Upper Tropospheric Water Vapor Transport from Indian to Sahelian Regions

Abstract

We present a study of upper tropospheric westward transport of air masses coming from the Indian monsoon zone over the period 1998–2008. The objective is to characterize upper tropospheric transport of water vapor from the Indian to Sahelian regions, and to improve the understanding of the dynamical mechanisms that govern water vapor variations in West Africa and the interconnections between India and the Sahel, focusing on the direct role of the Indian monsoon region on Sahel tropospheric water vapor and precipitation. The calculations of forward trajectories with LACYTRAJ (LACY TRAJectory code) and humidity fluxes show that a substantial part (40 to 70% at 300 hPa) of trajectories coming from the upper troposphere of the monsoon region crossed the Sahelian region in a few days (3–14 days), and water vapor fluxes connecting these two regions are established when the Indian monsoon begins at latitudes higher than 15° N in its south–north migration. The intensity and orientation of water vapor fluxes are related to the tropical easterly jet, but they are from the east above the high convection zones. Between 1998 and 2008, these fluxes between the 500–300 hPa pressure levels are associated with precipitation in Sahel only if they are from the east and with an intensity exceeding 8 kg·(m·s)−1.

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

“…Panel (c) shows a projection in the longitude-latitude-altitude space, which confirms this point. This is consistent with the results discussed by 40 in which particles trajectories in the TEJ become straight, although our analysis distinguishes regions in the TEJ with different particle fates. Indeed, each particle is just a representative for all the particles placed within each domain.…”

“…Trajectory analysis has also been used to examine meteorological phenomena like tropical moisture export 37 – 39 . When applied to West Africa, this type of Lagrangian approaches have shown that a large proportion of tropospheric air masses located over India between 500 and 300 hPa end up over West Africa, after following a direct path through the TEJ 40 .

Figure 2 Particle trajectories of the WAM system.

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

“…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.…”

“…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).…”

“…2.2). As transport is an important control factor for atmospheric moisture, such a dynamical clustering will give a useful rst overview of the characteristic moisture evolution of the de ned clusters (Nieto et al, 2006, Salih et al, 2015, Sy et al, 2018.…”