In order to quantify intraseasonal activity, it is interesting to define indexes for the different modes of variability presented in the documentation section Meteorological objects – Misva (aeris-data.fr) .

• The indices related to the REGIONAL INTRASEASONAL MODES – Misva (aeris-data.fr) i.e. the “Sahelian” mode and the quasi-biweekly zonal dipole (QBZD) are only computed for analysis purposes. They are therefore used less frequently, but can be useful for analysing exceptional events. They are available under Observations Séries Temporelles – Misva (aeris-data.fr) with the following choice of parameters for :
  • QBZD : Parameters = OLR-NOAA ; Type = QBZD ; Domain1 = West-Africa
  • Sahelian mode : Parameters = OLR-NOAA ; Type = QBZD ; Domain1 = West-Africa
• The indexes for African Easterly Waves (AEW) activity and Heat Low (HL), computed for analysis and forecasting purposes, are the most useful and are therefore presented below.

Heat Low (HL)

1. Description

As detailed in the Heat Low – Misva (aeris-data.fr) object, the HL exhibits a mode of intraseasonal variability with a period of around 15 days, marked by two opposite phases illustrated below for a situation in August 2021 using the product :

Prévision Synoptique Cartes prévues – Misva (aeris-data.fr) Parameters : PotTemp – 850 – Africa

• During the East phase (HLE) on 10 August (left): Strong warming over the Mediterranean allowing the extension of the HL towards the NE, and cooling over the Atlantic coast coupled with a strengthening of the Trade Winds (ventilation).
• The situation reverses during the West phase (HLW) on 17 August (right): Strong warming to the West with an HL overflowing off the Mauritanian-Moroccan coast (weakening of the Trade Winds).

The HL mode modulates convective activity over the Sahel, as shown below, hence the interest in analysing its evolution.

• During the HLE phase, the monsoon system strengthens to the east with a more powerful HL, resulting in enhanced convective activity.
• The HL then weakens with the strengthening of the ventilation to the east (corresponding to the arrival of a “cold surges” type of event (Vizy and Cook, 2009), causing the ITD to retreat and reducing convective activity to the east.
• The convection then propagates westwards (~7 ms-1) to reach the western Sahel during the HLW phase one week after the HLE phase, allowing the WAM to strengthen to the west as well as the convection.

2. Definition and interpretation

2.1 The HL index

This index is available under : Prévision synoptique Séries temporelles prévues – Misva (aeris-data.fr) Parameters : Heat-Low – Temp850-Index – WAfrica

The HL intraseasonal variability index is calculated as the spatial projection (regression) of potential temperature anomalies Ɵ₈₅₀ at 850 hPa on the spatial structure of the HL intraseasonal mode. Above an absolute value of 1, the index is significant and can be interpreted as follows:

• Positive index : We are in an HLE phase with a warm anomaly over the eastern Mediterranean and a cold anomaly near the coast of Morocco. When the index is greater than 1 (standard deviation), the HL is in an HLE event, i.e. a strong east phase.
• Negative index : We are in a west phase with a cold anomaly over the eastern Mediterranean and a warm anomaly near the coast of Morocco. When the index is below -1 (standard deviation), the HL is in an HLW event, i.e. a strong west phase. A cold wave (Cold Surge by Vizy and Cook 2009) forms over north-east Africa.

2.2 Meridional index at 200 hPa

Index available under : Prévision synoptique Séries temporelles prévues – Misva (aeris-data.fr) Parameters : MidLat-Jet – Vwind200-Index – WAfrica

This index corresponds to the mean meridional wind anomalies at 200 hPa (in ms-1) in the box [35N-55N; 30W-10W] over the near Atlantic at mid-latitudes. It characterises the activity of the quasi-stationary Rossby waves at mid-latitudes which, by penetrating North Africa, modulate the ventilation and thus force the intraseasonal oscillation of the HL at a period of around 15 days. The evolution of this index, which precedes that of the HL by a few days, can be interpreted as follows :

• Positive index : There is a southerly wind anomaly over the near Atlantic. The polar jet therefore tends to bend towards the south-east of England, inducing the formation of a west HLW phase over the next few days (2 on average). When the index is greater than 1 (standard deviation), there is a high probability that an HLW event will occur.
• Negative index : there is a northerly wind anomaly over the near Atlantic. The polar jet therefore tends to bend southwards in the middle of the Atlantic (~20W), inducing the formation of a HLE phase over the next few days (2 on average). When the index is less than -1 (standard deviation), there is a high probability that an HLE event will occur.

3. Illustration

Les figures ci-dessous montre les indices HL et de vent méridien sur le proche Atlantique pour la situation précédente avec l’occurrence d’une phase HLE le 10 août 2021 suivi d’une phase HLW une semaine plus tard en accord avec le modèle conceptuel du mode intrasaisonnier du HL. L’indice de vent méridien (à droite) précède de 2 jours celui du HL illustrant ainsi le forçage de l’oscillation du HL par les ondes de Rossby des moyennes latitudes.

We can therefore expect stronger convective activity to the east around August 10, which will then move westwards during the following week and reach the Atlantic around August 17. Numerical weather prediction (NWP) models are currently capable of forecasting the evolution of this HL index, making it possible to anticipate the displacement of this zone of enhanced convection, which the NWP models are having difficulty in forecasting.

4. Complementary products

These indices help to condense the information on the intraseasonal oscillation of the HL, but to analyse the situation in greater depth, the forecaster needs to examine other products. These are listed on page 9 of the “Daily Summary” available at Documents > Daily summary – Misva (aeris-data.fr).

Individually they can be accessed under :

4.1 Potential temperature at 850 hPa Ɵ₈₅₀

Prévision Synoptique Cartes prévues – Misva (aeris-data.fr) Paramètres : PotTemp – 850 – Africa

Maps needed to analyse the structure of the HL and forecast its development, as illustrated by the 1^st figure above.

4.2 Time-latitude Hovmöller of Ɵ₈₅₀

Prévision synoptique diagrammes Temps-Latitude – Misva (aeris-data.fr)

Available for 2 longitude bands to the East and West (Parameters: 10E-30E or 20W-10W), they show the penetration of mid-latitude Rossby waves over North Africa via Ɵ₈₅₀ anomalies, their inclination giving their propagation speed towards the South (typically 4 ms^-1 – oblique line). We note the phase opposition between East and West during HLE and HLW events. In the Sahelian band (10N-20N), the cold anomalies (blue) generally correspond to an increase in precipitation, whereas the warm disturbances reflect the strengthening of the HL preceding that of the monsoon flow.

4.3 Hovmöller of the meridional wind at 200 hPa

In addition, the hovmöller (longitude-time) in the latitude band [40N-60N] can be used to characterise mid-latitude Rossby wave activity. This product is available under :

Prévision Synoptique Hovmollers – Misva (aeris-data.fr) Paramètres : Vwind200  – 40N-60N  – Africa

African Easterly Waves (AEW)

1. Description

This index corresponds to the kinetic energy EKE = ½ (u’² + v’²) of the horizontal wind fluctuation at 700 hPa averaged over a given box (in latitude and longitude). The wind fluctuations u’ and v’ are obtained at each point in the domain after filtering in the 2-10 day band corresponding to the AEW frequency range, which maximises at AEJ level (~700 hPa).

This product is available over 6 months of analyses preceding the date in question, averaged 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
• Caption : Temporal evolution of the kinetic energy (EKE in m²s-² – red curve) of AEW at 700 hPa averaged over the box [18°W – 30°E; 8°N -15°N] over a period of 4 months for the analysis and over 10 days beyond the day in question for the ECMWF forecast. The black curve corresponds to the climatology. On the left for the latitude band [5°N-15°N] and on the right [10°N-20°N].

This product is also available for the 3 weeks of analyses preceding the date in consideration and the forecast for the following week, averaged from 18°W to 30°E, for 2 bands of continental latitudes [5°N-15°N] and [10°N-20°N] or for each capital in West Africa, under Prévision synoptique Séries temporelles prévues – Misva (aeris-data.fr)

• Parameters : AEW – EKE700 – WestAfrica or CAPITALES (to be selected)

2. Use

This index provides a direct indication of changes in AEW intensity over the past analysis period and the forecast period. Since AEW activity accounts for around 50% of precipitation variability, the EKE is an index that directly modulates rainfall and makes it possible to anticipate favourable or unfavourable periods for convective events. However, it is necessary to crosscheck this index with the analysis of other fields and variables characterising the AEW on the basis of its conceptual model, African Easterly Waves and Jet (AEW and AEJ) – Misva (aeris-data.fr), to ensure that they are consistent and to understand the situation.

NB:A similar index calculated with the American GFS model is available on the Ventrice website Monsoons – Michael J. Ventrice, Ph.D. (weebly.com). It is interesting to compare this with the same index proposed by MISVA calculated with the ECMWF model.

3. Illustration