African Easterly Waves during 2004—Analysis Using Objective Techniques

2007 ◽  
Vol 135 (4) ◽  
pp. 1251-1267 ◽  
Author(s):  
Gareth Berry ◽  
Chris Thorncroft ◽  
Tim Hewson
Keyword(s):  
Composite Structures ◽  
Multiple Scales ◽  
Data Interpretation ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Marked Variability ◽  
West African Coast ◽  
Ultimate Fate ◽  
Second Category

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.

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

2015 ◽  
Vol 143 (12) ◽  
pp. 4975-4995 ◽  
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.

scholarly journals Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic

2017 ◽  
Vol 145 (2) ◽  
pp. 599-616 ◽  
Author(s):  
Abdou L. Dieng ◽  
Saidou M. Sall ◽  
Laurence Eymard ◽  
Marion Leduc-Leballeur ◽  
Alban Lazar
Keyword(s):  
West African ◽  
Dynamical Structure ◽  
African Easterly Waves ◽  
The South ◽  
The West ◽  
Easterly Waves ◽  
West African Coast ◽  
African Coast ◽  
Tropical Depressions ◽  
The Relationship

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 40°W 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.

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

2009 ◽  
Vol 90 (8) ◽  
pp. 1137-1156 ◽  
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

The Potential Vorticity Structure and Dynamics of African Easterly Waves

2020 ◽  
Vol 77 (3) ◽  
pp. 871-890 ◽  
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.

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

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.

scholarly journals Revisiting the connection between African Easterly Waves and Atlantic tropical cyclogenesis

2017 ◽  
Vol 44 (1) ◽  
pp. 587-595 ◽  
Author(s):  
James O. Russell ◽  
Anantha Aiyyer ◽  
Joshua D. White ◽  
Walter Hannah
Keyword(s):  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Easterly Waves

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

2011 ◽  
Vol 139 (9) ◽  
pp. 2704-2722 ◽  
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.

scholarly journals North Atlantic Hurricanes Contributed by African Easterly Waves North and South of the African Easterly Jet

2008 ◽  
Vol 21 (24) ◽  
pp. 6767-6776 ◽  
Author(s):  
Tsing-Chang Chen ◽  
Shih-Yu Wang ◽  
Adam J. Clark
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Monsoon Trough ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
West African Coast ◽  
Atlantic Hurricanes ◽  
Population Ratio

Abstract A majority of tropical cyclones in the North Atlantic develop from African easterly waves (AEWs), which originate along both the southern and northern flanks of the midtropospheric African easterly jet (AEWS and AEWn, respectively). The purpose of this note is to identify the contribution of AEWSs and AEWns to North Atlantic tropical cyclones that develop from AEWs. Applying a manual backtracking approach to identify the genesis locations of AEWS, it was found that the population ratio of tropical cyclones formed from AEWSs to those formed from AEWns is 1:1.2. Because the population ratio of AEWSs to AEWns is 1:2.5, the conversion rate of the former AEWS to tropical cyclones is twice as effective as the latter waves. In addition, it was found that AEWns travel farther and take longer to transform into tropical cyclones than AEWSs, which is likely because the AEWns are drier and shallower than AEWSs. An analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough over West Africa provides moisture and enhances low-level vorticity for both AEWns and AEWSs as they move off the West African coast. The monsoon trough appears to be of particular importance in supplying AEWns with enough moisture so that they have similar properties to AEWSs after they have traveled a considerable westward distance across the tropical Atlantic.

scholarly journals Electrically Active Hot Towers in African Easterly Waves prior to Tropical Cyclogenesis

2010 ◽  
Vol 138 (3) ◽  
pp. 663-687 ◽  
Author(s):  
Kenneth D. Leppert ◽  
Walter A. Petersen
Keyword(s):  
Brightness Temperature ◽  
Vertical Motion ◽  
Meridional Wind ◽  
Tropical Cyclogenesis ◽  
Tropical Storms ◽  
African Easterly Waves ◽  
Reanalysis Dataset ◽  
Easterly Waves ◽  
Low Level ◽  
Upper Level

Abstract It has been hypothesized that intense convective-scale “hot” towers play a role in tropical cyclogenesis via dynamic and thermodynamic feedbacks on the larger-scale circulation. In this study the authors investigate the role that widespread and/or intense lightning-producing convection (i.e., electrically hot towers) present in African easterly waves (AEWs) may play in tropical cyclogenesis over the east Atlantic Ocean. The 700-hPa meridional wind from the NCEP–NCAR reanalysis dataset was analyzed to divide the waves into northerly, southerly, trough, and ridge phases. The AEWs were subsequently divided into waves that developed into tropical storms (i.e., developing) and those that did not develop into tropical storms (i.e., nondeveloping). Finally, composites were created using various NCEP variables, lightning data gathered with the Zeus network and worldwide lightning location network (WWLLN), and brightness temperature data extracted from the NASA global-merged infrared brightness temperature dataset. Results indicate that in all regions examined the developing waves seem to be associated with more widespread and/or intense lightning-producing convection. This increased convection associated with the developing waves might be related to the increased midlevel moisture, low-level vorticity, low-level convergence, upper-level divergence, and increased upward vertical motion found to be associated with the developing waves. In addition, the phasing of the convection with the AEWs as they move from East Africa to the central Atlantic shows some variability, which may have implications for tropical cyclogenesis.

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