African Easterly Waves and Jet (AEW and AEJ)

African Easterly Waves and Jet (AEW and AEJ)

Definition and relevance of the JEA and AEWs

African Easterly Waves (AEWs) are key synoptic disturbances over West Africa during the monsoon season, and especially over the Sahel, that forecasters must follow and predict. AEWs are disturbances that strongly modulate rainfall over the Sahel (30%) and favour the occurrence of convective systems within their structure. On the one hand, the current forecasting models do not correctly represent this coupling between AEWs and precipitating systems, making the forecasted rainfall not very reliable. On the other hand, numerical models represent AEWs and their propagation quite well. The approach adopted in MISVA and presented here for AEW consists in documenting :

  • The structure of an AEW – conceptual model: trough, ridge, PW anomaly, which allows to characterise an AEW of the day and to locate the convection.
  • The characteristics of an AEW: period, wavelength, area and mechanisms of genesis, AEW trains that allow to anticipate the arrival of an AEW 
  • Factors influencing the occurrence and intensity of an AEW (JEA, equatorial waves, seasonal context) that allow a forecaster to judge whether the considered AEW will favour strong propagative or rather less structured thunderstorm systems.

AEWs are generated via barotropic and baroclinic instability mechanisms associated with the occurrence of the zonal East African Jet (AEJ) characterised by an easterly wind reaching ~15 ms-1 at near 600-700 hPa.

Conceptual Model of the AEW structure

The figures on the right provide a schematic representation of the typical structure of an AEW for the 3 levels that the forecaster has to analyse (600-700 hPa, 850 hPa and surface). It is the result of meetings and discussions between forecasters, modelers and experimentalists, and represents a new consensus on the structure of AEWs. A detailed presentation of this conceptual scheme is available in section 2.1.3.3 of the chapter “Synoptic Systems” of the West African Forecaster’s Handbook (Handbook) Manuel du prévisionniste en Afrique de l’Ouest (Handbook) available on this site.

Panels 1 and 2 : levels 600-700 hPa

The dynamic signature of AEW is strongest at the AEJ level (600-700 hPa) and has been the subject of much theoretical researches and composite analyses. It is based on dynamical variables such as streamlines, meridional and zonal wind, vorticity, and key features such as troughs and ridges, the AEJ core reinforcement in the trough vicinity… Panel 1 shows the typical configuration of these dynamical criteria, including troughs tilted horizontally opposite to the meridional zonal wind shear (signature of the barotropic transfer of energy from the AEJ to the wave). Panel 2 illustrates another configuration (among many possible variants) where the trough to the south of the AEJ core is tilted in the direction of the meridional wind shear.

Panels 3 and 4 :

Forecasters often prefer the intermediate 850 hPa level for which the zonal wind is weak, allowing to identify closed circulations with streamlines. The cyclonic ones correspond to the 850 hPa vorticity maximum line, south of the AEJ, determining the “monsoon trough” favourable to thunderstorm development. Another closed circulation can be detected in the northern part of the trough as shown by panel 4. 

The anomaly approach promoted by MISVA completes and refines the wave structure: indeed the PW* precipitable water anomaly allows the identification of a doublet of dry and wet anomalies to the west and east of the trough respectively.

The mean flow in the layer below the AEJ (925-600 hPa) completes the description of the wave by visualising the meridional transport. Thus, during the AEW growing stage, the northerly advection (Harmatan burst) will reinforce the dry and warm anomaly to the west of the trough, and the southerly advection (monsoon burst) will reinforce the moist and cool anomaly to the east. This diagnosis is efficient to position the troughs and ridges, and also to detect the genesis of AEWs over the East Sahel.

Panel 5 :

The wave is also well marked at the surface and in the lower layers. We thus note :

  • Strengthening of the thermal low (HL hereafter) underneath the dry and warm anomaly to the west of the trough, with a retreat of the ITD (Inter Tropical Discontinuity) due to the northerly push.
  • Conversely, to the east of the trough, HL weakening, strengthening of the monsoon flow and of its thickness, and northward penetration of the ITD.

Panel 6 :

The diurnal cycle is strong at the surface and in the lower layers with a strengthening of the monsoon flow during the night which can move the ITD 300 to 500 km northward. It is thus recommended to plot the ITD at 06UTC when it is most intense and northern.

Panels 1 and 2 : Fast-moving Mesoscale Convective Systems (MCS) typically occur along the axis of the AEJ ahead of the trough with favourable conditions of strong vertical wind shear and pronounced dryness in the mid-troposphere. Faster than the wave, they can propagate up to the ridge. Slower convective systems are found further south, where the vertical wind shear is weaker and the monsoon layer is deeper.

Schematic of the various observable elements of an AEW, and likely relationships between these. Left‐hand panels show a
‘normal’ situation, as far as this exists, while right‐hand panels show common alternatives. For example, the structure in panel 2 would be
expected in an environment with additional barotropic shear, with a stronger easterly wind to the north (i.e. ∂u/∂y < 0).
Panel 1at 600-700 hPa
Panel 2 at 600-700 hPa
Panel 3 at 850 hPa
Panel 4 at 850 hPa
Panel 5 at surface during the day
Panel 6 at the surface during the night

Warning: In reality each AEW is unique with its own characteristics, so this conceptual scheme could be misleading if it was seen as the unique structure of all AEWs that would not reflect the very wide range of observations. The forecaster must therefore identify all the criteria proposed to characterise the current AEW and forecast its evolution, the conceptual scheme only serving to de guide et élément de comparaison.

Main characteristics

Origin / Characteristics

  • AEWs are generated via instability mechanisms (barotropic and baroclinic) along the African Easterly Jet (AEJ), an easterly wind reaching ~15 ms-1 around 600-700 hPa. 
  • The AEJ axis corresponds to the zonal band of strong meridional gradients of temperature (baroclinic) and humidity (hydroclinic) separating the humid and relatively cold Sudanese regions from the dry and hot Saharan ones. 
  • The mean position of the AEJ in summer is around 16°N, but its meridional variations are important (between 8°N and 20°N). The AEJ is a key player in the monsoon to be monitored, through its oscillations corresponding to AEWs, but also its vertical wind shear, which affects the organisation of convective systems.
  • AEWs correspond to oscillations of the AEJ, i.e. a dynamic signal in u,v, vorticity; and in PW precipitable water for the part north of the Sahel.
  • The AEW is a synoptic mode (period ~5 d; zonal wavelength ~3-4000 km) propagating westward (~9 ms-1 or ~800 km/d). However, AEWs are less propagative and slower to the east of the Sahel.
  • Robust and frequent (3-4 waves/month at a given location), occurring in packets of about 3 successive waves, often increasingly intense within the same wave train.
  • AEWs are initiated in the Eastern Sahel via forcing mechanisms that are still being investigated, by :
    • Equatorial Rossby waves,
    • Midlatitude Rossby waves penetrating over the West Africa,
    • and convection over the Darfour region.

Factors driving the intensity

  • AEJ – width and intensity : Wider and more intense the AEJ core is, stronger are AEWs.
  • Vorticity, PW and PW anomaly : A marked structure in dynamics (vorticity) and in precipitable water (anomaly PW*> 0, raw PW> 45mm) favour the convective activity.
  • Location within an AEW train: The energy propagates eastward allowing the new wave to be amplified by the previous one within the same AEW train.
  • Mid-latitude connection: South and north wind anomalies can sometimes extend from the Equator to Europe, forming a favourable environment for strong thunderstorm systems within the AEW.

Wave context 

  • The passage of a Kelvin wave favours the genesis of an AEW in its wake, via a convergence in the low levels and a strengthening of the AEJ.
  • The active phase of the MJO (VP200) corresponds to more efficient AEWs.
  • The wet phase of equatorial Rossby waves favours more efficient AEWs.

The dynamical activity of AEWs is measured by the kinetic energy associated with the AEW wind anomalies (EKE). In brief, the EKE increases when the AEJ is large and strong, after the passage of a Kelvin wave, in the active phase of the MJO and during the wet phase of an equatorial Rossby wave.

Les produits adaptés

Only listed. Interpretation and use provided in Product

The above conceptual model provides a methodological framework for monitoring and forecasting AEWs. But first it is essential to analyse AEW activity at larger scales.

AEW and AEJ at the season scale and for the current and future weeks

  • Seasonal forecast (ensembles) : AEJ position and intensity on a month scale Lien à mettre avec http://seasonal.meteo.fr/
  • Sub-seasonal forceast (ensembles) : AEJ and AEW activity indices for the previous and forthcoming weeks :
    • AEW activity : This product EKE_AEW corresponds to the kinetic energy of AEWs. It is available computed from 18°W to 30°E, for 2 bands of continental latitudes [5°N-15°N] and [10°N-20°N], under Observations Séries Temporelles – Misva (aeris-data.fr)
      • Parameters : EKE700 – AEW – WestAfrica
    • Evolution of the AEJ intensity and location
    • Contexte d’ondes équatoriales dans lequel les AEW vont évoluer :
      • Filtered Hovmöller of the different types of waves (VP200, SF200, PW*)

Structure of the AEJ and AEWs

Références

Handbook

  • JEA
    • Section 2.1.2.3 African Easterly Jet (AEJ) pages 44-45
    • Section 11.8 WASA-F: AEJ pages 433-435
  • AEW
    • Section 2.1.3 African Easterly Waves pages 45-62
    • Section 11.9 WASA-F: African Easterly Waves and Cyclonic Vortices pages 435-441

Illustrations and case studies

  • Handbook 
    • Section 2.2.2 African Easterly Wave Case Studies and Canonical Structures : pages 72-79
    • 2.2.2.1 An Archetypal AEW Event: Case Study CS02, 12-16 August 2012:  pages 72-73
    • 2.2.2.2 AEW Breaking Events : pages 73-75 
    • 2.2.2.3 Development of AEWs : pages 75-79
  • Bilingual website handbook case study (umr-cnrm.fr)
    • CS01: 1-10 August 2012 – life cycle, structure and passage over West Africa of a train of African Easterly Waves. It resulted in a breaking of the AEJ.
    • CS02: 13-16 August 2012 – life cycle, structure and passage over West Africa of a canonical African Easterly Wave (AEW).

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