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

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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The impact of moist processes on the African easterly jet-African easterly wave system

Quarterly Journal of the Royal Meteorological Society ◽

10.1002/qj.414 ◽

2009 ◽

Vol 135 (641) ◽

pp. 894-913 ◽

Cited By ~ 52

Author(s):

R. J. Cornforth ◽

B. J. Hoskins ◽

C. D. Thorncroft

Keyword(s):

Wave System ◽

African Easterly Jet ◽

African Easterly Wave ◽

Easterly Wave ◽

Moist Processes ◽

The Impact

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The Mean Meridional Circulation of the Atmosphere Using the Mass above Isentropes as the Vertical Coordinate

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0239.1 ◽

2013 ◽

Vol 70 (7) ◽

pp. 2197-2213 ◽

Cited By ~ 3

Author(s):

Gang Chen

Keyword(s):

Small Amplitude ◽

Meridional Circulation ◽

Surface Flow ◽

Department Of Energy ◽

Flux Divergence ◽

Near Surface ◽

Vertical Coordinate ◽

Mean Meridional Circulation ◽

The Mean ◽

Mean Circulation

Abstract The mean meridional circulation of the atmosphere is presented using the mass (more specifically, the pressure corresponding to the mass) above the isentrope of interest as the vertical coordinate. In this vertical coordinate, the mass-weighted mean circulation is exactly balanced by entropy sources and sinks with no eddy flux contribution as in the isentropic coordinate, and the coordinate can be readily generalized to the mass above moist isentropes or other quasi-conservative tracers by construction. The corresponding Eliassen–Palm (EP) flux divergence for the zonal-mean angular momentum is formulated in a hybrid isobaric–isentropic form, extending the conventional transformed Eulerian-mean (TEM) formulation to finite-amplitude nongeostrophic eddies on the sphere. In the small-amplitude limit, the hybrid isobaric–isentropic formulation reduces to the TEM formulation. Applying to the NCEP–U.S. Department of Energy (DOE) Reanalysis 2, the new formulation resolves the deficiency of the conventional TEM formulation for the near-surface flow, where the isentropic surface intersects the ground, and the mean circulation agrees well with the TEM above the near-surface layer. In the small-amplitude limit, this improvement near the surface can be partially attributed to the isentropic static stability over the isobaric counterpart, as the mass density in the near-surface isentropic layers gradually approaches zero. Also, the mean mass streamfunction can be approximately obtained from the EP flux divergence except for the deep tropics or the near-surface flow, highlighting the dominant control of potential vorticity mixing for the subtropics-to-pole mean circulations. It is then expected to provide a valuable diagnostic framework not only for global circulation theory, but also for atmospheric transport in the troposphere.

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Intermittent African Easterly Wave Activity in a Dry Atmospheric Model: Influence of the Extratropics

Journal of Climate ◽

10.1175/jcli-d-11-00049.1 ◽

2011 ◽

Vol 24 (20) ◽

pp. 5378-5396 ◽

Cited By ~ 19

Author(s):

Stephanie Leroux ◽

Nicholas M. J. Hall ◽

George N. Kiladis

Keyword(s):

Storm Track ◽

Atmospheric Model ◽

Convective Heating ◽

African Easterly Waves ◽

Easterly Waves ◽

Global Circulation ◽

African Easterly Jet ◽

African Easterly Wave ◽

The North ◽

Easterly Wave

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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Case Study of an Intense African Easterly Wave

Monthly Weather Review ◽

10.1175/mwr2884.1 ◽

2005 ◽

Vol 133 (4) ◽

pp. 752-766 ◽

Cited By ~ 144

Author(s):

Gareth J. Berry ◽

Chris Thorncroft

Keyword(s):

Life Cycle ◽

North Africa ◽

Multiple Scales ◽

West African ◽

Unstable State ◽

The West ◽

African Easterly Wave ◽

Easterly Wave ◽

West African Coast ◽

African Coast

The life cycle of an intense African easterly wave (AEW) over the African continent is examined using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, Meteosat satellite images, and synoptic observations. This system, the strongest AEW of 2000, can be tracked from central North Africa into the eastern Atlantic Ocean, where it is associated with the genesis of Hurricane Alberto. Synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity (PV), allowing exploration at the role of multiple scales in the evolution of this AEW. The authors’ analysis promotes the division of the AEW life cycle into three distinctive phases. (i) Initiation: The AEW development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in Sudan. This convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical North Africa. (ii) Baroclinic growth: A low-level warm anomaly, generated close to the initial convection, interacts with a midtropospheric strip of high PV that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. This interaction is reinforced by the generation of subsynoptic-scale PV anomalies by deep convection that is embedded within the baroclinic AEW structure. (iii) West coast development: Near the West African coast, the baroclinic structure weakens, but convection is maintained. The midtropospheric PV anomalies embedded within the AEW merge with one another and with PV anomalies that are generated by convection over topography ahead of the system. These mergers result in the production of a significant PV feature that leaves the West African coast and rapidly undergoes tropical cyclogenesis.

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