Identification of Processes Driving Low-Level Westerlies in West

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Identiﬁcation of Processes Driving Low-Level Westerlies in West Equatorial Africa

WILFRIED M. POKAM Department of Physics, Higher Teacher Training College, University of Yaounde 1, and Center for International Forestry Research, Central Africa Regional Ofﬁce, Yaounde, Cameroon

CAROLINE L. BAIN,ROBIN S. CHADWICK, AND RICHARD GRAHAM Met Ofﬁce Hadley Centre, Exeter, United Kingdom

DENIS JEAN SONWA Center for International Forestry Research, Central Africa Regional Ofﬁce, Yaounde, Cameroon

FRANCOIS MKANKAM KAMGA University of Mountain, Bangangte, Cameroon

(Manuscript received 8 August 2013, in ﬁnal form 16 January 2014)

ABSTRACT

This paper investigates and characterizes the control mechanisms of the low-level circulation over west equatorial Africa (WEA) using four reanalysis datasets. Emphasis is placed on the contribution of the divergent and rotational circulation to the total ﬂow. Additional focus is made on analyzing the zonal wind component, in order to gain insight into the processes that control the variability of the low-level westerlies (LLW) in the region. The results suggest that the control mechanisms differ north and south of 68N. In the north, the LLW are primarily a rotational ﬂow forming part of the cyclonic circulation driven primarily by the heat low of the West African monsoon system. This northern branch of the LLW is well developed from June to August and disappears in December–February. South of 68N, the seasonal variability of the LLW is controlled by the heating contrast between cooling associated with subsidence over the ocean and heating over land regions largely south of the equator, where ascent prevails. The heating contrasts lead to a Walker- type circulation with development of LLW as its lower branch. Thus, evidence is presented that the LLW are driven by differential heating. This contrasts with the traditional conceptual view that the Saint Helena high is the primary driver of low-level circulation off the Atlantic Ocean to WEA. Forest cover in WEA may modulate the latent heating that helps to drive the differential heating and maintain the LLW, and this interaction should be the focus of further study.

1. Introduction moisture advection throughout the year (McCollum et al. 2000; Matsuyama et al. 1994). This low-level moisture The economy of west equatorial Africa (WEA) (108S– advection is dominated by moist air from the Atlantic 108N, 98–308E) is dominated by natural resources and ag- Ocean. The strongest moisture inflow is registered during riculture and is highly dependent on climate. Over WEA, the second and main rainy season from September to the climate is strongly influenced by changes in low-level November, when the highest amount of rainfall is re- corded (Pokam et al. 2012). Pokam et al. (2012) found that at interannual time scales, for both year-to-year Denotes Open Access content. comparisons and wet minus dry composites, low-level moisture flux from the Atlantic Ocean controls the

Corresponding author address: Wilfried M. Pokam, Department of Physics, Higher Teacher Training College, University of Publisher’s Note: This article was revised on 7 August 2017 to Yaounde 1, P.O. Box 47, Yaounde, Cameroon. correct the name of the ﬁrst author, whose given name and E-mail: [email protected] surname was inadvertently reversed when originally published.

DOI: 10.1175/JCLI-D-13-00490.1

Ó 2014 American Meteorological Society Unauthenticated | Downloaded 10/08/21 02:34 AM UTC 4246 JOURNAL OF CLIMATE VOLUME 27 moisture content of the entire atmospheric column. southern site (Zhang et al. 2006). At the northern site, this Changes in associated low-level westerlies control the limit moves upward from 925hPa in April to 750hPa interannual variability of rainfall in the coastal region in August (Fontaine and Janicot 1992). In the vicinity (Nicholson and Dezfuli 2013). Because of the important of the equator, LLW are defined from August to January. role of the water cycle in climate variability and change The rest of the year, westerlies disappear and east- (Burde et al. 1996) and the heavy dependence of the erlies dominate throughout the entire atmospheric col- economy and livelihood of the region on water cycle umn (Zhang et al. 2006). (Molua and Lambi 2007), it is important to explore the There are well-documented examples of regions of low-level circulation driving this moisture advection from concentrated low-level westerly flow in West Africa the Atlantic Ocean to WEA. This is essential for ad- (Grodsky et al. 2003; PuandCook2010). However, vancing the physical understanding and modeling of there are few documented examples in WEA. This study climate in the region. It is also important for exploring focuses on the investigation of mechanisms governing the reasons behind the disagreement between climate LLW over WEA. Very little is known about the pro- model responses to expected future climate change cesses that control the height of these LLW and the (James et al. 2013), as Washington et al. (2013) have shown seasonal and interannual variability of their strength. that moisture flux is a useful quantity to understand model The purpose of this study is to investigate the processes rainfall biases over WEA. that control the depth, the intensity, and the seasonal The atmospheric circulation over WEA has been de- and interannual variability of LLW using reanalysis scribed in several previous studies. Broadly, the atmo- data. We will use the National Centers for Environ- spheric circulation is dominated by a large seasonal shift in mental Prediction–National Center for Atmospheric the position of the intertropical convergence zone (ITCZ), Research (NCEP–NCAR) reanalysis (NCEP-1; Kalnay which is determined by the northeast and southwest trade et al. 1996), the NCEP–U.S. Department of Energy winds, and the monsoon circulation from the Atlantic (DOE) Atmospheric Model Intercomparison Project (Fontan et al. 1992). The regional upper-level dynamics phase 2 (AMIP-II) reanalysis (NCEP-2; Kanamitsu are influenced by the high pressure cells over the Sahara et al. 2002), the European Centre for Medium-Range and the south of Africa that drive high-altitude easterlies Weather Forecasts (ECMWF) Interim Re-Analysis (Fontan et al. 1992). Nicholson and Grist (2003) found (ERA-Interim; Dee et al. 2011), and the Modern-Era that at midlevel (around 600–700 hPa), the annual cycle Retrospective Analysis for Research and Applications of easterlies are dominated by the north component of (MERRA; Rienecker et al. 2011). Over WEA, the den- the African easterly jet (AEJ-N) and the south compo- sity of the observation/meteorological station network is nent of the African easterly jet (AEJ-S). low and observations are sparse and sometimes unavail- In the lower troposphere, moist air from the Atlantic able (Aguilar et al. 2009). In the region, the reanalysis Ocean (Fontan et al. 1992), known as low-level equa- data rely strongly on the physical parameterizations in torial westerlies (Nicholson and Grist 2003), are asso- the global models used to create the reanalyses. There- ciated with the southeasterly trades on the northeastern fore, the reanalyses may differ because of different flank of the Saint Helena (South Atlantic) high. Because analysis systems and different model physics. Other dis- of Coriolis forces, the southeasterlies become westerlies crepancies may arise from the difference in spatial resolu- when crossing the equator. The low-level westerlies tion between the reanalyses or the number of observations (LLW) are defined throughout the year and are well used. Some aspects of the atmospheric dynamics may be developed from July to September (Nicholson and more visible in ERA-Interim and MERRA because of Grist 2003). Using soundings, the atmospheric circu- their finescale resolution (see section 2a)comparedtothe lation was described over the west coast of WEA, in- coarser resolution of NCEP-1 and NCEP-2. Use of more cluding one site in the Northern Hemisphere (Douala: than one reanalysis dataset reduces the susceptibility of 4.38N, 9.428E) (Fontaine and Janicot 1992), one near the results to errors in the underlying model used, and con- equator (Libreville: 0.238N, 9.278E), and one in the sistency between the reanalyses is an indicator (though Southern Hemisphere (Luanda: 8.488N, 13.148E) (Zhang not a guarantee) of robustness. et al. 2006). It appears that the annual cycle of LLW Theobjectiveofthisstudyistoidentifycommon varies from the southern to the Northern Hemisphere. At features between the reanalyses in the representation of the northern (southern) site, LLW are well developed the mean characteristics of the low-level circulation over from July to September (October–February) and are WEA, provide detailed information about the drivers of the strongest and the deepest in August (December– the low-level westerlies, and dissect the contributions of January). The upper limit of westerlies migrates from rotational and divergent flow. The related driving pro- around 950 hPa in July to 700 hPa in January in the cesses are investigated at both seasonal and interannual

Unauthenticated | Downloaded 10/08/21 02:34 AM UTC 1JUNE 2014 P O K A M E T A L . 4247 time scales. Focus is made on the drivers of the low-level b. Wind decomposition and heating calculations zonal flow from the Atlantic Ocean to WEA. This study In section 3 we explore the processes that control the also revises the conventional view that LLW are driven variability of LLW. Focus is made on the contribution of solely by the anticyclone in the southeastern Atlantic the divergent and the rotational (nondivergent) circulation to Ocean. The paper is organized as follows: After presenting the total flow. To achieve this, the horizontal wind fields were the data and methods used in section 2, section 3 presents partitioned into the rotational and divergent circulations. the structure of the low-level circulation over WEA as Through such an approach, predominance of a circulation seen from the four reanalyses. It shows the predominance type may be analyzed, allowing the identification of the of the divergent circulation on LLW. In the tropics, the driving processes across the domain. For example, the field divergent circulation is significantly sensitive to the dia- properties of the divergent circulation will be investigated batic heating field, particularly that associated with moist through the contribution of sensible heat flux, latent heating, processes (Annamalai et al. 1999). The main analysis of the and radiative cooling (e.g., as in Hagos and Zhang 2010). three-dimensional distribution of the diabatic heating is According to the Helmholtz theorem, the horizontal described in section 4 and is related to the structure of the velocity vector is partitioned into divergent and rotational divergent winds. Section 5 deals with the evaluation of components (Li et al. 2006), Vx and Vc, respectively, interannual variability of the LLW and the drivers. Section 6 presents the summary and conclusions. V 5 Vc 1 Vx , (1) 2. Data and methodology where a. Datasets Vc 5 k 3 $c and We used 15 yr of monthly-mean fields (January 1989– Vx 5 $x , (2) December 2003) from NCEP-1, NCEP-2, ERA-Interim, and MERRA. The period of study used for our in- where c is the streamfunction, x is the velocity potential, vestigation starts from January 1989 so that all the re- $ is the horizontal gradient vector, and k is the unit analyses products overlap with the ERA-Interim data. vector in the vertical direction. Substituting (2) into (1) The selected variables are the meridional, zonal, and and applying the divergence yield, vertical winds; air temperature; and specific humidity. In NCEP-1, these variables are on 17 pressure levels from D 5 =2x , (3) 1000 to 10 hPa on a 2.5832.58 spatial grid. The assimilation of data in the dynamical atmospheric model occurs where D is the horizontal velocity divergence. Given the through the use of three-dimensional variation data as- V(u, y) field, we can derive the divergence D and solve similation (3DVar). The 3DVar assimilates all observa- the Poisson equation (3) to determine the velocity po- tions that occur within a specific period at a single time. All tential x. We may then derive the divergent wind using variables are available every 6 h from 1948 to the present. (2) and the rotational wind as the difference between the NCEP-2 is an updated version of NCEP-1. NCEP-2 uses total low and the zonal flow. the same spatial and temporal resolution as NCEP-1. The diabatic heating Q1 (Yanai and Tomita 1998)is Many of the known errors in NCEP-1 are improved in calculated as a residual of the thermodynamic equation NCEP-2, which uses an improved forecast model and data and computed from assimilation system. The NCEP-2 products are available T ›u ›u ›u ›u from 1979 to the present. MERRA data span from 1979 Q 5 C 1 u 1 y 1 v , (4) to the present, and variables are available at 72 vertical 1 p u ›t ›x ›y ›p levels. MERRA has a horizontal resolution of 0.583 0.668. As with NCEP-1, a 3DVar approach is used to and the moisture sink Q2 is computed from the equation assimilate the observations into the dynamic atmospheric of moisture continuity, model. Much of the model output is archived at an ›q ›q ›q ›q hourly time scale. The ERA-Interim product is the Q 52L 1 u 1 y 1 v 2 › › › › , (5) most recent ECMWF reanalyses dataset and is available t x y p from 1989 onward. An updated version of the ECMWF T forecast is used at 1.58 horizontal resolution. The ERA- where is the air temperature (K) and Interim incorporates four-dimensional variational data R/C P p assimilation (4DVar), which is a temporal extension of u 5 T 0 the 3DVar. P

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21 FIG. 1. Seasonal means of the zonal wind (m s ) for (left)–(right) ERA-Interim, MERRA, NCEP-1, and NCEP-2 averaged between 108 and 158E for 1989–2003.

is the potential temperature; q is the mixing ratio of The apparent heat source Q1 consists of heating 2 water vapor (kg kg 1), u and y are the zonal and me- resulting from radiation, the release of latent heat 2 ridional wind components (m s 1), v is the vertical resulting from condensation, and the vertical con- 21 pressure velocity (Pa s ), P is pressure, P0 5 vergence of the vertical eddy transport of sensible heat. 1000 hPa, R and Cp are the gas constant and speciﬁc The term Q2 represents the apparent moisture sink heat at constant pressure of dry air, and L is the latent resulting from the net condensation and the vertical heat of condensation. Yanai and Tomita (1998) de- divergence of the vertical eddy transport of moisture.

ﬁned the term on the right-hand side of (4) and (5) The vertical proﬁles of Q1 and Q2 serve to show the as the apparent heating and apparent moisture sink, occurrence of eddy vertical transport processes and in respectively. The diagnosed monthly rate of heating turn indicate the strength of the activity of cumulus 21 q1 (K day ) due to apparent heating is then com- convection (Yanai et al. 1973). In the case of strong puted by vertical motion, leading to a strong contribution of

! eddy transport, the vertical profile of Q1 will differ Q from Q . The levels of peak Q and Q are separated q 5 1 3 86 400, (6) 2 1 2 1 (Wada 1969), and the vertical eddy transports of heat Cp and moisture are associated with cumulus convection 21 and turbulent motion. The greater the difference be- and the rate of equivalent heating q2 (K day ) due to the apparent moisture sink is computed by tween the height of these peaks, the stronger the convective activity (Wada 1969). The high convective ! activity over WEA (Jackson et al. 2009) may lead to Q 5 2 3 q2 86 400. (7) significant contributions of the eddy vertical flux terms Cp to Q1 and Q2.

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3. The seasonal mean cycle and MERRA. In NCEP-1 and NCEP-2, this local maximum is overlapped by the topography because of the a. Total circulation coarse resolution. Despite this, the main characteristics of The mean zonal winds averaged along the coastal LLW are well represented by all the reanalyses, though region, between 108 and 158E, from all the reanalyses are the finescale of ERA-Interim and MERRA captures shown in Fig. 1. As shown later, the core speed of the more dynamical details than NCEP-1 or NCEP-2. LLW in WEA is located over this area all year. During To summarize, we suggest that the LLW consist of two all seasons, the core speed of LLW is located around cells, located north and south of 68N, respectively. The 925 hPa. LLW are slightly shallower during the rainy cells differ in their annual cycles. The northern cell peaks seasons March–May (MAM) and September–November during JJA, whereas the equatorial cell peaks in SON. (SON). The upper boundary of LLW is around 850 hPa b. Rotational and divergent circulation during SON. It crosses above this level during December– February (DJF) and June–August (JJA). During MAM, The difference in the phases of the two cells of LLW the westerlies are weak with a mean core speed around suggests that they may be controlled by different mech- 2 1ms 1. The westerlies strengthen and extend northward anisms, which are analyzed now. 2 during JJA. The mean core speeds reach 4 m s 1 north of 2 1) NORTH OF 68N 88N and range from 2 to 3 m s 1 around the equator. During SON, the zonal winds weaken significantly north Figure 2 represents the mean seasonal circulation of 68N. They strengthen around the equator and are more pattern at 925 hPa. The zonal wind in contours enables us developed in the Southern Hemisphere in this period. to follow the seasonal variability of LLW. As shown be- The basic features of the LLW, as described above, can fore, the core peak in speed of LLW is around 925 hPa all be seen in the mean seasonal zonal circulation from year. During DJF, north of 68N, the zonal wind is domi- all the reanalyses. Some differences appear between the nated by northeasterlies. During MAM there is a north- reanalyses. During MAM, one notable feature in the ward shift of westerlies, from the Gulf of Guinea to structure of the LLW is the two distinct cells north and around 108N. Westerlies are more defined over West south of 68N in ERA-Interim and MERRA (first two Africa. This is well represented in all reanalyses. In the columns of Fig. 1). north of WEA (north of 58N, between 98 and 308E), The northern cell strengthens during JJA, with westerlies appear in NCEP-1 and NCEP-2. In ERA- a northward extension. Its northern boundary moves to Interim and MERRA, the westerlies are limited east of the north from 118N during MAM and lies between 168 158E. During JJA, westerlies strengthen over West Africa and 188N in JJA. This northern cell weakens/disappears with a strong latitudinal extension and move eastward to during SON and DJF. It is important to note that the the Ethiopian highlands (Fig. 2), reinforcing LLW in the seasonal cycle of this northern cell is similar to the cli- north of WEA. This feature indicates that the northern matological annual cycle of the West Africa monsoon cell of LLW in WEA is related to the development of the (WAM) system (Fontaine et al. 2002; Thorncroft et al. zonal circulation in the WAM. The dynamics associated 2011), whereas the seasonal cycle of the southern cell is with the WAM was fully described by Sultan and Janicot distinct and thus likely driven by other mechanisms. (2003). They state that, at the beginning of June, relative In contrast to the northern cell, the equatorial cell does vorticity centers develop along the intertropical front not show latitudinal migration across the seasons. Its (ITF), between 158 and 208N, which is concomitant with boundaries are located at 68N in the north and between the position of the Saharan heat low. By the end of June, 168 and 188S in the south all year. The differences in the the ITF moves northward, associated with the increase annual cycle of westerlies north and south of 68Nsuggest of the relative vorticity. This leads to the development of that the control mechanism in the north may differ from a cyclonic circulation in northern West Africa. The cy- that in the south. This duel-cell feature is not captured by clonic circulation contributes to the strength of the zonal either NCEP-1 or NCEP-2. In NCEP-1 and NCEP-2, circulation south of 158N. Around mid-July, the relative LLW appear as a single feature, with a northward mi- vorticity associated with the Saharan heat low is at its gration from 78N in DJF to a mean position of around strongest and is located in the western part of the 178NduringJJA(Fig. 1). The southern boundary also Northern Africa. During the same period, there is an migrates during the year. NCEP-1 and NCEP-2 fail to advection of absolute vorticity from the Gulf of Guinea to depict the two cells potentially because of their coarse the coastal region in West Africa where absolute vorticity resolution compared to ERA-Interim and MERRA. is equal to zero (Tomas and Webster 1997). This leads to During MAM, to the north of the Cameroon highlands, the development of zonal wind shear around the zero there is a local maximum of westerlies in ERA-Interim contour in absolute vorticity with strong low-level

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FIG. 2. Seasonal means of the circulation at 925 hPa for (left)–(right) ERA-Interim, MERRA, NCEP-1, and NCEP-2 for the period 1989–2003. Contours and shading indicate the zonal wind speed. The zero contour indicates the boundaries of westerlies (positive values on solid lines). westerlies over West Africa driven by the rotational flow. northern WEA weaken significantly in the retreat phase This enhancement of the heat low rotational circulation of the monsoon. Over West Africa, the westerlies are coincides with a peak of the monsoon zonal winds over localized along the coastal region during SON (Fig. 3). a large part of West Africa. During DJF, westerlies disappear north of 58N, where The mean seasonal flow in Fig. 2 has been split into its the circulation becomes dominated by easterlies driven rotational and divergent components using the method by the rotational winds. This feature is well captured by described in section 2, and they are shown in Figs. 3 and 4. all the reanalyses. However, the rotational wind is The figure clearly shows that the strong zonal circulation slightly stronger in the ERA-Interim and NCEP-1. with higher westerlies (Fig. 2) is part of the rotational 2) SOUTH OF 68N circulation of the monsoon system (Fig. 3). The domi- nance of westerlies in the rotational circulation compo- Figure 2 shows that over the Atlantic Ocean, the cir- nent appears during MAM (Fig. 3), in the early phase of culation is dominated by strong southeasterlies in all the WAM. After the monsoon onset at end of June, the seasons. The southeasterlies are driven by the rotational zonal circulation in the WAM system intensifies and is circulation, which weakens from the ocean to the land characterized by the northern extension of the westerlies (Fig. 3). The southeast–northwest orientation of this (Fig. 3), ultimately controlled by the location of the rotational flow suggests that it is related to the low-level Saharan heat low (Sultan and Janicot 2003). Benguela jet (Nicholson 2010). The low-level jet (LLJ) This monsoon enhancement is associated with the is located on the northeastern flank of the South Atlantic strengthening of westerlies in the northern WEA, pri- high. The LLJ is primarily driven by large-scale geo- marily driven by the rotational circulation. The northern strophic balance resulting from the surface pressure cell of LLW peaks during JJA (Fig. 3). Westerlies in the gradient over the southern Atlantic Ocean. From July to

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FIG.3.AsinFig. 2, but for the rotational circulation. White boxed areas indicate grid points under the topography.

October, the increases in thermal contrast between land Figs. 3 and 4 reveals that, although the rotational flow and ocean lead to the development of thermal winds, dominates the total circulation over the ocean, LLW at which superimposed upon the geostrophic flow strengthen 925 hPa, from the coast to the WEA, are driven by the the LLJ. The LLJ is well developed during this period divergent circulation in all seasons. The divergent zonal (Nicholson 2010) and in turn acts to strengthen the ro- wind speed is about the same magnitude in all the re- tational circulation (Fig. 3). analyses lending confidence to the result. However, South of 68N over WEA, the contribution of the ro- the area of maximum winds is more longitudinally ex- tational wind to the LLW is quite weak in the reanalyses. tendedinNCEP-1andNCEP-2thaninERA-Interim It is nonexistent in MERRA. Only NCEP-1 shows any and MERRA. substantial contribution, with core speed barely reach- 2 3) VERTICAL COMPOSITION OF THE CIRCULATION ing 1 m s 1 in the vicinity of the equator. However, the zonal component of the divergent circulation consists The vertical cross section of the zonal circulation is primarily of westerlies (Fig. 4). Broadly, the divergent now investigated to identify the dominant flow (di- circulation strengthens from the southeast Atlantic to vergent or rotational) through the depth of westerlies. the continent. During MAM, the westerly component of Figure 5 represents the longitude–height cross section of the divergent winds weakens, with core speeds around the mean seasonal zonal component of the winds. 2 1ms 1 located along the coast over the ocean sector. The total, divergent, and rotational circulations are This feature strengthens during JJA and the area of averaged between 08 and 108S. The profiles during maximum zonal wind extends over the continent, east of MAM and SON represent how these seasons corre- 2 208E(Fig. 4). The core speed reaches 4 m s 1. In SON, spond to weak and deep westerlies, respectively, in the the patterns are more like those in JJA than in the other Southern Hemisphere over WEA. The differences be- seasons but with a slight decrease of the zonal compo- tween MAM and SON in strength, depth, and eastward nent over the continent. The comparison of Fig. 2 with excursion of the total flow clearly appears in all the

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FIG.4.AsinFig. 3, but for the divergent circulation. reanalyses. In MAM, LLW are defined around 108E and To summarize, over WEA two cells of LLW are de- 2 up to 925 hPa, with core speeds less than 1 m s 1. fined north and south of 68N. The northern cell is well During SON, LLW are well developed over 850 hPa developed during JJA and is related to the development 2 with core speeds greater than 2 m s 1. Figure 5 clearly shows of the zonal circulation driven by the heat low of the that the development of the LLW during SON is pri- WAM system. This cell weakens during the other marily due to the reinforcing of the divergent circulation. seasons and disappears in DJF. The southern equato- This is consistent with the predominance of the easterly rial cell is driven by the divergent circulation through flow in the rotational circulation throughout the tropo- a Walker-type circulation with ascent (descent) over spheric column. In all reanalyses, the divergent compo- the continent (ocean). The divergent flow is well de- nent of the zonal wind is far greater than the rotational veloped during JJA and is maintained in SON. The component (Fig. 5, second row of each reanalysis) in the equatorial cell weakens in MAM. area of LLW. The divergent circulation drives a Walker- type circulation, characterized by the ascending air over 4. Drivers of divergent circulation the land and downward motion over the ocean. The related strengthening of low-level descent over the ocean In the previous section, we found that, over WEA and the divergence below 850 hPa leads to stronger south of 68N, LLW are driven by the divergent circula- westerlies around 925 hPa. In MAM, the Walker cell is tion. The divergent circulation is modulated by the dis- weak. This leads to weak and shallow LLW. The divergent tribution of the diabatic heating (Johnson et al. 1985). In circulation in SON is associated with a Walker-type circula- regions of cooling, which are an energy sink, the vertical tion. The related strengthening of the low-level branch of this energy flux is downward. In regions of heating, which are circulation leads to strong and deep LLW (particularly visible an energy source, the vertical energy flux is upward. in MERRA). All the reanalysis agree well on the control of Thus, in upper (lower) layers in heat source regions, the the divergent circulation on the depth of the LLW. horizontal energy flux is divergent (convergent). However,

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FIG. 5. MAM and SON of the (top) mean zonal total, (middle) divergent, and (bottom) rotational circulation in longitude–pressure section, averaged between 08 and 108S for (a) NCEP-1, (b) NCEP-2, (c) ERA-Interim, and (d) MERRA. The vertical velocity is sig- niﬁcantly smaller than the zonal wind; thus, for clarity in the plot the vertical velocity has been scaled up by a factor of 600. Shaded areas 2 indicate westerly winds, greater than 1 m s 1. White boxed areas east of 108E below 850 hPa indicates grid points under the topography. 2 Units for westerly winds are meters per second and for vertical velocity are 101 Pa min 1.

in heat sink regions, the horizontal transport of energy circulation and the differential heating. Herein the struc- is convergent (divergent) in the upper (lower) layers ture of the diabatic heating in relation to the seasonal (Johnson et al. 1985). This leads to a low-level circula- variation of the divergent circulation is investigated. tion from cooling to heating regions. These basic prin- Hereafter, the diabatic heating and the moisture sink ciples underline the direct link between the divergent refer to q1 and q2 as deﬁned in section 2.

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21 FIG. 6. Diabatic heating (contours; K day ) and the divergent zonal circulation (vectors; 2 ms 1) in (a) NCEP-1, (b) NCEP-2, (c) ERA-Interim, and (d) MERRA in MAM and SON in longitude–pressure section, averaged between 08 and 108S. The vertical velocity is signiﬁcantly smaller than the divergent zonal wind; thus, for clarity in the plot vertical velocity has been scaled up by a factor of 600. White boxed areas east of 108E below 850 hPa indicate grid points below the topography.

Unauthenticated | Downloaded 10/08/21 02:34 AM UTC 1JUNE 2014 P O K A M E T A L . 4255 a. Diabatic heating and divergent flow 700–925 hPa is chosen as the cooling centers and peaks within this layer. One striking feature of the pattern in Figure 6 represents the longitude–pressure section of MAM is the location of a major source of heating over the diabatic heating averaged from the equator to 108S. the north of Africa, spanning from the WAM region During MAM, the major heat source is found over the eastward to the Ethiopian highlands. Dry air and dry soil 2 continent, between 108 and 308E. Heating of 1–2 K day 1 prevail over the Saharan region in this period. The occurs from 700 hPa upward. A low-level maximum in heating shown in Fig. 7 reflects the predominance of the heating appears to the east of WEA, between 308 and sensible heat flux from the surface (Fontaine et al. 2002). 2 408E, with heating rates exceeding 2 K day 1. Below the Dry convection associated with the cyclonic region of midlevel heating over WEA, a weak cooling lower than the heat low occurs, with the peak of upward motion at 2 1Kday 1 is discernable. Cooling generally prevails over 850 hPa (Sultan and Janicot 2003). This feature in the ocean upward of 925 hPa. A well-defined cooling combination with the subsidence over the ocean and the 2 maximum with rate exceeding 1 K day 1 is located associated lower branch of the overturning (Fig. 6) leads around 850 hPa. The cooling over the ocean is about to dominant meridional flow in the divergent circulation the same magnitude in all the reanalyses. However, the (Fig. 7). LLW weaken during this season (Figs. 4, 5). In heating over the continent is doubled in ERA-Interim SON, the heating decreases significantly over West Af- and MERRA compared to NCEP-1 and NCEP-2. In rica and is shifted south to the Gulf of Guinea. The SON, the features are like in MAM but much more major source of heating occurs south of 58N over the pronounced. The contrast between large positive values continent with a maximum along the coast in WEA and of heating rate over the continent and the negative values over East Africa. Over the ocean, the cooling strengthens over the ocean is reinforced (Fig. 6). The values of q1 are and extends northward to the Gulf of Guinea. The di- at least twice those of MAM, except in MERRA over the vergent circulation at 925 hPa is dominated by south- land. The strong low-level heating east of 308Eexpands westerlies, which reinforces the zonal circulation with upward to the deep upper-level heating over WEA, with stronger acceleration over the coast along 128E(Figs. the peak within the layer of 500–600 hPa (Fig. 6). A 5, 7). Thus, the development of the LLW over WEA 2 heating maximum greater than 3 K day 1 occurs over the during SON is controlled by the heating contrast between coast, east of 108E, from the surface up to 850 hPa. This the cooling over the ocean and the heating over the increases the low-level contrast of heating between the WEA, which strengthens a Walker-type circulation with ocean and the continent. This is consistent with the re- ascent over the continent and descent over the ocean inforcement of the low-level peak of cooling over the driven by the divergent wind. The related strengthening 2 ocean, with rates exceeding 2 K day 1 (Fig. 6). of low-level descent at 850 hPa and the lower branch Also represented in Fig. 6 is the vertical section of the of the overturning, seen previously in Fig. 6,leadsto divergent circulation (as in the middle panels of Fig. 6). a stronger LLW at 925 hPa with the peak over the coastal The vertical structure of the heating is associated with region. a Walker-type circulation with rising air over the warm There are two dry seasons, DJF and JJA. During DJF, continent and descent over the cool ocean. The stronger the vertical heating profile is the same as in SON but is divergent circulation during SON is consistent with the slightly weaker (not shown). However, during JJA, when greater heating (cooling) contrast over the continent driest conditions prevail over WEA, the cooling over the (ocean). The maximum in heating over the coast may Atlantic Ocean strengthens significantly, whereas heating contribute to the local acceleration of the westerlies. over WEA weakens. Based on the previously mentioned link between hori- From the above analysis of Figs. 6 and 7, four key zontal divergent circulation and vertical distribution of regions are identified as the main contributors to the the diabatic heating, we argue that the low-level peak of development of the LLW. The regions consist of the cooling over the ocean and the heating in the coastal cool region over the southeast Atlantic Ocean (Fig. 8: area make key contributions to the strength of LLW. region A), the low-level heating over the coast (region This increase in the strength of the LLW can feedback B), the upper-level heating over WEA (region C), and on the strength of the convection and diabatic heating the low-level heating over East Africa (region D). Re- over WEA. gion C covers the center of the Congo Basin, corre- To illustrate the relation between low-level contrast in sponding to the humid dense forest area of southeast the heating and the strength of LLW, Fig. 7 presents the Cameroon, southern Central African Republic, east of patterns of seasonal mean rate of diabatic heating be- the Congo Republic, and a major part of Democratic tween 700 and 925 hPa and divergent circulation at Republic of Congo. Region B covers southern Came- 925 hPa. To represent the low-level cooling, the layer of roon to the northwest of Angola and covers Gabon,

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FIG. 7. Distributions of MAM and SON minus MAM means of the vertical integrated dia- 2 batic heating (shading; K day 1) between 700 and 925 hPa and the divergent zonal circulation 2 (vectors; m s 1) at 925 hPa in (a) NCEP-1, (b) NCEP-2, (c) ERA-Interim, and (d) MERRA. Wind vectors below the topography are masked.

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formation by condensation would actually lead to a positive heating, so this would not explain the increased cooling at this level. Instead, the enhanced radiative cooling from the top of boundary layer clouds (Wood 2012; Slingo et al. 1982) leads to the peak of cooling around 850 hPa (Fig. 9a). The heating below 925 hPa is a result of vertical convergence of sensible heat ﬂux from

the surface. During SON, the negative values of q1 are reinforced. However, there is no signiﬁcant change in the

values of q2. This feature suggests that the longwave cooling at the top of the stratocumulus modulates the seasonal variability of the low-level cooling over the ocean. FIG. 8. Regions where the vertical proﬁles of the diabatic heating Along the coast in WEA (region B), our main interest and the moisture sink are examined. is the heat source below 850 hPa (Fig. 6). Figure 9b

shows the mean vertical profiles of q1 and q2 over re- Equatorial Guinea, and the southeast of the Congo gion B. During MAM, q1 is positive at low levels with Republic. The regions act as follows: regions A and B a peak at 925 hPa; q2 values are also positive. This in- modulate the lower branch of the overturning and the dicates the predominance of sensible heat flux and the acceleration of LLW over the coast, while heating in release of latent heat by condensation in the lower region D causes wind convergence over region C, troposphere. During SON, the decrease of q2 values enhancing convective heating, which in turn controls the below 850 hPa probably reflects high evaporative ascending branch of the Walker-type circulation. cooling from the wet soil resulting from an increase in rainfall over the coast (Jackson et al. 2009). However, b. Principal factors impacting heating 21 q1 exhibits an increase of heating of around 2.5 K day To identify the principal factors contributing to the at the surface. This indicates that the increase in sen- cooling or heating over the four regions identified in Fig. sible heating is larger than the heat loss from increased

8, the vertical profiles of q1 and q2, defined in section 2, evaporation. are analyzed. In Fig. 9 the profiles for each region be- The vertical profile inland over WEA (region C of tween MAM and SON are shown. Although in Fig. 9 the Fig. 9c) during MAM is characterized by positive values mean vertical profiles of q1 and q2 from the surface to of q1 and q2 throughout the troposphere, except below 300 hPa are shown, a specific layer is of interest for each 850 hPa, where q2 is negative. Above 850 hPa, q1 and q2 location. In regions A, B, and D the lower-tropospheric values are positive, with the vertical separation of the heating or cooling appears to be more important, whereas peaks of q1 (500–600 hPa) and q2 (700 hPa) characterizing in the region C it is in the upper troposphere. It is im- the presence of cumulus-convective vertical transport portant to note that over region A (region B) only sea (Yanai et al. 1973). During SON, large positive values of

(land) grid points are taken into account during the q1 and q2 occur at upper levels, indicating the enhance- computation of the vertical proﬁles. ment of the condensation process that leads to an in-

The vertical distribution of q1 over the southeast crease in the release of latent heat. This is consistent with Atlantic Ocean (region A; Fig. 9a) during MAM is a strengthening in convective activity, which is most de- characterized by cooling throughout the troposphere, veloped during this period (Jackson et al. 2009). All the except the layer below 925 hPa. The q2 profile indicates reanalyses agree well on the predominance of the release that moisture is being evaporated at the sea surface and of latent heat in the upper levels and the associated then mixed through the boundary layer by turbulent deeper moist convection in SON. processes. The peak of the apparent moisture source The profile in Fig. 9d is located over East Africa. at 925 hPa indicates the peak of where the turbulent During MAM, q1 is strongly positive in the lower- transports deposits moisture from the surface. This pro- tropospheric layer and decreases with height, becoming cess contributes to the prevalence of low-level stratocu- negative above 700 hPa. The q2 values are also positive mulus cloud cover (Rozendaal et al. 1995), with deep at low levels and become negative in the layer of 700– stratocumulus-topped boundary layer (Wood 2012). The 850 hPa. During SON, large positive values of q1 and q2 turbulent moisture transport does not lead to a change of occur below 700 hPa. These profiles are indicative of phase; therefore, q1 changes, except at the top of the lower-level sensible heat and latent heat release by con- boundary layer, where clouds form. However, this cloud densation that strengthens in SON.

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21 21 FIG. 9. Seasonal mean vertical proﬁles of the diabatic heating (K day ) and the moisture sink (K day ) averaged over regions in Fig. 8.

5. Interannual variability the southern part of WEA. This result is consistent among the reanalyses. As shown in the previous sec- In addition to the long-term seasonal mean circu- tion, south of 68N, the seasonal mean variability of lation, the year-to-year variability of the contribution the LLW is driven by the divergent circulation and the of divergent and rotational circulation to LLW is in- rotational wind is dominated by easterlies. Hence, the vestigated. The patterns of standard deviation of the predominant impact of the rotational wind on LLW monthly-mean total, divergent, and rotational zonal interannual variability is a dampening effect on the circulation for DJF, MAM, JJA, and SON (not shown) strength of the equatorial cell of LLW. This is well show that there is weak interannual variability of the illustrated for MAM and SON in Fig. 10.Forallre- divergent circulation. The year-to-year variability of analyses, although the divergent circulation dominates LLW appears to be driven by the interannual variability the strength of LLW, its variability is weak compared of the rotational circulation over both the northern and to the variability of rotational ﬂow. As a consequence,

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FIG. 10. The 1989–2003 time series of annual mean total, divergent and rotational wind at 925 hPa averaged between 58N and 108S and from 98 to 158E over WEA. the trend of LLW shows an increase (decrease) during This is well illustrated in ERA-Interim and MERRA. years of weak (strong) rotational flow. This rotational flow may originate from the Mascarene During MAM and SON, the rotational wind over the high over the southwestern Indian Ocean (Findlater south of WEA originates from East Africa (Fig. 3). This 1969). This suggests that there may be a teleconnection flow originates from the southwestern Indian Ocean and between LLW and Indian Ocean at interannual time turns into northeasterlies south of the Ethiopian high. scales.

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6. Summary and conclusions (sink) is located over the continent (ocean). Note that at this time low-level stratocumulus cloud is the preva- The seasonal means and the interannual variability of lent cloud type over the southeast Atlantic Ocean. The the low-level circulation over west equatorial Africa have cooling associated with the heat sink over the ocean been described in some detail using NCEP-1, NCEP-2, peaks at 850 hPa, because of the enhanced radiative ERA-Interim, and MERRA. For the characterization of cooling at the top of boundary layer clouds. Over WEA, the processes that control the low-level circulation, diabatic heating prevails, associated with the release of a focus is made on the contribution of divergent and latent heat by condensation. Low-level heating over nondivergent circulation to the total flow. East Africa, driven by the release of latent heat by The increased resolutions of ERA-Interim and MERRA condensation and surface sensible heat, contributes to provide a clear depiction of two distinct cells of LLW the atmospheric heating over WEA. This land–sea north and south of 68N. The northern cell is related to the contrast in heating drives a Walker-type circulation with development of the zonal circulation driven by the heat ascent over WEA and descent over the ocean. The low within the WAM system. It is well developed during strengthening of the descent at 850 hPa, as well as the JJA. South of 68N, over the Atlantic Ocean, the circula- consequent horizontal low-level flow below, leads to tion is dominated by strong southeasterly rotational wind well-developed LLW at 925 hPa. During SON, the in all seasons. It is related to the Benguela jet, which is cooling over the Atlantic is reinforced. A low-level peak primary driven by the surface pressure gradient associ- of heating appears over the coastal region in WEA as ated with the South Atlantic high. From July to October, a consequence of the enhanced release of both latent the increased thermal contrast between ocean and land heat by condensation and surface sensible heat. The superimposes thermal winds upon this system. The low-level heating over East Africa also strengthens. This rotational circulation is the strongest during this period. contributes to the strong heating throughout the depth Further analysis shows that, during the small (March– of the troposphere in WEA. This reflects the strong in- May) and the big (September–November) rainy seasons, fluence of latent heat release by condensation within the the divergent circulation drives a Walker-type circula- deep moist convection. The strengthening of the heating tion, with ascent over the continent due to the diabatic contrast between the land and the ocean reinforces the heating. During the big rainy season, the zonal heating Walker cell and in turn the LLW. contrast between the diabatic cooling over the ocean and The LLW have been shown to have a large impact on the diabatic heating over the continent intensifies. The moisture inflow and precipitation variability over WEA maritime subsidence is reinforced, and the related (Nicholson and Dezfuli 2013). The LLW enhance the strengthening of low-level descent around 850 hPa leads zonal circulation and therefore promote ascent through to deep and strong LLW that peak at 925 hPa. There is the orographic lifting effect of the highlands, thus lead- a good agreement between reanalysis on this increase of ing to the enhancement of rainfall in the highland region heating contrast and the related strengthening of LLW. (Vondou et al. 2010). In addition to this effect, the These differences in the drivers of LLW north and south strengthening of LLW in MAM intensifies the conver- of 68N are consistent with the idea that the meteorolog- gence associated with the ITCZ and promotes its north- ical processes controlling each region differ. The region ward excursion. This acts to link ascent associated with the north of 68N is controlled by the same processes that in- ITCZ and the rain belt, which in turn contributes to the fluence Sahelian West Africa (Nicholson and Grist 2003). intensification of rainfall (Nicholson and Dezfuli 2013). Our results on the LLW over WEA contradict the In WEA, 45.5% of the area is covered by the forest conventional view that the low-level circulation from (de Wasseige et al. 2009, 2012), which maintains higher the Atlantic Ocean is associated exclusively with the evaporation rates than other cover types, including open South Atlantic high (Fontan et al. 1992; Nicholson and water (Sheil and Murdiyarso 2009). This suggests that Grist 2003). This study has demonstrated that the low- forest cover may significantly contribute to the condensed level winds over the southeast Atlantic Ocean, driven by water over region C and in turn to LLW. the high, remain southeasterly all year. The wind turns It is important to note that, in MAM, the main source into southwesterlies near the coast, driven by the heat- of heating over the continent is located over the WAM ing contrast between land and ocean. The seasonal region and the Ethiopian highlands. During this season, variability of the strength and height of LLW is con- southerlies dominate in the divergent flow over the trolled by the seasonal evolution of this heating contrast. ocean and LLW are weak. In SON, the major source of Our hypothesis on the drivers of the LLW south of heating moves southward over WEA and East Africa. 68N can be described as follows: During MAM, when the Over the ocean the zonal (meridional) component of the LLW are the weakest, the major source of heat source divergent flow strengthens (weakens). These changes to

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