Saharan dust and the African easterly jet-African easterly wave system: Structure, location and energetics

Abstract A dynamical model is constructed of the northern summertime global circulation, maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (Thorncroft et al.; Leroux and Hall). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward-propagating 3–5-day disturbances over West Africa. These simulated waves display intraseasonal intermittency as the observed AEWs also do. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.

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The Role of Convectively Coupled Atmospheric Kelvin Waves on African Easterly Wave Activity

Monthly Weather Review ◽

10.1175/mwr-d-12-00147.1 ◽

2013 ◽

Vol 141 (6) ◽

pp. 1910-1924 ◽

Cited By ~ 23

Author(s):

Michael J. Ventrice ◽

Chris D. Thorncroft

Keyword(s):

Vertical Wind Shear ◽

Leading Edge ◽

Active Phase ◽

Kelvin Waves ◽

Boreal Summer ◽

African Easterly Jet ◽

African Easterly Wave ◽

Easterly Wave ◽

West African Coast

Abstract The role of convectively coupled atmospheric Kelvin waves (CCKWs) on African easterly wave (AEW) activity is explored over tropical Africa during boreal summer. Examination of the pre-Alberto AEW in 2000 highlights the observation that the convective trigger for the initiation of the AEW was generated by a strong CCKW and that the subsequent intensification of the AEW at the West African coast was associated with a second CCKW. Composite analysis shows that, generally, AEW activity increases during and after the passage of the convectively active phase of strong CCKWs. The increase in AEW activity is consistent with convective triggering at the leading edge of the convective phase of the CCKW. This convective triggering occurs in a region where the background low-level easterly vertical wind shear is increased by the CCKW. As the AEW propagates westward through the convectively active phase of the CCKW, it can develop in an environment favorable for convection. It is also shown that this phase of the CCKW is characterized by enhanced meridional vorticity gradients in the core of the African easterly jet suggesting that enhanced mixed barotropic–baroclinic growth may also be responsible for enhanced AEW activity there.

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Saharan Dust and the Nonlinear Evolution of the African Easterly Jet–African Easterly Wave System

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0118.1 ◽

2016 ◽

Vol 74 (1) ◽

pp. 27-47 ◽

Cited By ~ 7

Author(s):

Dustin F. P. Grogan ◽

Terrence R. Nathan ◽

Shu-Hua Chen

Keyword(s):

Life Cycle ◽

Linear Growth ◽

Nonlinear Evolution ◽

Meridional Circulation ◽

Flux Divergence ◽

Nonlinear Stabilization ◽

African Easterly Jet ◽

African Easterly Wave ◽

Easterly Wave ◽

Mean Meridional Circulation

Abstract The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume. The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.

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An Analysis of the Environments of Intense Convective Systems in West Africa in 2003

Monthly Weather Review ◽

10.1175/2010mwr3321.1 ◽

2010 ◽

Vol 138 (10) ◽

pp. 3721-3739 ◽

Cited By ~ 22

Author(s):

Stephen D. Nicholls ◽

Karen I. Mohr

Keyword(s):

West Africa ◽

Regional Scale ◽

Convective Available Potential Energy ◽

Potential Temperature ◽

Saharan Dust ◽

Convective System ◽

Low Level ◽

Convective Systems ◽

African Easterly Wave ◽

Easterly Wave

Abstract The local- and regional-scale environments associated with intense convective systems in West Africa during 2003 were diagnosed from soundings, operational analysis, and space-based datasets. Convective system cases were identified from the Tropical Rainfall Measuring Mission (TRMM) microwave imagery and classified by the system minimum 85-GHz brightness temperature and the estimated elapsed time of propagation from terrain greater than 500 m. The speed of the midlevel jet, the magnitude of the low-level shear, and the surface equivalent potential temperature θe were greater for the intense cases compared to the nonintense cases, although the differences between the means tended to be small: less than 3 K for surface θe and less than 2 × 10−3 s−1 for low-level wind shear. Hypothesis testing of a series of commonly used intensity prediction metrics resulted in significant results only for low-level metrics such as convective available potential energy and not for any of the mid- or upper-level metrics such as the 700-hPa θe. None of the environmental variables or intensity metrics by themselves or in combination appeared to be reliable direct predictors of intensity. In the regional-scale analysis, the majority of intense convective systems occurred in the surface baroclinic zone where surface θe exceeded 344 K and the 700-hPa zonal wind speeds were less than −6 m s−1. Fewer intense cases compared to nonintense cases were associated with African easterly wave troughs. Fewer than 25% of these cases occurred in environments with detectable Saharan dust loads, and the results for intense and nonintense cases were similar. Although the discrimination between the intense and nonintense environments was narrow, the results were robust and consistent with the seasonal movement of the West African monsoon, regional differences in topography, and African easterly wave energetics.

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