ATMOSPHERIC SCIENCE LETTERSAtmos. Sci. Let. 11: 169–174 (2010)Published online 17 March 2010 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/asl.262

Modulation of the African easterly jet by a mesoscaleconvective systemZhuo Wang1* and Russell L. Elsberry2†

1Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, USA2Department of Meteorology, Naval Postgraduate School, Monterey, CA, USA

*Correspondence to:Zhuo Wang, Department ofAtmospheric Sciences, Universityof Illinois, Urbana, IL61801, USA.E-mail: zhuowang@illinois.edu

†The contribution of Russell L.Elsberry to this article wasprepared as part of his officialduties as a United States FederalGovernment employee.

Received: 30 November 2009Revised: 17 January 2010Accepted: 28 January 2010

AbstractThe modulation of the African easterly jet (AEJ) by a mesoscale convective system (MCS)is examined in a numerical simulation. An AEJ with a strong and confined core is simulatedbefore the formation of the MCS north of the AEJ axis, and the jet is ‘split’ with twoseparate cores after the passage of the MCS. Our diagnosis suggests that the MCS maybe triggered by a wave propagating south of the AEJ axis. A momentum budget analysisindicates that the meridional circulation associated with the MCS weakens the jet to itssouth and forms the secondary jet to its north. Copyright 2010 Royal MeteorologicalSociety

Keywords: African easterly jet; mesoscale convective system; mesoscale numericalstimulation

1. Introduction

African easterly waves (AEWs) are the dominantsynoptic-scale weather systems over West Africa dur-ing boreal summer. The precipitation systems associ-ated with AEWs are critical to local agriculture (e.g.Omotosho, 1985), and the wave activities are alsoclosely related to tropical cyclone formation over theAtlantic (e.g. Landsea, 1993). AEWs have the typicalwavelength of 2500–4000 km and propagate west-ward at 6–10 m s−1 along the African easterly jet(AEJ). The relationship of the AEWs with the AEJ hasbeen extensively studied via observational, theoretical,and numerical modeling approaches. Burpee (1972)noted that the mid-tropospheric reversal of meridionalgradient of potential vorticity in association with theAEJ satisfies the Charney and Stern’s (1962) necessarycondition for instability. The definitive observationalstudy by Reed et al. (1977) described the structure andproperties of AEWs through the analysis of the GlobalAtmospheric Research Program (GARP) AtlanticTropical Experiment (GATE) data. Hall et al. (2006)provided a comprehensive review of the idealizedstudies of AEW development in relation to the AEJ.In their idealized simulation, Hall et al. consideredthe zonally varying nature of the AEJ and includedthe surface westerlies and the mean meridional cir-culation, which modifies the dynamical modes com-pared with the previous studies that assumed a zonallyuniform basic state. Hsieh and Cook (2005, 2007)emphasized the importance of convectively inducedbarotropic instability for the generation of AEWs.

Evidence for a second wave track on the north-ern flank of the AEJ over West Africa dates backto Carlson (1969), who found a low-level, relativelycloud-free vortex at about 20 ◦N. Spectral analyses byBurpee (1974) of surface meridional winds with peri-ods of 3.6–5.7 days indicated a maximum amplitudearound 20 ◦N. Similarly, Reed et al. (1977) found asurface circulation center near 20 ◦N in the GATE dataset. Various studies based on ECMWF analyses orreanalysis have indicated a circulation north of the AEJ(Nitta et al., 1985; Reed et al., 1988; Pytharoulis andThorncroft, 1999; Fink et al., 2004). Different fromthe southern waves that have maximum amplitude atthe jet level (600–700 hPa), the northern waves areusually confined below 850 hPa (Reed et al., 1988).

Based on the observations during the JET2000field Campaign (Thorncroft et al., 2003), Parker et al.(2005) present a schematic of the thermodynamicstructure of the AEJ. The AEJ is at the equatorwardend of a sloping baroclinic zone that tilts southwardwith height below the jet level. The baroclinic zoneseparates the low-level warm, moist air in the southfrom the mid-level hot, dry air from the Sahara Desert.Equatorward of the AEJ, thunderstorms and mesoscaleconvective systems (MCSs) are common due to theabundant moisture in the low-level equatorial west-erlies that are not capped by an inversion. In undis-turbed conditions, smaller convective towers exist dur-ing the day beneath the sloping baroclinic zone. Indisturbed conditions, squall-type systems will developwith strong downdrafts that are produced owing to thevertical wind shear and dry, mid-tropospheric air. Fink

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170 Z. Wang and R. L. Elsberry

et al. (2006) documented rapidly moving, organizedconvective systems in a highly sheared environmentwith a deep and dry mid-troposphere in the WestAfrican Sudanian zone, which is consistent with theconceptual model in Parker et al. (2005).

The JET2000 observations were the best avail-able observations to illustrate the structure of theAEJ before the African Monsoon MultidisciplinaryAnalysis (AMMA) in 2006. Thorncroft et al. (2003)describe the JET2000 experiment and provide prelim-inary results. In particular, they analyzed a strong,confined jet core on 28 August 2000 and a split ofthe jet on 29 August 2000 after the passage of aconvective system. The objective of this study is tounderstand how the convective system was related tothe AEWs and how it interacted with the AEJ througha numerical model simulation. The mesoscale modelis briefly described in Section 2. The model simulationis presented and analyzed in Section 3, followed by asummary and discussion in Section 4.

2. WRF model configuration

The WRF model (Skamarock et al., 2005) used inthis study is a fully compressible, nonhydrostaticmesoscale model. The WRF model was implementedwith 41 vertical levels and a triply nested domainwith 81, 27, and 9 km horizontal grid spacings.The three domain dimensions are: 40 ◦W to 30 ◦E,7 ◦S to 29 ◦N; 25 ◦W to 22 ◦E, 2 ◦N to 25 ◦N; and9 ◦W to 11 ◦E, 6 ◦N to 18 ◦N. On the two outerdomains, the Betts–Miller–Janjic cumulus parame-terization scheme is used. On the innermost domain,convection is explicitly calculated on the grid scale.Other physics options include the Noah land surfacescheme, Yonsei University planetary boundary layerscheme, rapid radiative transfer model (RRTM) long-wave radiation, and the Dudhia shortwave radiationschemes.

The model was initialized with ECMWF 6-hourlyanalysis data (Uppala et al., 2005) with T106 (about1.125◦ × 1.125◦) horizontal resolution, and was inte-grated for 2 days from 0000 UTC 28 August to 0000UTC 30 August 2000. The initial conditions at 0000UTC, which is local midnight at the longitude of theJET2000 observations, is chosen in part not to shockthe model planetary boundary layer as would occurduring the day.

3. Simulation of the AEJ and its modulationby an MCS

The 3-hourly accumulated precipitation superposed onthe 850 hPa streamlines is shown in Figure 1. At 0900UTC 28 August (Figure 1(a)), which is around thetime of the first JET2000 flight on 28 August, a well-defined easterly wave circulation is present south of

the AEJ. The wave has a northeast–southwest hori-zontal tilt, which suggests barotropic energy conver-sion from the basic flow to the wave disturbance. Tothe east of the wave axis, two precipitation regionsare approaching the prime meridian. As no precipi-tation was observed during the JET2000 flight on 28August, the WRF model may be initiating precipita-tion too early in the diurnal cycle. To the northeast ofthe southern wave, another wave (or a cyclonic vor-tex) is centered around (18 ◦N, 3 ◦E) to the north ofthe AEJ axis. This northern wave is much smaller inspatial size and weaker in amplitude than the southernwave. It is confined below 800 hPa, in contrast withthe southern wave that has the maximum amplitudearound 700 hPa. This is consistent with previous stud-ies (e.g. Reed et al., 1988; Pytharoulis and Thorncroft,1999)

Nine hours later (Figure 1(b)), precipitation southof the jet is enhanced and extends to the west of thewave trough axis. The wave north of the jet axis hasweakened significantly. Propagating at a faster speed,the northern wave has moved to the same longitude as

Figure 1. 850 hPa streamlines and accumulated 3-hourlyprecipitation (shading; units: mm/day) in the WRF simulation at(a) 0900 UTC 28 August, (b) 1800 UTC 28 August, and (c) 0300UTC 29 August. The dashed lines represent the wave troughaxis, where meridional flow is zero. The black thick contoursare the axis of the 600 hPa easterly jet, which is defined as thelocal maximum of easterlies above 10 m s−1.

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Modulation of the African easterly jet 171

the southern wave, instead of lagging to the east. Weakprecipitation is scattered around 6 ◦E to the north of thejet axis. The overall precipitation pattern is similar tothat observed in TRMM 3B42 3-hourly accumulatedprecipitation (not shown).

At 0300 UTC 29 August (Figure 1(c)), the precipi-tation region south of the jet has moved ahead (west)of the wave trough axis, which suggests that the con-vective system embedded within the wave propagatesfaster than the wave. The scattered precipitation to thenorth of the jet is enhanced and has organized intoa nearly zonally oriented band along 15 ◦N between1 ◦E and 8 ◦E. East of the prime meridian, the origi-nal jet is displaced southward, and a secondary jet hasformed to its north, which is consistent with the splitjet observed in JET2000 (Thorncroft et al., 2003).

To better illustrate the jet structural change, verti-cal cross-sections of the zonal and meridional windfields along 2 ◦E are shown at 0900 UTC 28 August(Figure 2(a)) and 0300 UTC 29 August (Figure 2(b)).At 0900 UTC 28 August, the jet has a well-definedcore with a maximum of 19.4 m s−1 at 650 hPa along13–14 ◦N. This simulated jet is about 2 m s−1 weakerthan that observed in JET2000 field experiment, butabout 2 m s−1 stronger than the jet in ECMWF analy-sis. Southerlies prevail below 400 hPa south of 10 ◦Nand extend farther north below 850 hPa, and norther-lies are largely collocated with the easterlies above theboundary layer.

At 0300 UTC 29 August (Figure 2(b)), the jet hastwo separate cores, with one located at 18 ◦N and theother broader core south of 15 ◦N. Between the two jetcores, a weak easterly flow extends below 800 hPa,which is consistent with that observed (figure 11 inThorncroft et al., 2003). Associated with the changes

in the easterly jet structure, the mid-level souther-lies extend to 15 ◦N and low-level southerlies extendbeyond 25 ◦N. Meanwhile, the mid-level northerliesare strengthened, and shift north of 15 ◦N in associa-tion with the northern jet core. The meridional windstructure suggests convergence around the jet level(600 hPa) and divergence below and above, which isconsistent with a typical stratiform divergence profile(Mapes and Houze, 1995). A similar wind structureis also present along other longitudes between 0 ◦Eand 7 ◦E, where the secondary jet exists (Figure 1).The WRF model simulation reproduces the structuralchange of the AEJ similar to that observed duringthe JET2000 field experiment. The major discrepancyfrom the JET2000 observation is the location of thejet core at 0900 UTC 28 August, which is displacedabout 3◦ northward in the model simulation.

To examine what causes this change in the jetstructure, a momentum budget is calculated for thezonal velocity at 0130 UTC 29 August based on thefollowing equation:

∂u

∂t= −u

∂u

∂x− w

∂u

∂z+ v

(f − ∂u

∂y

)− ∂φ

∂x+ R

(1)The term on the left hand side of Equation (1) is

the local time tendency of the zonal wind. The firsttwo terms on the right hand side are zonal and verticaladvections, and the last two terms are pressure gradientforce and the residual term. The residual term includessurface friction, turbulent mixing, and other sub-gridscale processes. The third term on the right handside of Equation (1) is associated with the meridionalcirculation and can be regarded an ‘effective Coriolis’force. As shown in Figure 3(a), a negative tendency is

Figure 2. Pressure–latitude cross-sections of zonal wind (shading; units: m s−1) and meridional wind (contoured at 2 m s−1

intervals with negative values dashed) along 2 ◦E in the WRF simulations at (a) 0900 UTC 28 August 2000 and (b) 0300 UTC 29August 2000.

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172 Z. Wang and R. L. Elsberry

found north of 15 ◦N with a positive tendency to thesouth at the jet level (∼650 hPa). A reversed tendencypattern is found below and above the jet level, which isconsistent with the weakening of the original jet southof 15 ◦N and the formation of the secondary jet core tothe north. The effective Coriolis force associated withthe meridional circulation (Figure 3(b)) resembles thepattern of the meridional circulation (Figure 2(b)), andit is the major contributor to the zonal wind tendencyother than below the jet level to the south of 15 ◦N.It is concluded that the southward flow at the jet levelcontributes to the formation of the secondary jet coreas the flow turns westward, whereas the Coriolis effecton the northward flow contributes to a weakeningof the original easterly jet and appears to displace itsouthward.

The meridional circulation is associated with theprecipitation zone around 15 ◦N (Figure 1(c)). TheMCS north of the jet axis thus plays an important rolein the structural change of the AEJ. To examine howthis MCS is initiated, the vertically integrated moistureconvergence from 1800 UTC 28 August to 0000UTC 29 August (time period before and during theformation of the MCS) are shown in Figure 4. In thevicinity of the southern wave, moisture convergenceis mainly confined to the west of the trough axisand is slightly in advance of the precipitation, whichthen contributes to the westward propagation of thisconvective system. East of the southern wave trough,the southerly flow extends poleward of the jet axis,and another region of moisture convergence andprecipitation occurs where the moist southerly flowmeets the dry northerly flow in the north. As themoisture convergence is primarily due to the low-levelflow convergence, it is considered that the associated

dynamic ascent triggers vigorous convective cellsembedded within the MCS. The simulated verticalmotion, temperature, and moisture profiles associatedwith the MCS to the north of the jet axis resemblethose of tropical squall systems (not shown).

Based on this control simulation, sensitivity testswere conducted with initial conditions and lateralboundary forcing derived from the ECMWF reanaly-sis (ERA-40) NCEP–NCAR reanalysis, and analysesfrom the Global Forecast System (GFS). The hor-izontal resolution of the GFS analyses is 1◦ × 1◦,and the horizontal resolution of the ERA-40 andNCEP–NCAR reanalyses is 2.5◦ × 2.5◦. These threesensitivity tests failed to produce the secondary jetcore around 2 ◦E on 29 August. The simulations withthe NCEP/NCAR and ERA reanalyses both failed toproduce an MCS. Although the simulation with theGFS analyses produced a weak MCS, it was in thelongitude range of 6–10 ◦W, which is to the west ofthe MCS in the control run. However, this MCS alsocreated a secondary jet to its north, which indicatesthe importance of the MCS in the split jet formation.The southern wave in the simulations driven by theNCEP/NCAR and GFS analyses had a different prop-agation speed and structure compared with the controlrun. These sensitivity tests suggest that the predictionof the AEWs, MCSs, and the split jet is sensitive tothe initial conditions and the lateral boundary forcing.

4. Summary and discussions

A triply nested WRF model simulation is used tounderstand the AEJ structural change observed during

Figure 3. Pressure-latitude cross-sections along 2 ◦E of (a) zonal velocity (contoured at 2 m s−1 intervals) and zonal velocitytendency (shading; units: 10−4 m s−2); (b) effective Coriolis force v(f − uy) (shading; units: 10−4 m s−2) and vorticity (f − uy)(contours; units: 10−5 s−1). The zonal velocity in (a) is at 0300 UTC 29 August, and all the other fields are at 0130 UTC 29 August.

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Modulation of the African easterly jet 173

Figure 4. Meridional wind at 600 hPa (contoured at 2 m s−1

intervals) and vertically integrated moisture convergence fromthe surface to 200 hPa (shading; units: 10−3 kg s−1) in the WRFsimulation: (a) 1800 UTC 28 August, (b) 2100 UTC 28 August,and (c) 0000 UTC 29 August. The black thick lines are the jetaxis, which is defined as the local maximum of easterlies above10 m s−1.

the JET2000 field experiment. The WRF model sim-ulation forced by the T106 ECMWF analysis datareasonably simulates the AEJ structure, and the south-ern and the northern waves with respect to the jet axis.Two convective systems are produced in the modelsimulation, one associated with the southern wave andthe other located to the north of the jet axis. After thepassage of the northern MCS, the simulated jet had asplit structure as observed in JET2000.

Momentum budget analysis suggests that the changeof the jet structure is due to the meridional circulationassociated with the MCS north of the original jet. TheMCS has the typical squall line structure, with conver-gence at the jet level and divergence below and above.The ‘effective Coriolis’ force [v(f − ∂ u/∂ y)] asso-ciated with the jet-level southward flow north of theMCS contributes to the formation of the secondary jet,whereas the northward flow at the jet level south of theMCS weakens the original jet. In this simulation, thisMCS is triggered by mass and moisture convergencewhere the northward flow associated with the southernwave meets a southward flow on the northern side ofthe jet. In this sense, the southern wave modulates thejet structure indirectly through the MCS. As an AEWsouth of the jet is not always accompanied by an MCSto its northwest, other factors must also play a role inthe formation of the MCS.

Previous studies have suggested that the AEJ haslarge day-to-day variations (e.g. Berry et al., 2007).Based on the ECMWF reanalysis and TRMM 3B423-hourly precipitation data, the same scenario in whicha convective system north of the jet axis weakensthe original jet and generates a secondary jet coreto its north occurred six times over West Africaalong various longitudes during August 2000. Theseadditional cases suggest that MCSs play an active rolein modifying the structure of the AEJ.

AEWs are closely related to tropical cyclone forma-tion over the Atlantic and eastern Pacific (e.g. Landsea,1993; Thorncroft and Hodges, 2001). In particular,Chen et al. (2006, 2008) found that more tropicalstorms originate from the northern waves than thesouthern waves, but it takes longer time for the north-ern waves to transform into a tropical storm probablydue to their drier condition. Mesoscale convectionnorth of the jet may increase the column moisturecontent and modify the thermodynamic environmentwhere the northern waves form, and may thus createa more favorable condition for downstream develop-ment. Further work is warranted to explore variousissues such as the recovery time for the AEJ, howthe split jet and MCSs north of the jet axis affect thenorthern waves, and the possible interactions betweenthe northern waves and the southern waves as theyapproach the Atlantic Ocean.

Acknowledgements

This study was supported by the Office of Naval ResearchMarine Meteorology section. Computing resources were pro-vided by the Arctic High Performance Computing Center.

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