Interest
Statistical analysis of the variability of different dynamic, thermodynamic and precipitation variables of the West African monsoon reveals several modes of intraseasonal variability. Unlike the equatorial waves, these modes do not correspond to normal modes, but to empirical principal modes derived from a principal component analysis of observations or analyses over the longest possible period. As these modes are associated with a modulation of the convective activity of the WAM, it is interesting for the forecaster to detect them from the analyses and forecasts. It is possible to extract indices for each type of mode that summarise the evolution of its activity. Three main modes of WAM variability are presented below:
- The HL mode in the 14-day range
- The QBZD mode in the 10-25-days range
- The Sahel mode in the 10-25-days range
Structure and characteristics of regional modes
1. The HL mode
Conceptuel model
In addition to a strong diurnal cycle, the Heat Low (HL) exhibits an intraseasonal mode of variability with a period of about 15 days, marked by two opposite phases illustrated by the figure opposite.
- During the Eastern phase (HLE) the ventilation by the 850 hPa northerly wind is enhanced over the Mauritanian-Moroccan coast and on the contrary weakened over the eastern Mediterranean. The result is a strengthening of the HL in the East and, on the contrary, its weakening in the West with a shift of its position towards the East, moving it away from the Atlantic coast. This is consistent with the strengthening of the trade winds and the Azores high, which is more intense and tighter.
- The situation reverses during the Western phase (HLW) with in particular an enhanced ventilation over the eastern Sahel. This phase is marked by an extension of the Azores high over Europe and its weakening over the Atlantic.
Impact on convection
The HL mode modulates the convective activity over the Sahel as illustrated by the composite below.
- During the HLE phase the monsoon system strengthens to the east with a more intense HL, resulting in an increase in convective activity.
- Then the HL weakens with the strengthening of the ventilation to the East (corresponding to the arrival of a “Cold surges” type event of Vizy and Cook, 2009), making the ITD retreat and diminishing of the convective activity to the East.
- The convection then propagates westward (~7 ms-1) to reach the western Sahel during the HLW phase one week after the HLE phase, allowing a strengthening of the WAM in the west and of the convection.
Properties of the HL mode
- Period ~15 days. Opposition between the East and West Sahel.
- HLE phase : Weak ventilation to the east with a strengthening of the HL, the monsoon flow and the convective activity.
- HLW phase : Strong cooling over eastern Mediterranean propagating southwards towards the eastern Sahel (~4 ms-1) which it reached 4 days later, corresponding to a cold surge event that reduced convective activity. On the other hand, over the western Sahel, the LH strengthens and spreads as it approaches the Atlantic coast and convective activity intensifies.
- Between the 2 phases, the convection reinforcement moves from East to West.
- The swing between HLE and HLW phases is favourable for the onset of the monsoon over the western Sahel.
- This intraseasonal mode of the HL is forced by the midlatitudes Rossby waves which precede the HL mode by 4 days (see object “Interactions with midlatitudes” for more details).
- It also interacts with equatorial waves (Rossby waves and Sahel mode) – (see object “Equatorial waves” for more details).
2. The quasi‐biweekly Zonal Dipole Mode (QBZD)
The quasi-biweekly zonal dipole (QBZD) is the dominant mode of variability over West Africa in the 10-25 day range. The “dipole” aspect refers to a quasi-stationary oscillation with a phase opposition between the African rains and the rains over the western Atlantic and Central America. The figure opposite shows its composite structure.
When convection is minimal over West and Central Africa, solar radiation reaching the surface is strong, which increases surface temperatures and decreases surface pressures. This creates an east-west pressure gradient at the latitudes of the ITCZ and the Saharan thermal low, leading to increased moisture advection over the continent.
The arrival of a positive pressure signal from the Atlantic, associated with the structure of a Kelvin wave, amplifies the zonal component of the low-level winds and the moisture advection over the continent, leading to an increase in convective activity over West and Central Africa. Then the opposite phase of the dipole develops (Mounier et al., 2008). This mode is also present in spring.
It is a mode coupled with the surface and modulated by the passage of Kelvin waves.
3. The Sahel mode
The “Sahel mode” is the second mode of 10-25 day variability (Figure opposite) and was detected by Sultan et al. (2003).
The convection enhancement in the African ITCZ is associated with a propagative mode
- first appearing over Central Africa (Sahel+ phase, Fig. (g) opposite),
- moving northwards toward the Sahelian latitudes for 2-4 days (b) and (d),
- then propagating westward toward the eastern tropical Atlantic (f), (h) and (a),
- and exits Africa, leaving behind ( to the East) a phase of reduced convection over the Sahel (e), (g) and (b).
This structure is associated with a cyclonic circulation located to the northwest of the convective strengthening pole (blue area in the dipole structure), increasing the moisture advection towards this pole.
The activation of this mode is related to the arrival of equatorial Rossby waves (ER) over East Africa from the Indian monsoon.
4. Coupling between HL and Sahel modes and equatorial Roosby waves
The Sahel mode can be partly explained by the intraseasonal variability of the midlatitudes via the Saharan Heat Low (cf. HL mode) for about 1/3 of the cases (Roehrig et al. 2011) and by the dynamics of equatorial Rossby waves for another 1/3 of cases (Janicot et al. 2010).
The figure below shows the scenario of favourable interaction between equatorial and midlatitudes Rossby waves leading via the HL mode to a strengthening of the Sahel mode.
- Phase HLE :
- Ventilation enhanced on the Mauritanian-Moroccan coast and on the contrary weakened on the Eastern Mediterranean.
- The HL strengthens and shifts eastward
- Quasi-stationary Rossby wave over Europe at 700 hPa associated with a cyclonic/anticyclonic circulation sequence
- Cyclonic circulation over Syria propagates toward Egypt, Sudan, Chad (curved black arrow)
- Phase HLE+5 = Phase Sahel+ :
- Strengthening of monsoon flow southeast of maximum HL (red)
- and strengthening of the cyclonic circulation at 700 hPa over Chad
- => Strengthening of the convection (symbol Cb +) corresponding to the Sahel+ phase
- Phase HLE+8 = HLW :
- Reversal of the situation with respect to the HLE phase
- strong ventilation to the East corresponding to a “cold surge” event reducing convection to the East
- Strengthening HL and monsoon flow to the West
- Propagation of the cyclonic eddy anomaly at 700 hPa associated with an ER wave
- Westward propagation of convective strengthening over the Sahel
- Phase HLE+12 = Phase Sahel- :
- Collapse of convection to the east following the cold surge
- Strengthening of convection in the west
- The western part of the HL collapses (in blue HL-) due to convective activity
- the cyclonic eddy anomaly at 700 hPa arrives over Atlantic
Adapted products
MISVA
- Pot. Temperature Anomalies (10E-30E) & (20W-10W)
- Daily Summary : page 9 provides a summary of all products suitable for HL monitoring and forecasting Documents > Daily summary – Misva (aeris-data.fr)
- Indexes of intraseasonal activity
- HL-Index: 10-25 days > HL > indexes – Misva (aeris-data.fr)
- Parameters: HL-Index
- QBZD INDEX QBZD index – Misva (aeris-data.fr)
- SAHELIAN MODE INDEX Sahelian mode index – Misva (aeris-data.fr)
- HL-Index: 10-25 days > HL > indexes – Misva (aeris-data.fr)
Références
Handbook
Chapter 7 Subseasonal Forecasting
- Section 7.1.3 Detection of the Main Modes of Subseasonal Variability of Convection pages 258-260
- Section 7.1.5 Other Convectively Coupled Signals and Links with Equatorial Waves pages 265-269
- 7.1.5.1 Signals Between 10 and 25 Days
Articles
Chauvin F, Roehrig R, Lafore JP. 2010. Intraseasonal variability of the Saharan heat low and its link with midlatitudes. J. Climate. 23: 2544-2561.
Mounier F, Janicot S, Kiladis GN. 2008. The West African Monsoon Dynamics. Part III: The Quasi-Biweekly Zonal Dipole. J. Climate. 21: 1911-1928.
Roehrig R, Chauvin F, Lafore J-P. 2011. 10-25 day intraseasonal variability of convection over the Sahel: a role of the Saharan heat low and midlatitudes. J. Climate 24: 5863-5878
Sultan B, Janicot S, Diedhiou A. 2003. The West African Monsoon Dynamics. Part I: Documentation of Intraseasonal Variability. J. Climate. 16: 3389-3406.
Vizy EK, Cook KH. 2009. A mechanism for African monsoon breaks: Mediterranean cold air surges. J. Geophys. Res. 114 : D01104. doi :10.1029/2008JD010654.
Vizy EK, Cook KH. 2014. Impact of cold air surges on rainfall variability in the Sahel and wet African tropics: a multi-scale analysis. Clim. Dyn. 43: 1057-1081. doi: 10.1007/s00382-013-1953-z.