MID-LATITUDES

Definition and role of the mid-latitude circulation

The West African Monsoon (WAM) system is not isolated and interacts directly with the circulation of neighboring regions and more particularly with the wave systems of the equatorial zones and mid-latitudes. The forcings induced by these waves increase the predictability of the WAM, hence the importance for the forecaster to monitor and characterize them.

The mid-latitude circulation of the Northern Hemisphere is characterized by the occurrence of eastward propagating Rossby waves associated with disturbances. These are stronger during winter, however during the WAM the activity of these waves is weaker, or even null, but they circulate further south and can therefore interact with the WAM. This is particularly true during the transition periods at the beginning and end of the monsoon season.

Archetype of Rossby waves interacting with the WAM

The animations below illustrate the dominant structure of the mid-latitude Rossby waves interacting with the WAM during the African monsoon period (JJAS) from a composite analysis. It allows to propose a conceptual model of forcing of the WAM by these Rossby waves via the Saharan thermal low (HL thereafter).

Dynamic signature at 200 hPa: The opposite animation of wind and geopotential fluctuations over a period of 20 days illustrates the oscillations of the polar westerly jet in the 40-60°N latitude band during interaction events with the WAM. The wind disturbances propagate slowly (~3 ms-1) eastward across the Atlantic and then bifurcate toward the Mediterranean when arriving over Europe. They correspond to strong oscillations of the meridian wind. These Rossby waves are quasi-stationary but propagate their energy rapidly eastward (~30 ms-1), they are also quasi-barotropic with a stronger potential temperature signature Ɵ at 850 hPa.

*Animation of geopotential (colors in m) and wind (arrows in ms*^-1) fluctuations at 200 hPa of a mid-latitude Rossby wave. The thick isolines represent the core of the polar westerly jet (zonal wind > 20 ms^-1 in 10 ms^-1 steps).

Thermal signature at 850 hPa : The corresponding animation for the potential temperature Ɵ at 850h shows the strong impact of meridional wind disturbances with temperature oscillations up to 3° (negative and positive respectively for northerly and southerly wind disturbances following a strong baroclinicity). These anomalies propagated southward over Libya and Egypt, towards the Eastern Sahel. The cold phase of this penetration of Rossby waves corresponds to the “cold surges” documented by Vizy and Cook (2009) which strongly reduces the convective activity. These Rossby waves strongly modulate the HL structure (intensity, extension, position) identified by the black isolines (Ɵ850 ≥300K) in the animation.

These Rossby waves precede the HL oscillations by 4 days contributing to the appearance of a 14-day variability mode of the HL characterized by a modulation of its intensity with a phase opposition between east and west. This mode is described in the object Heat Low” of this online documentation.

*Animation of the signature in Ɵ fluctuations (colors) and wind (arrows) at 850 hPa of a mid-latitude Rossby wave and its impact on the HL (thick isolines above 28°C).*

The mid-latitude Rossby waves have different impacts on the monsoon system:

HL mode forcing
“Cold surge” events
Interaction with AEW
Extra-tropical dry air intrusions
Tropical Plumes

Main characteristics

Roosby Waves

Associated with the exit zone of the Polar Westerly Jet over the Atlantic. These waves are more intense as the intensity of the polar jet is stronger.
Quasi-stationary and quasi-barotropic waves with a wavelength of ~4-5000 km. NB: the quasi-stationary character enhances their impact on the WAM.
Slow eastward propagation (~3 ms-1), but rapid westward energy propagation (group velocity ~30 ms-1)
Strong meridional wind oscillations at 200 hPa (5-10 ms-1) and Ɵ at 850 hPa (3°K).
Forcing of the HL mode via modulation of the meridional wind (ventilation) as it enters the Sahel.
More active at the beginning and end of the monsoon season with a more southerly trajectory allowing a better interaction with the WAM.
Strong variability of its activity depending on the year and sometimes a less quasi-stationary character. This is still an open question requiring more research and attention from forecasters.

Forcing of the HL mode and cold surges

Period ~15 d. Opposition between East and West Sahel
HLE phase : Weak ventilation in the East with a strengthening of the HL, the monsoon flow and the convective activity.
HLW phase : Strong cooling over the East Mediterranean spreading towards the East Sahel (~4 ms-1) which it reaches 4 days later, corresponding to a “Cold surge” event that reduces convective activity. On the other hand, over the Western Sahel, the LH strengthene and extends towards the Atlantic coast and the convective activity intensifies.
Between the 2 phases, the convection strengthening moves from East to West.
It should be noted that the combination of a mid-latitude Rossby wave with the Sahelian mode (equatorial Rossby wave type) can explain up to 1/3 of the cases of intense events over the Sahel.
The switch between HLE and HLW phases is favourable to the onset of the monsoon over the western Sahel.

Interactions with AEWs

The interaction with the Rossby waves can suddenly amplify the AEW when the southern disturbance of the Rossby wave is in phase with the monsoon surge behind the AEW trough, or/and the opposite situation in front of the trough.
In case of phase opposition of the two waves (Rossby and AEW propagating in opposite directions) this can smother the AEW (temporarily).

Extra-tropical dry air intrusions

These are pockets of very dry air (RH < 10%) observed over several tropical regions in the free troposphere and of extratropical origin, which can inhibit convection or favour its organisation in the case of squall lines.They originate from regions located around 50°N, on the anticyclonic side of the polar jet, at high levels (200-250 hPa). They start to descend over Europe, moving southward along the 330 K isentropes, reaching the Sahelian middle troposphere a few days later (Roca et al. 2005).

Tropical plumes

At the synoptic scale, during the boreal winter, the penetration of upper mid-latitude lows into the low latitudes (so-called “upper cold drops” marked by a strong potential vortex anomaly (PV)) generate a “tropical plume” on their southern flank, over the Atlantic Ocean and north-West Africa.
The diagram opposite shows the conceptual model presented in the Handbook (section 2.1.4, Fig. 2.20).
These precipitation events over West Africa during the dry season are sometimes referred to as the “Mango” rains and correspond to an ITD northward penetration and a greening of the Sahel.
It should be noticed that numerical models have a good ability to predict such events.

Schematic depictions of the large‐scale circulation associated with dry‐season precipitation over West Africa associated with low‐latitude upper‐level disturbances from the extratropics. (a) Cases with a direct influence form the upper trough, mostly affecting the western Sahel. Thick black lines delineate the low‐latitude upper trough and the thick arrow indicates the associated STJ streak. Thin grey arrows show mid‐level moisture transports from the deep tropics. Stippled regions indicate high clouds, and hatching delineates the major precipitation zone. Light (dark) grey shading depicts the region of convective instability under the coldest air at upper levels (positive quasi‐geostrophic forcing for mid‐level ascent). The dashed lines bound a region of upper‐level inertial instability along the anticyclonic shear‐side of the jet. Source: Knippertz (2007)

Les produits adaptés

Wind at 200 hpa
- Prévision Synoptique Cartes prévues – Misva (aeris-data.fr)
  - Domain / Parameters / level : Africa – Wind – 200
Hovmoller Meridional Wind at 200 hPa
- Prévision Synoptique Hovmollers – Misva (aeris-data.fr)
  - Parameters: Vwind-200 – 40-60N – Africa
Merid. Wind Anomalies at 200 hPa dans le proche Atlantique [35N-55N, 30W-10W]
- 10-25 days > HL > indexes – Misva (aeris-data.fr)
  - Parameters: V200 – 35N55N – 30W10W
In addition: Prévision Synoptique Cartes prévues – Misva (aeris-data.fr)
- 600 hPa Wind Map
  - Parameters: Wind – 600 – Regional
- Meridional transport
  - Parameters: LowLev-Diag – 925-600_Raw – Synoptic

Références

Chauvin F, Roehrig R, Lafore JP. 2010. Intraseasonal variability of the Saharan heat low and its link with midlatitudes. J. Climate. 23: 2544-2561.

Knippertz P. 2007. Tropical–extratropical interactions related to upper‐level troughs at low latitudes. Dyn. Atmos. Oceans 43: 36–62. doi: 10.1016/j.dynatmoce.2006.06.003.

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.

Roca R, Lafore JP, Piriou C, Redelsperger JL. 2005. Extra‐tropical dry air intrusions into the West African monsoon mid‐troposphere: an important factor for the convective activity over Sahel. J. Atmos. Sci. 62: 390–407.

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.

Handbook