The Influence of the MJO on Upstream Precursors to African Easterly Waves

Abstract

The Madden-Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the ''trigger region'' in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region.In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa.

West African monsoon intraseasonal activity and its daily precipitation indices in regional climate models: diagnostics and challenges

“…Considering more specifically the distribution within the two frequency bands, most of RCM simulations seem rather inaccurate in comparison with the reanalysis. First, the large Eastern African low-frequency activity shown in all RCMs simulations (variance field in 10-90-day) is consistent with the analysis of Alaka and Maloney (2012) showing that the MJO dry and wet phase oscillations induce strong PW fluctuations over this region. Although this MJO-like behavior seems fairly consistent with ERAI fields and the findings of Alaka and Maloney (2012) east of 10°E -Central to Eastern Africa, the low-frequency activity appears to be overestimated in all RCMs over the rest of the continental Sahel.…”

“…First, the large Eastern African low-frequency activity shown in all RCMs simulations (variance field in 10-90-day) is consistent with the analysis of Alaka and Maloney (2012) showing that the MJO dry and wet phase oscillations induce strong PW fluctuations over this region. Although this MJO-like behavior seems fairly consistent with ERAI fields and the findings of Alaka and Maloney (2012) east of 10°E -Central to Eastern Africa, the low-frequency activity appears to be overestimated in all RCMs over the rest of the continental Sahel. This is reminiscent of some results on the WAM intraseasonal-scale analysis with CMIP5 GCMs by Roehrig et al (2013) and implies that, even in RCMs, there is still an overrepresentation of the impacts of larger-scale phenomena (from oceanic basins, tropical waves such as MJO).…”

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

“…The westerly anomalies also increase cyclonic shear on the equatorward flank of the AEJ, enhancing wave development and transient convective activity. MJO-induced waves often trigger convection over the Darfur region [195], where AEWs frequently originate [69]. The Rossby waves appear to be the more important part of the MJO-induced response over Africa, especially on time scales of 25 to 90 days [59,60,63].…”

ESMValTool (v1.0) – a community diagnostic and performance metrics tool for routine evaluation of Earth system models in CMIP

“…Associated with enhanced convection in the tropics, the MJO exerts a significant influence on monsoon precipitation, e.g. on the South Asian Monsoon (Pai et al, 2011) and on the west African monsoon (Alaka and Maloney, 2012). The eastward propagation of the MJO into the West Pacific can trigger the onset of some El Niño events (Feng et al, 2015;Hoell et al, 2014).…”