautier 1993). The availability of satellite photos has Palmer 1951). The first mention of the convergence led to the last two parameters being frequently uti- of the trade winds between the two hemispheres may lized as a matter of convenience (Waliser and Gautier have been in a paper by Brooks and Braby (1921) 1993). The use of so many different parameters has entitled “The Clash of the Trades in the Pacific.” This been justified by the long-held assumptions that 1) feature, identified by streamline confluence and not the pressure minimum and rainfall maximum are horizontal wind convergence, became known as the collocated with each other and with the wind conver- intertropical front (ITF). When the importance of gence, 2) maximum cloudiness is roughly collocated wind convergence in tropical weather was realized with maximum rainfall, and 3) longwave radiation is in the 1940s and 1950s (Barry and Chorley 1992), at a minimum at that location. Unfortunately, these the trade wind convergence was designated the in- assumptions, especially the first one, do not stand tertropical convergence zone, initially abbreviated up to close scrutiny. Even over the oceans the zone of as ITC (Fletcher 1945) (Fig. 1). minimum pressure does not generally coincide with Most of the work that led to the development of that of the wind convergence or the rainfall maxi- the ITCZ concept was based on conditions over the mum (Tomas and Webster 1997; Toma and Webster Pacific. Later, Bjerknes et al. (1933) described the ITF 2010). Over West Africa even the maximum in surface over all three tropical oceans and also attempted to convergence is some 350 km south of the surface wind trace its course over land in Africa and Southeast discontinuity between easterly and westerly winds Asia. Petterssen (1941, 1958) further promoted the (Hastenrath 1988). global picture of an ITCZ but defined it as the meet- Notably, most definitions emphasize the conver- ing of the trade winds. He used it to explain the basic gence of the trade winds. This may be appropriate rainfall pattern that exists over much of the tropics: over some oceans sectors. However, for the most bimodal rainfall seasonality in the equatorial lati- part, the trade winds do not exist over the tropical tudes with the twice-equatorial transit of the ITCZ, landmasses. Thus, over Africa depictions of the unimodal in the outer tropics when the ITCZ reaches surface ITCZ instead represent the meeting of the its extreme latitudinal positions. Palmén and Newton northeasterlies and southwesterly monsoon flow. (1969) subsequently published a global picture of However, the term is all too frequently applied to surface streamlines with the land portions essentially the rainfall maximum. Recognizing that the surface representing a subjective compromise between the convergence zone and the rainfall maximum are not locations shown by the analyses of Mintz and Dean closely coupled, many authors now avoid the use of (1952) and Riehl (1954). Variants of their maps are

338 | FEBRUARY 2018 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC generally relatively clear skies and little precipitation. Ramage (1971) was particularly harsh in his criti- cism of the ITCZ concept. He states that the original concept of the intertropical front or ITF confused tropical meteorologists, who noted that “the worst weather” and the ITF do not coincide. He concluded that the confusion was compounded when the term ITCZ came into use and that “confusion became chaos” when the tropical meteorologists started to use the terms ITF and ITCZ “indis- Fig. 1. Idealized schematic of the ITCZ, as historically described. criminately and interchangeably.” Notably, Ramage refused to use repeated in nearly every climatological textbook, either term in his book. as well as in a multitude of articles and textbooks Hastenrath (1988) openly called for an abandon- from a broad range of disciplines. The portion over ment of the “outmoded notions” of the global coin- Africa was generally adopted as the January and July cidence of the various parameters. In their widely locations of the ITCZ (Fig. 2). Eventually, the picture used climatology textbook, Barry and Chorley (1992) emerged of a global zone in which the pressure mini- distinguished between an ITCZ over the ocean and mum, cloud and rainfall maxima, and low-level wind an ITF over land. They mentioned that the forma- convergence coincided. tion is discontinuous in time and space and not well These ideas were not without controversy. developed in the doldrums. Notably, the term ITCZ Trewartha (1943) pointed out the ambiguity of does not appear in two other widely used textbooks, the ITCZ and the rather capricious approach to Wallace and Hobbs’s (2006) Atmospheric Science delineating it. He states that there is “considerable and Krishnamurti’s (1979) Compendium of Tropical vagueness in exactly defining this front, some writers Meteorology. Over Africa, the latter text refers instead making it synonymous with the doldrums, others to a “wind separation line.” Moreover, detailed studies locating it at the equatorial margins of the trades.” of the global ITCZ, such as that of Schneider et al. Riehl (1979), in his classical book on tropical meteo- (2014), consider only ocean regions. rology, focused attention on the pressure field (i.e., the In summary, the origins of the ITCZ paradigm equatorial trough). He argued against the use of the for Africa are murky, but it clearly emerged from at- term ITCZ, stating that convergence is intermittent tempts to parallel midlatitude concepts. However, its and that the term relates back to “old times when it was thought that the meeting of northern and southern air masses occurred right at the equator.” One fact that Riehl cited as contrary to this picture was the independent seasonal displacement of the rainfall maximum, com- pared to the displacement of the pressure minimum and surface convergence. Palmén and Newton (1969) pointed out that, in what

they termed the trade con- Fig. 2. The ITCZ (dotted line) over Africa in Jul–Aug and Jan (from Nicholson fluence zone, there were 2011). The dashed line is the Congo air boundary.

AMERICAN METEOROLOGICAL SOCIETY FEBRUARY 2018 | 339 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC concept specifically to East Africa, attributing the seasonal cycle of rainfall there to the movement of the ITCZ toward and away from the equator. The association between the ITCZ and rainfall seasonality, as presented by Petterssen (1941), was also extrapolated to Africa in the form of a diagram similar to that in Fig. 4 (Miller 1971; Flohn 1964). This paradigm is used extensively not only in the meteorological literature Fig. 3. Weather zones over West Africa (from Nicholson 2009). but also appears in sources related to disciplines as disparate as geology, development was haphazard. Most of those who argued ecology, paleoclimate, and history. for and implemented the concept were midlatitude Eventually, the view for the extreme seasons meteorologists. Tropical meteorologists, and espe- published by Bjerknes et al. (1933) was expanded to cially those who actually worked in Africa, harshly the often-reproduced diagram in Fig. 5, showing the criticized the ITCZ paradigm and its applications, ITCZ in four seasons. The original source appears with many suggesting that it was completely wrong. to be Thompson’s (1965) atlas, The Climate of Africa, Unfortunately, the use of this paradigm has persisted. but this depiction further appeared in the French doctoral dissertation of Dhonneur (1974), which is THE ITCZ AND THE CYCLE OF SEASONS also often cited as the source. Over West Africa, where OVER AFRICA. Hubert (1926) may have been the the concept is best developed, various authors have first to identify an entity equivalent to the ITCZ over added further details of the link to the precipitation Africa. He used the term ITF, defining it as the meeting regime. Note that the image of the ITCZ over Africa of two air masses, the dry northeast (NE) harmattan in Fig. 5 differs markedly from the image based on the and the moist southwest (SW) monsoon. At some point the diagram in Fig. 3 depicting weather zones with respect to the surface ITCZ over West Africa was published, but the origin of the initial model has been attrib- uted to various sources (Trewartha 1961)—Solot (1943), Walker (1957, 1958), and Hamilton et al. (1945)—all of whom served as meteorologists in Africa, as well as A Pilot’s Primer of West African Weather (Knight and Smith 1944). Over West Africa, where the aforementioned meteorologists worked, the diagram more or less cor- rectly depicts the spatial relationship between the ITCZ and the rainfall zones. However, as described later, the ITCZ and the rainfall zones are decoupled. Griffiths (1972) appears to have been the first to associate a latitudinal displacement of these zones with the seasonal cycle over West Africa. Henderson et al. (1949) may have Fig. 4. Rainfall as a function of latitude and month over eastern Africa been the first to apply the ITCZ along a transect at 32°E (modified from Flohn 1964).

340 | FEBRUARY 2018 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC Palmén and Newton (1969) streamline maps (Fig. 2). (GATE) in 1974, the launch of the Tropical Rainfall Meanwhile, many other significantly different images Measuring Mission (TRMM) satellite in 1997, and of the African ITCZ have appeared in the literature. the African Monsoon Multidisciplinary Analysis As early as the 1950s, the application of the concept (AMMA) research project and field campaign (Janicot to Africa was highly criticized. Crowe (1951) showed et al. 2008; Redelsperger et al. 2006). The emphasis how difficult it is to utilize the ITCZ to explain rainfall is now on the West African monsoon, the phases of seasonality at coastal stations of Africa. Tschirhart which are described by Thorncroft et al. (2011). Zhang (1959), a meteorologist working in equatorial Africa, et al. (2006) and Nicholson (2009) independently criticized the ambiguity concerning the nature of described the monsoon in the boreal summer and the ITF and further claimed that the ITCZ and ITF presented markedly similar structures. The latter is should be considered distinct entities, with the term shown in Fig. 6. The point of both representations is ITCZ limited to the ocean. Obasi (1976), a Nigerian that the wind discontinuity, usually termed the ITCZ, working in Kenya, stated that the ITCZ plays no role represents a secondary cell of vertical motion that is in forecasting in East Africa. Leroux (2001) provides a completely independent of the main region of vertical scathing review of the concept over Africa. He points ascent (although they merge in some years). The main out the lack of an accepted definition, the subjective ascent and the rainfall maximum lie between the axes analyses used to formulate charts of the ITCZ, the of the midtropospheric African easterly jet and the highly divergent images of it that have been published, upper-tropospheric tropical easterly jet. Although both the quite varied parameters used to define it, and the the surface wind discontinuity and the zone of maxi- plethora of labels attached to it. In discussing the sur- mum rainfall shift latitudinally with the seasons, as face zone where the NE harmattan encounters the SW well as from year to year, they fluctuate independently, monsoon, he patently states that “to call it an ITCZ is with the latter exhibiting a much greater longitudinal completely wrong,” and laments that terms “such as displacement (Grist and Nicholson 2001). the ITF and ITCZ…are inappropriate but hallowed Unfortunately, no such comprehensive paradigm has by use.” been published for equatorial Africa. However, several Our picture of the meteorology over West Africa things are noteworthy about the situation in equatorial has fortunately changed dramatically, as a result of Africa. First, even the classical picture of Africa’s circu- such milestones as the Global Atmospheric Research lation (Fig. 5) shows no ITCZ over western equatorial Program (GARP) Atlantic Tropical Experiment Africa. Second, precipitation there is associated primarily with mesoscale convective complexes and they are so intense and frequent that Zipser et al. (2006) designate

Fig. 5. Mean monthly location of the ITCZ over Africa Fig. 6. Schematic illustration of the revised picture of (from Dhonneur 1974). the West African monsoon (Nicholson 2009).

AMERICAN METEOROLOGICAL SOCIETY FEBRUARY 2018 | 341 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC vector winds, divergence, and vertical motion. The dataset commences in 1979 and runs to the present, providing a spatial resolution of roughly 80 km (0.75° latitude–longitude) and a time step of 6 h (Dee et al. 2011). Each of the variables was also examined using the 40-yr version of the ECMWF Re-Analysis (ERA- 40; Uppala et al. 2005) and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset (Kalnay et al. 1996; Kistler et al. 2001). As the results were completely consistent among the three datasets, only the results based on ERA-Interim are presented. Most analyses presented here examine the Fig. 7. Rainfall stations and transects used in the analysis. four months of January, April, August, and October, with January and August representing the annual extremes and April and October being used to rep- this region as having the most intense thunderstorms resent the equatorial rainy seasons. A more difficult on Earth. Moreover, the dominant factor appears to be decision is what level to utilize to represent surface orographic forcing over the highlands surrounding the conditions. In Northern Hemisphere sectors the ter- Congo basin (Jackson et al. 2009). Finally, in eastern rain is relatively low and 1,000 hPa is appropriate, equatorial Africa a previously neglected factor is the but in much of the Southern Hemisphere the surface low-level Turkana jet. This appears to be a major factor elevation exceeds 1,000 m. As a compromise, the in the region’s aridity (Nicholson 2016) and appears to 925-hPa level is utilized. However, similar results are have a major influence on rainfall seasonality as well. achieved if 1,000 hPa is considered. Nevertheless, the bulk of the literature on the region’s climate and interannual variability refers to seasonality as linked to a twice-annual passage of the ITCZ. The remainder of this article challenges that concept.

ANALYSIS OF THE ITCZ PARADIGM FOR EQUATORIAL AFRICA. The core of the ITCZ paradigm over Africa is an annual north–south migration, with a twice-annual equatorial passage corresponding to the two equatorial rainy seasons. Accordingly, there is a corresponding north–south migration of the rain belt associated with low-level convergence. This section examines circulation and rainfall, in order to see whether or not this paradigm can explain the seasonal cycle in equatorial latitudes. Here, for the sake of argument, the commonly accepted definition of the ITCZ over Africa is adopted (i.e., the location where the dry northeasterly harmattan meets the moist southerly flow of the monsoon). Some of the analyses presented will consider the entire equatorial sector, but the focus is on the central and western equatorial regions. The meteorology of these regions is more poorly understood than that of the eastern equatorial regions, about which considerable research has been carried out.

Data. Here, the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis Fig. 8. Mean rainfall (mm month−1) during Apr and Nov (ERA-Interim) dataset is utilized to examine over the period 1979–2014.

342 | FEBRUARY 2018 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC Rainfall is evaluated from an independent gauge dataset compiled by the author. The original dataset is described in Nicholson (1986) and Nicholson et al. (2000). Numerous stations have been added and the re- cords updated (Nicholson et al. 2017). The dataset includes a total of 2,091 stations within the analysis sector (Fig. 7). Rainfall data have been converted to a ½° grid, using a natural-neighbor technique (Watson 1999). Mean rainfall results for April and November, based on the Fig. 9. Mean vector winds and divergence at 925 hPa in Jan, Apr, Aug, and years 1979–2014, are shown Nov. Only every third vector in the analysis is plotted. in Fig. 8.

Low-level winds and divergence. As indicated, the com- residing at roughly 20°S (Fig. 2), the wind shift and mon explanation for the seasonal cycle in equatorial zone of convergence clearly lie at roughly 6°–10°N. Africa is the north–south migration of the ITCZ. Notably, these locations are strongly in accord with Accordingly, it moves between the two hemispheres the ITCZ locations indicated in Fig. 5. and transits the equator twice a year, producing Thus, in all cases the ITCZ remains well north of the bimodal seasonal cycle in equatorial regions. the equator. In each of the four months a contigu- An examination of low-level winds and divergence ous zone of convergence, located close to the ITCZ, indicates that this is not the case. extends across the continent. This zone does migrate Figure 9 shows the mean vector winds and with the seasons, in accordance with the shifts in the divergence at 925 hPa during four months of the year. wind regime seen in Fig. 9. However, its migration During August the southwesterly “monsoon” prevails bears little relationship to that of the rain belt. This over West Africa up to roughly 20°N. The ITCZ (i.e., is clearly seen from a comparison of Figs. 8 and 9. the interaction of the monsoon with the northeast In both April and November, the ITCZ and associ- harmattan) is clearly evident in the east, but much ated convergence lie well to the north of the equato- less so west of ~10°E, where the low-level circulation rial locations where this feature is assumed to bring around the Saharan heat low (not shown) interrupts rainfall. Moreover, the latitudinal span of the rain the pattern. At 925 hPa a continuous zone of conver- belt is roughly 3–4 times greater than the latitudinal gence is roughly coincident with the ITCZ. In April, span of the low-level convergence. the core of the first equatorial rainy season, the flow More interesting is the pattern of divergence in over northern Africa shows some similarity to that of the equatorial locations during April and November. August, but with the pattern displaced equatorward Instead of low-level convergence, divergence prevails by roughly 6°–10° of latitude. The ITCZ is situated at over much of the region, especially across the Congo roughly 10°–12°N and is marked by strong conver- basin (Fig. 9) (see also Jackson et al. 2009). Over gence. In November, the core of the second equatorial eastern Africa, the divergence patterns in both rainy season, the southwesterly monsoon is extremely months bear a resemblance to that of January, the weak, but southerly flow gives way to the northeasterly heart of the dry season. Yang et al. (2015) similarly harmattan at roughly 8°–10°N. However, the main noted prevailing low-level divergence over parts of area of convergence is somewhat farther north, well East Africa during the rainy seasons. Much of the within the northerly flow, and convergence is weaker divergence is associated with the low-level Turkana than in April. In January, when the ITCZ is generally jet (Nicholson 2016), seen in Fig. 9 at roughly 0°–5°N represented as having traversed equatorial Africa and and 35°–40°E (Nicholson 2016).

AMERICAN METEOROLOGICAL SOCIETY FEBRUARY 2018 | 343 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC In summary, during the equatorial rainy seasons rain belt is relatively constant throughout the sea- the low-level convergence associated with the ITCZ son. In the second rainy season, a latitudinal shift is lies well to the north of the regions experiencing evident between October and November, but the rain rainfall at these times. Further, the low-level winds belt shows little change in latitude between November are divergent, on average, over much of the region and December. The situation is very different in cen- during these seasons. Thus, the observed patterns tral equatorial Africa, over the Congo basin (25°E). of wind further contradict the ITCZ paradigm as an During the first rainy season only a small northward explanation for the seasonal cycle in Africa’s equato- shift is apparent, but the latitudinal extent of the rial latitudes. rain belt steadily decreases between March and May. During the second rainy season at both longitudes a Rainfall and vertical motion. A further tenet of the ITCZ latitudinal shift is evident in each month. Only here paradigm is a progressive north–south shift of the and in this season is there a strong semblance of the rain belt during the course of the year, following the classic scenario of the ITCZ. path of the overhead sun. Figure 10 shows the lati- The ITCZ paradigm further relates rainfall to tudinal profile of rainfall during the two equatorial ascent produced by the low-level convergence. The rainy seasons along the two transects shown in Fig. 7. patterns of vertical velocity during the two equato- Profiles are shown for three months of each season: rial rainy seasons, as represented by April and No- March–May for the first season and October–December vember, belie this scenario. Figure 11 depicts vertical for the second season. These show some degree of velocity via omega (vertical velocity in pressure north–south displacement, but only one of the four coordinates) along the two transects at 16° and 25°E, cases truly fits the classic ITCZ paradigm. as well as with latitudinal profiles of topography and Over western equatorial Africa (16°E) there is little rainfall. Although shallow areas of ascent around latitudinal displacement of the rain belt between 10°N are associated with the surface positions of March and April. Only between April and May is the ITCZ, the ascent extends only to the midtro- there a northward shift apparent. The width of the posphere and prevails over less than 10° of latitude. Shallow areas of ascent are also evident in association with topographic peaks. These appear at ~21°N and ~13°S along the transect at 16°E. At 25°E, they appear around 14°N in both months and also at ~12°S in November only. Within the latitudinal span of the rain belt (very roughly 5°N–10°S in April and 5°N–15°S in November) the situation is quite different. Low- level subsidence prevails in both months and along both transects. These areas of low-level subsidence largely correspond to areas of low-level divergence or at most very weak convergence (Fig. 9). Aloft, above 850 hPa, strong ascent that reaches into the

Fig. 10. Rainfall vs latitude at 16° and 25°E along the two transects shown upper troposphere spans in Fig. 7 during the first (Mar–May) and second (Oct–Dec) equatorial rainy the latitudes of the rain belt. seasons. At the bottom of each panel is the surface elevation (m). It should be noted that the

344 | FEBRUARY 2018 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC correspondence between vertical motion and rainfall rain belt during the course of the first rainy season is not a model-generated result, since the rainfall and a progressive southward movement of the rain dataset is independent of the reanalysis dataset. The belt within the course of the second rainy season. deep column of ascent appears to be decoupled from This pattern is apparent only at 25°E and only in the shallow areas of ascent associated with the ITCZ the second rainy season. Figure 10 suggests that the around 10°N. The decoupling is particularly clear in changes during the first rainy season might be better November. This further indicates that the ITCZ should described as a progressive contraction of the rain belt, not be considered to be the bearer of seasonal rainfall as the northern edge shows relatively little change over Africa’s equatorial region. from month to month. If the ITCZ paradigm is incorrect, the obvious SUMMARY AND CONCLUSIONS. The ITCZ question is what does produce the seasonal cycle over Africa has long been a point of contention. over equatorial Africa, a region of extraordinarily Much of the picture of the ITCZ over equatorial intense thunderstorms and convective activity. That Africa has been derived from vague ideas concern- question is beyond the scope of this article, but sev- ing the circulation over Africa and much of the eral potential factors have been identified in other original work was conjecture. Tropical meteorolo- studies. An important one is the storms that develop gists have long suggested that it is not an appropriate in the highlands surrounding the Congo basin; paradigm for the seasonal cycle over equatorial Af- rica, yet the use of this paradigm persists. The structure of the motion field during the rainy seasons of the boreal spring and autumn was examined over central and western equatorial Africa. In both locations, the structure showed little similarity to the classic ITCZ paradigm, which entails surface convergence leading directly to ascent and hence rainfall. Low-level subsidence un- derlies much of the region of maximum rainfall. It results from the divergence of the mountain breezes over the surrounding highlands (Jackson et al. 2009). Ascent associated with the tropical rain belt over Africa commences higher up in the atmosphere. Clearly, the latitudinal progression of the equatorial rainy season does not follow that of a surface convergence zone. The ITCZ paradigm further suggests a progres- Fig. 11. Mean vertical motion in Apr and Nov at 16° and 25°E, along the tran- sive northward shift of the sects shown in Fig. 7. Data are from ERA-Interim.

AMERICAN METEOROLOGICAL SOCIETY FEBRUARY 2018 | 345 Unauthenticated | Downloaded 09/27/21 06:52 PM UTC these move into the basin at night when katabatic Berhane, F., and B. Zaitchik, 2014: Modulation of daily flow prevails (Jackson et al. 2009). The East African precipitation over East Africa by the Madden–Julian highlands, on the eastern rim of the Congo basin, oscillation. J. Climate, 27, 6016–6034, https://doi are particularly important in the initiation of me- .org/10.1175/JCLI-D-13-00693.1. soscale convective systems that propagate westward Bjerknes, J., H. Solberg, and T. Bergeron, 1933: Physi- and traverse the equatorial latitudes (Hartman 2017, kalische Hydrodynamic, mit Anwendung auf die manuscript submitted to Mon. Wea. Rev.). The dynamische Meteorologie. Springer, 797 pp. intense development of these storms over the Congo Brooks, C. E. P., and H. W. Braby, 1921: The clash of the basin (i.e., central and western equatorial Africa) is trades in the Pacific. Quart. J. Roy. 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