Moisture Variability and Multiscale Interactions during Spring in West Africa

Abstract

The West African monsoon (WAM) is a vital source of rainfall for the African Sahel. In addition to the agricultural benefit of its rains, it benefits public health because bacterial meningitis outbreaks end with the monsoon onset. Outbreaks occur between December and May, a period of low humidity. Knowledge of the onset of high humidity could aid in predicting where the outbreaks will cease. Therefore, this study investigates the variability of atmospheric moisture during the spring over West Africa, characterizing the sources of moisture, as well as circulation patterns and relative influences of tropical and midlatitude systems. A conceptual model of the evolution of the premonsoon period is presented. The meridional and temporal variability of surface moisture during the spring is modulated by multiscale interactions, as illustrated for the period from mid-April to early May 2009. As westward-propagating, synoptic disturbances move across West Africa, a corresponding peak occurs in the surface relative humidity. With the passage of each disturbance a new and more humid regime is established. Filtered anomalies of outgoing longwave radiation (OLR) indicate that Kelvin waves, equatorial Rossby waves, and possibly the MJO contributed to the initiation and intensification of the synoptic disturbances. During the last of the disturbances, whose passage raised the relative humidity above 40%, a critical threshold for meningitis, an extratropical cyclone also contributed to moisture influx over the Sahel. Analysis of the period 2000–09 shows the relative influences of synoptic and subseasonal circulations on the onset of high relative humidity over the Sahel during the spring.

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

“…Their surface air parcel trajectory analysis, both horizontally and verti cally, revealed that the northeastern Atlantic, the Gulf of Guinea, the South Atlantic, North Africa, and the Mediterranean Sea are moisture sources for WA during the February-June period. Mera et al (2014) also found the levels of the atmosphere from which the air origi nated as the seasons progressed. Using the ECMWF global dataset at 600 hPa, Spinks and Lin (2015) Fig.…”

“…The study also found that the cold tongue regulates the timing and intensity of the coastal rainfall in the spring, while the SHL brings moisture into the continent. Mera et al (2014) studied the variability and transport of surface moisture in WA using NCEP global model analyses, satellite, and sta tions observations during the spring. Their surface air parcel trajectory analysis, both horizontally and verti cally, revealed that the northeastern Atlantic, the Gulf of Guinea, the South Atlantic, North Africa, and the Mediterranean Sea are moisture sources for WA during the February-June period.…”

Intraseasonal variability of low-level moisture transport over West Africa

“…The cold tongue regulates the timing and intensity of coastal rainfall in the spring, whereas the heat low brings moisture into the continent. Mera et al (2014) studied the variability and transport of surface moisture in WA using NCEP global model analyses, satellite, and station observations during the spring, which revealed that the Northeast Atlantic, the Gulf of Guinea, the South Atlantic, North Africa, and the Mediterranean Sea are moisture sources for WA during the February-June period.…”

Disagreements in Low-Level Moisture between (Re)Analyses over Summertime West Africa

“…Processes that are known to influence the position of the ITD include interactions with the extratropics Vizy and Cook 2009;Knippertz and Todd 2010;Roehrig et al 2011; Denotes Open Access content. Roberts and Knippertz 2014), changes to the strength of the SHL (Parker et al 2005), African easterly waves (AEWs) on the African easterly jet (AEJ) (Berry and Thorncroft 2005;Couvreux et al 2010;Bain et al 2011), convectively generated cold pools (Flamant et al 2007;Cuesta et al 2010;Marsham et al 2008Marsham et al , 2013bGarcia-Carreras et al 2013;Roberts and Knippertz 2014), and eastward-propagating convectively coupled Kelvin waves (Mounier et al 2007;Mekonnen et al 2008;Mera et al 2014). Increased boundary layer moisture south of the ITD, in regions where the monsoon flow is deep, allows for the production of mesoscale convective systems (MCSs).…”

“…This makes the ITD especially important in the Sahel, whose inhabitants are heavily dependent on rain-fed agriculture. Additionally, the collapse of seasonal outbreaks of meningitis that affect the Sahel have been shown to be strongly linked to increases in humidity associated with northward surges of the monsoon flow (Mera et al 2014;Pandya et al 2015). These impacts on West African meteorology mean that there is great value in understanding the behavior of the ITD in models, observations, and such widely used products as operational analysis and reanalysis datasets.…”