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  • African Easterly Waves (AEWs)

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    Here are the current Papers & Articles under the research topic African Easterly Waves. Click on the title of a paper you are interested in to go straight to the full paper. Papers and articles covering the basics (ideal for learning) are shown in Green.

    What is an African Easterly Wave?

    African Easterly Waves - An Educational Guide

    African Easterly Wave Variability and Its Relationship to Atlantic Tropical Cyclone Activity
    2000 paper. Abstract:
    Automatic tracking of vorticity centers in European Centre for Medium-Range Weather Forecasts analyses has been used to develop a 20-yr climatology of African easterly wave activity. The tracking statistics at 600and 850 mb confirm the complicated easterly wave structures present over the African continent. The rainy zone equatorward of 158N is dominated by 600-mb activity, and the much drier Saharan region poleward of 158Nismore dominated by 850-mb activity. Over the Atlantic Ocean there is just one storm track with the 600- and 850-mb wave activity collocated. Based on growth/decay and genesis statistics, it appears that the 850-mb waves poleward of 158N over land generally do not get involved with the equatorward storm track over the ocean.Instead, there appears to be significant development of 850-mb activity at the West African coast in the rainy zone around (108N, 108W), which, it is proposed, is associated with latent heat release.Based on the tracking statistics, it has been shown that there is marked interannual variability in African easterly wave (AEW) activity. It is especially marked at the 850-mb level at the West African coast between about 108and 158N, where the coefficient of variation is 0.29. For the period between 1985 and 1998, a notable positive correlation is seen between this AEW activity and Atlantic tropical cyclone activity. This correlation is particularly strong for the post reanalysis period between 1994 and 1998. This result suggests that Atlantic tropical cyclone activity may be influenced by the number of AEWs leaving the West African coast, which have significant low-level amplitudes, and not simply by the total number of AEWs

    Analysis of African Easterly Wave Structures and Their Role in Influencing Tropical Cyclogenesis
    Published Sept 2009
    Composite structures of African easterly waves (AEWs) that develop into named tropical cyclones in the Atlantic are compared and contrasted with non developing AEWs using the 40-yr ECMWF Re-Analysis(ERA-40) data and satellite brightness temperature between 1979 and 2001. Developing AEWs are characterized by a more distinctive cold-core structure two days before reaching the West African coast. As they move westward, the convective activity increases further in the vicinity of the Guinea Highlands region. At the same time the AEW trough increases its vorticity at low levels consistent with a transformation toward a more warm-core structure before it reaches the ocean. As the AEW moves over the ocean convection is maintained in the trough, consistent with the observed tropical cyclogenesis. The non developing AEW has a similar evolution before reaching the coast except that the amplitudes are weaker and there is less convective activity in the Guinea Highlands region. The non developing AEW composite has a more prominent dry signal just ahead of the AEW trough at mid- to upper levels. It is argued that the weaker west coast development (i.e., reduced convective activity and reduced spin up at low levels) combined with the closer proximity of the trough to mid- to upper-level dry air aloft are consistent with the non development.The most intense non developing AEWs were characterized by more intense convection and stronger mid-and low-level synoptic circulations at the West African coast than the developing AEWs. The analysis strongly suggests that the lack of development was due to the presence of dry mid- to upper-level air just ahead of the AEW trough that may have been enhanced because of equatorward advection of dry air by the AEW itself.

    Analysis of Strengthening and Dissipating Mesoscale Convective Systems Propagating off the West African Coast

    Published Aug 2014.

    A large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the role of the ocean surface conditions in their evolution.Satellite infrared brightness temperature and meteorological radar data over seven summer seasons be-tween 1993 and 2006 are used to subjectively select 20 cases of strengthening and dissipating MCSs in the vicinity of the Senegal coast. With these observed MCSs, a lagged composite analysis is then performed using Interim ECMWF Re-Analysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR).It is shown that the strengthening MCS is generally preceded by prior passage of an AEW near the West African coast. This previous wave trough is associated with a convective cyclonic circulation in the low and middle troposphere, which enhances the southwesterly flow and then provides humidity to the strengthening MCS, located in the vicinity of the subsequent AEW trough. This is favored by the contraction of the wavelength associated with the two troughs. The sea surface contributes to the MCS enhancement through surface evaporation flux. But this contribution is found to be less important than advection of humidity from the previous wave trough. These conditions are almost not found in the dissipating MCS cases, which dissipate in a dry environment dominated by a subsident and anticyclonic circulation, with generally no interaction with a previous wave trough.

    Case Study of an Intense African Easterly Wave

    Published Sept 2004.

    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 of2000, 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 amid tropospheric 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 tropica lcyclogenesis.

    Characterization of convective systems and their association with African easterly waves
    Published May 2017.
    This study investigates the relationship between African easterly waves (AEWs) and different types of deep
    convection. It is known that AEWs impact the development of deep convection over tropical North Africa and tropical cyclone
    formation over the eastern Atlantic. However, the process of how AEWs interact with deep convection is not well understood.
    Composite analysis based on a 24-year data set of cloud systems (CS) from the International Satellite Cloud Climate Project
    shows that the relationship changes with various types of convection over this region. This phase change relationship analysis
    may shed light into the dynamics of AEWs and improve the ability of forecasters to anticipate associated rainfall over the
    Sahel. Weak and disorganized convective systems (WDCSs; 50 km < radius < 100 km) are most common within the southerly
    phase of the AEWs over East Africa. Mesoscale convective systems (MCSs) with cloud radii >100 km increase in frequency
    within and to the west of the AEW-trough zone. MCSs are common features of summer in northwestern Africa. Our results
    indicate that the association between AEWs and deep convection is different and changes across North Africa. Weak AEWs
    over East Africa have a stronger relationship with WDCSs, while mature AEWs over West Africa have more MCS activity.
    This evolution suggests that the organization of convection from WDCS to MCS may play a critical role in AEW development.
    This hypothesis contrasts the traditional view that treats convection uniformly.

    Do West African thunderstorms predict the intensity of Atlantic hurricanes?
    Published April 2015.
    Since 85% of all major Atlantic hurricanes originate as thunderstorm clusters in equatorial Africa, we have investigated the connection between these African thunderstorms and the consequent development of these disturbances into tropical storms. We have analyzed Meteosat infrared cloud top temperature data to determine the areal coverage of cold cloud tops over a 6 year period from 2005 to 2010.In addition, hurricane statistics from the same period (intensity, date of generation, location, and maximum winds) were obtained from the National Hurricane Center database. We first show that the areal coverage of cold clouds (with brightness temperatures Tb<50°C) in tropical Africa is a good indicator of the monthly number of African Easterly Waves (AEWs) leaving the west coast of tropical Africa. Furthermore, the AEWs that develop into tropical storms have a significantly larger area covered by cold cloud tops compared with non developing waves. Finally, we show that on a storm-by-storm basis, the cold cloud coverage in West Africa is positively correlated (r= 0.57) with the accumulated cyclone energy of the future tropical cyclones that develop out of these waves

    Effects of Saharan Dust on the Linear Dynamics of African Easterly Waves
    Published Nov 2015.
    The direct radiative effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs) are examined analytically and numerically. The analytical analysis combines the thermodynamic equation with a dust continuity equation to form an expression for the dust-modified generation of eddy available potential energy GE. The dust-modified GE is a function of the transmissivity and spatial gradients of the dust, which are modulated by the Doppler-shifted frequency. The expression for GE predicts that for a fixed dust distribution, the wave response will be largest in regions where the dust gradients are maximized and the Doppler-shifted frequency vanishes. The numerical analysis uses the Weather Research and Forecasting (WRF) Model coupled to an online dust model to calculate the linear dynamics of AEWs. Zonally averaged basic states for wind, temperature, and dust are chosen consistent with summertime conditions over North Africa. For the fastest-growing AEW, the dust increases the growth rate from;15% to 90% for aerosol optical depths ranging from t51.0 to t52.5. A local energetics analysis shows that fort51.0, the dust increases the maximum barotropic and baroclinic energy conversions by; 50% and; 100%, respectively. The maxima in the generation and conversions of energy are collocated and occur where the meridional dust gradient is maximized near the critical surface—that is, where the Doppler-shifted frequency is small, in agreement with the prediction from the analytical analysis.

    Internal processes within the African Easterly Wave system
    Published 2014.
    The internal processes within an African Easterly Wave (AEW) system, involving mass,dynamic and water vapour fields are investigated using ERA-I reanalysis, in order to highlight the interactions between convection and AEWs. The budgets of heat, moisture and momentum are analysed during the different phases of AEWs detected using synoptic-scale precipitable water anomalies as proposed by Poanetal.(2013). The strong climatological meridional gradient of moisture present in the Sahel impacts the shape of the apparent heat source and humidity sink. AEW events over the Sahel are associated with a meridional shift of the intertropical convergence zone (ITCZ). Large exchanges of momentum by small-scale convective transport are also highlighted between the low- and mid-levels, contributing to the reinforcement of the AEW circulation at 600 hPa and the damping of the monsoon flow. This also appears as a possible mechanism for the vertical tilt of the meridional wind associated with AEWs. Heat budget computation, in the southern flank of the West African Heat-Low (HL) region where such AEWs occur, reveals that the heating anomalies are mainly driven by the horizontal advections. The vertical circulation acts as a precursor, which initiates the heat transport in the lower troposphere. However, weaker, turbulent mixing also participates in the development of these anomalies, especially in the boundary layer. These budgets ultimately allow the distinct contributions of diabatic and adiabatic processes to be determined.

    The Genesis of African Easterly Waves by Upstream Development
    Published Oct 2013.
    A genesis mechanism for African easterly waves (AEWs) is proposed. In the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development, it is proposed that new AEWs can be generated upstream of existing AEWs. A local eddy kinetic energy budget of the AEW that ultimately became Hurricane Alberto (2000) demonstrates that upstream development explains its genesis more convincingly than previous theories of AEW genesis. The energetics and ageostrophic secondary circulation of a composite AEW are consistent with a new AEW forming as a result of this mechanism. Some strengths and weaknesses of upstream development as a paradigm for AEW genesis are discussed with respect to other potential mechanisms.

    The Influence of the MJO on Upstream Precursors to African Easterly Waves
    Published Oct 2011.
    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 theAfrican 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 intra-seasonal 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.

    The Role of Convectively Coupled Atmospheric Kelvin Waves on African Easterly Wave Activity
    Published Oct 2012.
    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 2000highlights 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.

    Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic
    Published Feb 2017.
    In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems). By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 408W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.

    West African Storm Tracks and Their Relationship to Atlantic Tropical Cyclones
    Published June 2017.
    The automatic tracking technique used by Thorncroft and Hodges has been used to identify coherent vorticity structures at 850 hPa over West Africa and the tropical Atlantic in the 40-yr ECMWF Re-Analysis. The presence of two dominant source regions, north and south of 15°N over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity. There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall, and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2–6-day-filtered meridional wind, which provides a synoptic-scale measure of African easterly wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual time scales.


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