Effects of Saharan Dust on the Linear Dynamics of African Easterly Waves

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The radiative effects of Saharan mineral dust (SMD) aerosols on the structure, location and energetics of the African easterly jet–African easterly wave (AEJ‐AEW) system are examined for July–September 2006. Experiments are conducted with and without SMD using the Weather Research and Forecasting (WRF) model, which is radiatively coupled to an interactive dust model. The SMD‐modified heating field shifts the AEJ northward, upward and westward, and enhances its zonal asymmetry. These SMD‐induced changes to the AEJ are manifest in the AEWs: the northern and southern tracks of the AEWs shift northward (like the AEJ); and the zonal‐scale of the AEWs expands and their westward propagation increases. The SMD also strengthens the energetics of the AEJ‐AEW system. The domain and temporally averaged baroclinic energy conversion, which is an order of magnitude larger than the barotropic conversion, increases by a factor of 2.5. The eddy kinetic energy and generation of available potential energy increase by factors of 1.5 and 2.7, respectively. The implications of an SMD‐modified AEJ‐AEW system for West African precipitation and tropical cyclogenesis in the eastern Atlantic Ocean are discussed.

“…The SMD‐induced increase in the energetics terms was also found in the idealized linear stability analysis of Grogan et al . ().…”

Section: African Easterly Wavesmentioning

confidence: 76%

“…Grogan et al . () showed, based on their idealized WRF‐dust model, that

G_{E}^{*}

is due to SMD‐modified baroclinic effects and SMD‐modified eddy APE. Their equation for

G_{E}^{*}

can, after rearrangement of terms, be written in the following symbolic form:

G_{E}^{*} = A \frac{\partial \overset{γ}{true}}{\partial y} + B \frac{\partial \overset{γ}{true}}{\partial p},

where

\overset{γ}{true}

is the dust mass mixing ratio and A and B are functions of the background wind, SMD, transmissivity, and eddy fluxes.…”

Section: African Easterly Wavesmentioning

confidence: 99%

“…Indeed, as shown by Grogan et al . (, ) and Nathan et al . (), SMD can affect AEW growth via two additional feedback mechanisms.…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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

Bercos‐Hickey

Nathan

Chen

2017

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The characteristics of African easterly waves coupled to Saharan mineral dust aerosols

Grogan

Thorncroft

2019

The characteristics of African easterly waves (AEWs) and their relationship with synoptic‐scale plumes of Saharan mineral dust (SMD) are examined using 37 summers (July–September, 1980–2016) of 3‐D fields from the Modern‐Era Retrospective analysis for Research and Applications, version 2 (MERRA‐2). The AEWs and SMD plumes are obtained by projecting the kinematic, thermodynamic and dust fields onto the 2–6‐day time‐filtered aerosol optical depth (AOD) near the west coast of North Africa (15°N, 15°W). Results show that the kinematic fields of the AEWs have significant structure north of the African easterly jet (AEJ). For the SMD, signals occur in two regions relative to the AEW over Africa: in the northerlies ahead of the AEW trough in the Sahel (10–20°N), and in the southerlies behind the AEW trough in the Sahara (20–27°N). The SMD signals are emitted on the flanks of a low surface‐pressure system associated with the northern circulation, mixed vertically in ascent regions below 600 hPa and removed by dry and wet deposition processes. Over the eastern Atlantic Ocean, the kinematic fields of the AEWs weaken, which modifies the propagation of the SMD signals. Consequently, the SMD signals from the two sources merge in the Saharan air layer (SAL) over the Atlantic. The direct radiative effects of SMD on the AEWs are also investigated over Africa. The perturbation diabatic heating rate is combined with the perturbation temperature field to infer the structure of the diabatic generation term in the energetics of the AEWs. The implied energetics show that an SMD‐induced generation region of eddy potential energy occurs at mid‐levels (850–650 hPa) north of the AEJ, which replenishes energy that is converted by baroclinic processes.

Dynamical study of three African Easterly Waves in September 2021

Jonville,

Flamant,

Lavaysse

2024

Three convectively active African easterly waves (AEWs) that propagated south of the African easterly jet were observed over the northeast Atlantic Ocean in September 2021. Their evolution is studied using a suite of theoretical frameworks, as well as the European Centre for Medium‐range Weather Forecast reanalyses and satellite‐derived brightness temperature observations. The environment of these AEWs was sampled during the Cloud–Atmospheric Dynamics–Dust Interactions in West Africa campaign near Cape Verde with the goal to assess their potential for developing into tropical cyclones. We highlight the processes that inhibited the development of the first AEW (which evolved into tropical disturbance Pierre‐Henri) and that played a role in the development of the later two into tropical storms Rose and Peter on September 19, 2021. The three AEWs developed a so‐called “marsupial protective” pouch. For Peter and Rose, the pouch was associated with a vertically aligned vortex at low levels and efficiently protected the convective systems inside from dry and dusty air intrusion. The development of this low‐level vortex is associated with an interaction with the monsoon trough for Rose and with a vorticity center associated with a wave propagating north of the African easterly jet (AEJ) in the case of Peter. The presence of a dust flux toward the convective core near the surface is highlighted for Rose and Peter in spite of the presence of the protective marsupial pouch. On the other hand, Pierre‐Henri interacted positively with both the monsoon trough and an AEW north of the AEJ but failed to develop into a tropical cyclone. The wave north of the AEJ brought Saharan air layer air masses inside the pouch that led to a drying of the circulation that may explain the decrease in convective activity.