Characteristics of African easterly waves associated with tropical cyclogenesis in the Cape Verde Islands region in July–August–September of 2004–2008

Abstract African easterly waves (AEWs) are identified in numerical model analyses using an objective technique based on the 700-hPa streamfunction field. This method has been developed to (i) reduce the amount of manual data interpretation, (ii) reduce the likelihood of unrelated phenomena being identified as AEWs, and (iii) facilitate completely objective comparisons between AEWs with different structures on multiple scales, in order to describe their variability. Results show this method performs well when compared to methods of AEW identification used in previous studies. The objective technique is used to analyze all AEWs that originated over tropical North Africa during July–September (JAS) 2004. Results indicate that the “average” AEW in this period bears a close resemblance to composite structures from previous research. However, there is marked variability in the characteristics and ultimate fate of AEWs. Most AEWs of JAS 2004 are first identified east of the Greenwich meridian and develop as they move westward. Mature structures over the African continent varied, ranging from isolated potential vorticity maxima confined equatorward of the objectively defined African easterly jet to broad cross-jet structures symptomatic of both baroclinic and barotropic growth. As many as 80% of the cases fell into the second category. After leaving the West African coast, 45% of the AEWs in JAS 2004 were associated with tropical cyclogenesis in either the Atlantic or Pacific Ocean basins.

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The Saharan Air Layer and the Fate of African Easterly Waves—NASA's AMMA Field Study of Tropical Cyclogenesis

Bulletin of the American Meteorological Society ◽

10.1175/2009bams2728.1 ◽

2009 ◽

Vol 90 (8) ◽

pp. 1137-1156 ◽

Cited By ~ 104

Author(s):

Edward J. Zipser ◽

Cynthia H. Twohy ◽

Si-Chee Tsay ◽

K. Lee Thornhill ◽

Simone Tanelli ◽

...

Keyword(s):

Field Study ◽

Tropical Cyclogenesis ◽

African Easterly Waves ◽

Easterly Waves ◽

Saharan Air Layer

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The Potential Vorticity Structure and Dynamics of African Easterly Waves

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-19-0019.1 ◽

2020 ◽

Vol 77 (3) ◽

pp. 871-890 ◽

Cited By ~ 1

Author(s):

James O. H. Russell ◽

Anantha Aiyyer

Keyword(s):

Potential Vorticity ◽

Deep Convection ◽

Wave Structure ◽

Wave Model ◽

Tropical Cyclogenesis ◽

African Easterly Waves ◽

Moist Convection ◽

Easterly Waves ◽

Stratiform Clouds ◽

Diabatic Forcing

Abstract The dynamics of African easterly waves (AEWs) are investigated from the perspective of potential vorticity (PV) using data from global reanalysis projects. To a leading order, AEW evolution is governed by four processes: advection of the wave-scale PV by background flow, advection of background PV by the AEW, diabatic forcing due to wave-scale moist convection, and coupling between the wave and background diabatic forcing. Moist convection contributes significantly to the growth of AEWs in the midtroposphere, and to both growth and propagation of AEWs near the surface. The former is associated with stratiform clouds while the latter with deep convection. Moist convection helps maintain a more upright AEW PV column against the background shear, which makes the wave structure conducive for tropical cyclogenesis. It is also argued that—contrary to the hypothesis in some prior studies—the canonical diabatic Rossby wave model is likely not applicable to AEWs.

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Variability and Evolution of African Easterly Wave Structures and Their Relationship with Tropical Cyclogenesis over the Eastern Atlantic

Monthly Weather Review ◽

10.1175/mwr-d-15-0106.1 ◽

2015 ◽

Vol 143 (12) ◽

pp. 4975-4995 ◽

Cited By ~ 17

Author(s):

Alan Brammer ◽

Chris D. Thorncroft

Keyword(s):

Coastal Region ◽

Lower Troposphere ◽

Absolute Number ◽

West African ◽

Tropical Cyclogenesis ◽

African Easterly Waves ◽

The West ◽

Easterly Waves ◽

West African Coast ◽

African Coast

Abstract African easterly waves (AEWs) are objectively tracked between West Africa and the tropical Atlantic based on the CFSRv2 data for 1979 to 2012. The characteristics of the troughs of the AEWs at the West African coast are explored and related to whether they favor tropical cyclogenesis over the eastern Atlantic. A logistic regression model was used to determine the optimum combination of predictors that relate AEW characteristics to tropical cyclogenesis. The most skillful model for genesis over the eastern Atlantic consisted of four variables of the AEWs dynamics over the coastal region and the absolute number of days from the peak in the AEW season. Using this diagnostic an equal number of favorable developing and nondeveloping waves were compared through a composite difference analysis. Favorable developing waves had significantly higher moisture content in the lower troposphere to the northwest of the trough as they exited the West African coast compared to favorable nondeveloping waves. Trajectory analysis for all the waves revealed that as the AEWs transition over the West African coast the troughs are typically open to the environment ahead and to the northwest of the trough. For developing waves this means that moist air is ingested into the lower levels of the system, while for nondeveloping waves dry air is ingested. At this point in the AEW life cycle this difference may be fundamental in determining whether a favorable wave can develop or not.

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Favorable monsoon environment over eastern Africa for subsequent tropical cyclogenesis of African easterly waves

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0339.1 ◽

2021 ◽

Author(s):

Kelly M. Núñez Ocasio ◽

Alan Brammer ◽

Jenni L. Evans ◽

George S. Young ◽

Zachary L. Moon

Keyword(s):

Large Scale ◽

West African Monsoon ◽

Eastern Africa ◽

Tropical Cyclogenesis ◽

African Easterly Waves ◽

Easterly Waves ◽

Convective Systems ◽

Mesoscale Convective ◽

Vertically Aligned ◽

Somali Jet

AbstractEastern Africa is a common region of African easterly wave (AEW) onset and AEW early-life. How the large-scale environment over east Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared to non-developing AEWs. Using a 21-year August-to-September (1990-2010) climatology of AEWs, differences in the large-scale environment between developers and non-developers are identified and are propose to be used as key predictors of subsequent tropical cyclone formation and could informtropical cyclogenesis prediction. TC precursors when compared to non-developing AEWs experience: an anomalously active West African Monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet (AEJ). These large-scale conditions are linked to near-trough attributes of developing AEWs which favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, deeper wave pouch, and arguably more AEW and Mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or west Africa. Developing AEWs are more likely to be southern-track AEWs than non-developing AEWs.

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Interactions of Diabatic Heating in Convective Superbursts with Energy Conversion Processes in the Genesis of Cape Verde Hurricanes from African Easterly Waves

Monthly Weather Review ◽

10.1175/2011mwr3621.1 ◽

2012 ◽

Vol 140 (3) ◽

pp. 748-773 ◽

Cited By ~ 3

Author(s):

Robert S. Ross ◽

T. N. Krishnamurti ◽

S. Pattnaik

Keyword(s):

Energy Conversion ◽

Diabatic Heating ◽

Developmental Stages ◽

Cape Verde ◽

African Easterly Waves ◽

Easterly Waves ◽

Multidisciplinary Analysis ◽

Mesoscale Convective ◽

Barotropic Energy Conversion ◽

Energy Equations

Abstract This paper defines a mechanism for the genesis of tropical cyclones from African easterly waves (AEWs) over the eastern Atlantic, the so-called Cape Verde storms. Convective “superbursts” produce strong diabatic heating, which then strengthens the African easterly jet (AEJ), leading to enhanced barotropic energy conversions, which occur at the critical developmental stages of the system. Diabatic heating is calculated using the Ertel isentropic potential vorticity (IPV) equation, while energy conversions are determined using energy equations first derived by Lorenz. The genesis mechanism is developed from studying Hurricane Bill (2009), as well as Tropical Storm Debby, Hurricane Helene, and a nondeveloping AEW, all from the 2006 NASA African Monsoon Multidisciplinary Analysis (NAMMA) field experiment, using the NCEP Final (FNL) analyses and the Advanced Research Weather Research and Forecasting model (WRF-ARW) simulations. A striking and singular maximum in the diabatic heating due to the convective superburst is shown to precede by 24–36 h a pronounced maximum in positive barotropic energy conversion, which is demonstrated to occur simultaneously with the strengthening of the AEJ. The maximum in barotropic energy conversion is documented to occur in the developmental stages of the system, typically in the depression or early storm stages. A physical mechanism is developed to explain how a mesoscale convective superburst can lead subsequently to an enhanced synoptic-scale AEJ over the eastern Atlantic, an enhanced jet that is critical to the genesis mechanism. The findings agree with cited idealized studies by other investigators who found that moist AEWs grow 3 times stronger than dry waves as a result of faster AEJ development and larger barotropic energy conversions.

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Revisiting the connection between African Easterly Waves and Atlantic tropical cyclogenesis

Geophysical Research Letters ◽

10.1002/2016gl071236 ◽

2017 ◽

Vol 44 (1) ◽

pp. 587-595 ◽

Cited By ~ 14

Author(s):

James O. Russell ◽

Anantha Aiyyer ◽

Joshua D. White ◽

Walter Hannah

Keyword(s):

Tropical Cyclogenesis ◽

African Easterly Waves ◽

Easterly Waves

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The Madden–Julian Oscillation’s Influence on African Easterly Waves and Downstream Tropical Cyclogenesis

Monthly Weather Review ◽

10.1175/mwr-d-10-05028.1 ◽

2011 ◽

Vol 139 (9) ◽

pp. 2704-2722 ◽

Cited By ~ 56

Author(s):

Michael J. Ventrice ◽

Chris D. Thorncroft ◽

Paul E. Roundy

Keyword(s):

Large Scale ◽

Tropical Cyclogenesis ◽

Tropical Atlantic ◽

Madden Julian Oscillation ◽

African Easterly Waves ◽

Tropical Africa ◽

Easterly Waves ◽

African Easterly Wave ◽

Easterly Wave ◽

Equatorial Rossby Waves

The influence of the Madden–Julian oscillation (MJO) over tropical Africa and Atlantic is explored during the Northern Hemisphere summer months. The MJO is assessed by using real-time multivariate MJO (RMM) indices. These indices divide the active convective signal of the MJO into 8 phases. Convection associated with the MJO is enhanced over tropical Africa during RMM phases 8, 1, and 2. Convection becomes suppressed over tropical Africa during the subsequent RMM phases (phases 3–7). African convective signals are associated with westward-propagating equatorial Rossby waves. The MJO modulates African easterly wave (AEW) activity. AEW activity is locally enhanced during RMM phases 1–3 and suppressed during RMM phases 6–8. Enhanced AEW activity occurs during periods of enhanced convection over tropical Africa, consistent with stronger or more frequent triggering of AEWs as well as more growth associated with latent heat release. Enhanced AEW activity occurs during the low-level westerly wind phase of the MJO, which increases the cyclonic shear on the equatorward side of the AEJ, increasing its instability. Atlantic tropical cyclogenesis frequency varies coherently with the MJO. RMM phases 1–3 show the greatest frequency of tropical cyclogenesis events whereas phases 7 and 8 show the least. RMM phase 2 is also the most likely phase to be associated with a train of three or more tropical cyclones over the tropical Atlantic. This observed evolution of tropical cyclogenesis frequency varies coherently with variations in AEW activity and the large-scale environment.

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Merger of African easterly waves and formation of Cape Verde storms

Quarterly Journal of the Royal Meteorological Society ◽

10.1002/qj.2439 ◽

2014 ◽

Vol 141 (689) ◽

pp. 1306-1319 ◽

Cited By ~ 12

Author(s):

Isaac Hankes ◽

Zhuo Wang ◽

Gan Zhang ◽

Cody Fritz

Keyword(s):

Cape Verde ◽

African Easterly Waves ◽

Easterly Waves

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Spatial and Temporal Variability of the Three-Dimensional Flow around African Easterly Waves

Monthly Weather Review ◽

10.1175/mwr-d-16-0454.1 ◽

2017 ◽

Vol 145 (7) ◽

pp. 2879-2895 ◽

Cited By ~ 4

Author(s):

Alan Brammer ◽

Chris D. Thorncroft

Keyword(s):

Correlation Analysis ◽

West Africa ◽

Trajectory Analysis ◽

Cape Verde ◽

Spatial And Temporal Variability ◽

African Easterly Waves ◽

Environmental Characteristics ◽

Easterly Waves ◽

Source Regions ◽

The Impact

This study presents a large-scale trajectory analysis of African easterly waves (AEWs) across West Africa and the eastern Atlantic. Back trajectories were initialized at multiple pressure levels from in and around the vortex center of the AEW troughs to reveal the source regions of environmental inflow. The trajectory analysis highlights a changing influence of environmental air on AEW troughs. Over West Africa, monsoonal flow dominates with source regions of air from the southwest and east to northeast influencing the trough. As the AEW troughs leave West Africa, flow from the northwest becomes increasingly important. Cluster analysis highlighted that the contribution of trajectories from the northwest increased as the AEW troughs move westward and that this cluster also had high variability in environmental characteristics. Correlation analysis of outgoing longwave radiation around the troughs with environmental characteristics 72 h earlier was conducted on 443 AEWs. This analysis reveals that the impact of the various source regions on convective activity within the AEW troughs is consistent with the cluster trajectory analysis. While the AEW troughs were over West Africa, convection was sensitive to midlevel equivalent potential temperature [Formula: see text] around the trough and to the northeast of the trough axis. Over the West African coast and Cape Verde basin, the correlation analysis captures the changing flow regime with sensitivity to [Formula: see text] west of the trough axis at midlevels and northwest at low levels. These results highlight that the cool and dry low-level northerly trade winds over the Canary and Cape Verde basin can be a crucial influence on AEWs as they leave West Africa.

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