Transport pathways across the West African Monsoon as revealed by Lagrangian Coherent Structures

Abstract

The West African Monsoon (WAM) system is the main source of rainfall in the agriculturally based region of the Sahel. Understanding transport across the WAM is of crucial importance due to the strong impact of humidity and dust pathways on local cloud formation. However, the description of this transport is challenging due to its 3D complex nature. Lagrangian Coherent Structures (LCS) simplify transport description across the WAM by providing a geometrical partition of the troposphere into domains. Air parcels within each domain have similar dynamical characteristics. LCS make it possible to achieve an integrated vision of transport pathways across this system. Using this approach we unveil new connections in the WAM system. In particular, we identify transport pathways between the Tropical Easterly Jet (TEJ) and the African Easterly Jet (AEJ). Furthermore, the clockwise circulation associated with the divergent upper part of the Sahara heat low is clearly delimitated. Additionally, we show the presence of mixing regions in the AEJ and the lower part of the TEJ that are linked to pathways to sources of dust and humidity.

Analysis of stable water isotopes in tropospheric moisture during the West African Monsoon

“…By contrast, the northwesterly subtropical clusters R3 (Harmattan) and S2 (extratropical mid-level dry intrusions) remain around the mixing curves with only slight tendencies towards the Rayleigh curve. This emphasizes the existence of a subtropical mixing barrier that hinders the isotopic exchange between subtropical and tropical transport clusters as discussed in Yang and Pierrehumbert (1994) and Niang et al (2020). The resulting contrast between the e ects of mixing and microphysical processes are well represented in the contours of the full ensemble.…”

“…The discussion of Fig. 5.1 already showed that COSMO iso simulates markedly di erent isotopic distributions in the Sahelian free troposphere during the three monsoon phases documented in (Sy et al, 2018, Niang et al, 2020, the isotopic composition of cluster S1 merges with the signals of R1. By contrast, the northwesterly subtropical clusters R3 (Harmattan) and S2 (extratropical mid-level dry intrusions) remain around the mixing curves with only slight tendencies towards the Rayleigh curve.…”

“…This emphasizes the complex coupling of the mid-tropospheric AEJ with the development of the Sahelian precipitation (Sy et al, 2018, Niang et al, 2020. Further, the low-level SHL is associated with a mid-tropospheric high pressure system leading to a divergent ow within an anticyclonic circulation over North Africa (arrow V in Fig.…”

“…The complex mechanisms of the WAM are associated with distinct transport pathways, which lead to high levels of air mass mixing in the Sahelian troposphere (e.g. Niang et al, 2020). These large-scale atmospheric features of the WAM system are found to be in close connection with the precipitation and low-level moisture over West Africa (Lélé and Leslie, 2016).…”

“…For instance, the AEJ appears rather dry, as it remains constantly around 600 hPa, and yet it is associated with an easterly moisture ux in the mid-troposphere (Cadet and Nnoli, 1987). On the one hand, upper tropospheric water vapor that was lifted within the Indian Monsoon can be mixed into the mid-tropospheric AEJ (Sy et al, 2018, Niang et al, 2020. This water vapor is then transported to the Sahel, where it might represent only a minor contribution compared to the lower tropospheric moisture uxes, but appears to be correlated with the Sahelian precipitation (Sy et al, 2018).…”