Description
These 2 diagnostics are complementary and are both available under : Prévision Synoptique Cartes prévues – Misva (aeris-data.fr)
- Parameters : Mean-Vwind – 925-600 – WestAfrica
- Legend : Mean meridional wind in the 925-600 hPa layer (colour, en m s–1) and mean horizontal wind vector in the 925-850 hPa layer.
This map provides 2 informations on 2 different layers :
- Colour : Meridional wind averaged in the 925-600 hPa layer, allowing to estimate the intensity of the meridional mass flux,
- northerly if red which dries the Sahel – decreasing PW – corresponding to a Harmattan flow,
- o southerly if blue which humidifies the Sahel – increasing PW – corresponding to the monsoon flow.
- Arrows : show the mean horizontal circulation at low level over a thinner layer (925-850 hPa) to avoid the heart of the JEA caracterized by a strong zonal wind, and to remain in the most energetic layer of the monsoon flow.
- Parameters : PW – over 2 domains : Africa or WestAfrica
- Legend : Fields of (a) PW (colour in mm) superimposed on wind vectors at 925 hPa (m s-1), and (b) corresponding PW* and wind anomalies.
This diagnostic makes it possible to identify PW* anomalies resulting from the mean meridional transport in the 925-600 hPa layer, hence the interest in looking at the consistency between these 2 diagnostics.
For example, in the case illustrated above at the end of winter, there is a wet upwelling (blue PW* anomaly) which could cause some rain on the coast and over the south of the Ivory Coast and Ghana, of the “mango rain” type, since PW exceeds 45 mm. We therefore need to examine PW and its anomaly PW* together.
In this case, the monsoon surge is weak, and the forcing clearly comes from mid-latitude Rossby waves highlighted by the meridional transport diagnostic, with 2 strong, thick Harmattan surges (in red) to the east and west and a southerly upwelling (in blue) between the 2, which sucks in the monsoon.
Use
Mean meridional wind (MT for Meridional Transport) : This is an efficient diagnostic tool for identifying strong monsoon surges (positive areas in blue) or northerly surges above a thin monsoon layer (negative areas in red). These monsoon and northerly surges are almost in phase with the wet and dry phases respectively of an easterly or equatorial Rossby wave, in accordance with the conceptual model (section 2.1.3.3; Figure 2.10 of the Handbook). This diagnostic also makes it possible to identify the mid-latitude Rossby waves which circulate eastwards and influence the weather over Africa, and the monsoon surges originating from the Gulf of Guinea and circulating westwards.
PW and PW* :PW* is suitable for identifying AEWs and equatorial and mid-latitude Rossby waves. PW, on the other hand, is used to characterise monsoon intensity and identify areas favourable to rainfall above a threshold (40-45 mm).
Illustrations
African Easterly Wave (AEW)
Case study CS02 of the Handbook (page 724 of the French version) of an AEW archetype (13-16 August 2012) available in full on the site handbook (umr-cnrm.fr), provides several illustrations of the use of meridional transport. The figure below gives an example for 15 August at 00 UTC.
Combined with the PW* map (Fig. a), the meridional transport diagnostic (Fig. b) allows :
- to facilitate the identification of troughs (continuous red line) separating the northern meridian transports (in red to the west) and the southern meridian transports (in blue to the east),
- and of ridges (blue to the west and red to the east). For example, the dotted red lines indicate an intense ridge of high pressure and a mean anticyclonic circulation over the Atlantic and southern Senegal, corresponding to a zone of inhibited convection (Figure opposite showing the corresponding Meteosat IR image).
- NB: This is the average position of the Talwegs and ridges in the 925-600 hpa layer. Given the slope of the Talweg, it will be slightly (~2°) further east at the AEJ level (~700 hPa), and further west at the surface.
- Identify the mid-latitude Rossby waves that circulate eastwards and influence the weather over Africa by modulating the Heat Low (HL) and the ventilation,
- and monsoon surges from the Gulf of Guinea moving westwards.
- When the monsoon surge (in blue) comes into phase with the southerly upwelling (in blue) associated with a mid-latitude Rossby wave (Fig. b), the AEJ amplifies abruptly, generating strong or even extreme rainfall events, as seen over Mali and Burkina Faso (IR image opposite), and in some cases leading to the AEW breaking.
- The meridional transport map also makes it possible to detect the genesis of a new AEW (trough in red on Fig. b) over Sudan with a dipole of north/south low-level wind anomalies, while the dipole of dry/wet anomalies has not yet formed (PW* map (a)).
(a) PW* (mm – colour) and 925 hPa horizontal wind anomalies (m s−1); (b) mean meridional wind in the 925–600 hPa layer (m s−1 – colour) and mean horizontal wind vector in the
925–850 hPa laye Source: Figure 11.13 of the Handbook.
Equatorial Rossby Wave
The figure below illustrates a wet phase with excess rainfall over the central and western Sahel spreading westwards from 16-24 August 2019. The PW product shows a penetration of the ITD and monsoon far into the continent, with a large area with PW>45 mm favourable to rainfall. This is confirmed by a large PW* anomaly covering the entire Sahel and propagating westwards while strengthening.
The daily sequence opposite of the mean meridional wind product in the 925-600 hPa layer from 20 to 25 August can be used to identify circulations associated with waves observed during this wet period.
- On 20 August, a large cyclonic vortex (in red) reflected the presence of an equatorial Rossby wave from the Indian monsoon, confirmed by the Hovmöller diagram of the VP velocity potential superimposed on the equatorial wave contribution (Figure below, Rossby wave in red).
- The arrival of this Rossby wave favours the triggering of a train of smaller-scale easterly waves (smaller cyclonic vortices in red) which propagate more rapidly. Subsequently, the signature of the Rossby wave is masked by that of the AEW, hence the need to extract the different types of waves.
- Note that crossing with a Kelvin wave (in blue) also encourages the development of AEWs.
- The easterly wave train then amplifies as it propagates westwards. Its signature is strong in PW*, but even clearer in meridian transport (MT), showing the interest of combining the 2 products PW and Mean-Vwind.
