Analysis of Low-Level Atmospheric Moisture Transport Associated with the West African Monsoon

Abstract

The major objective of this study is to re-evaluate the ocean-land transport of moisture for rainfall in West Africa using 1979-2008 NCEP-NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface-850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April-June), the northward flux forms a "moisture river" transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (JulySeptember), the southerly transport weakens, but westerly transport is enhanced and extends to 20°N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July-September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan-Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1-4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.

The Niger River Basin Moisture Sources: A Lagrangian Analysis

“…This concords with the penetration of VIMF to the south of the NRB carrying moisture from the Gulf of Guinea (Figure 5b), which is more appreciable in August (Figure 8b). In general the African Sahel is influenced throughout the monsoon period by southerly winds transporting moisture from the Gulf of Guinea [50]. These results are in agreement with similar findings for the NRB by Stohl and James [13].…”

Intraseasonal variability of low-level moisture transport over West Africa

“…The observational rainfall dataset used in this study was provided by the national meteorological centers of countries across the region (see Lélé et al 2015). As many as 180 rain gauge-based precipitation observations over WA were used.…”

“…Because the water cycle is an integral part of the global energy cycle, it therefore plays a fundamental role in determining the large-scale circulation and precipitation patterns. In addition, the variability of water availability is one of the most limiting parameters for sustaining life, agriculture, and economic development in the Sub-Saharan WA countries (Lélé et al 2015). Therefore, understanding the WAM climate fluctuations requires an accurate estimation of the hydrological cycle over the region and its linkages to regional-scale features.…”

Interaction between moisture transport and Kelvin waves events over Equatorial Africa through ERA‐interim

“…Along the Gulf of Guinea there is some evidence of a convection belt which The position of the flux reveals that moisture transport over Central Africa is regulated by an enhanced evaporation coming from tropical Atlantic Ocean at the beginning of Kelvin wave's active phase which positively interacts with convection over Atlantic. Previous studies (Lélé et al, 2015) showed that Atlantic Ocean is the mayor source of water vapor and the propagated characteristic is mostly progresses from equator to north and south of the continent and this created a bimodal regime over the continental area. But this cannot explain the fact that propagation characteristics suddenly changed with season, this may be related to the presence of Kelvin waves over the region which influenced propagation of dynamic structures which progressed in opposite direction.…”

“…They showed that the full column vertically integrated moisture convergence exhibits a bimodal distribution with maximum during the rainy seasons. Lélé et al (2015) evaluated the ocean-land transport of moisture for rainfall in West Africa and showed a pronounced south-west incursion of the flux over West Africa during April-June, while during July-September, the southerly transport weakens, but westerly transport is enhanced. Most studies evaluated the variability of moisture transport and it relationship with precipitation, large-scale or small-scale but the interaction between moisture and equatorials waves like Kelvin waves has not yet been studied and clearly understood in this region.…”