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

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.

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.

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.

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 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 Environmental Conditions for Tropical Cyclogenesis Associated with African Easterly Waves

SOLA ◽  
2013 ◽  
Vol 9 (0) ◽  
pp. 120-124 ◽  
Author(s):  
Masaki Satoh ◽  
Ryosuke Nihonmatsu ◽  
Hiroyasu Kubokawa
Keyword(s):  
Environmental Conditions ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Easterly Waves

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.

Sign in / Sign up

Export Citation Format

Share Document