Trains of African easterly waves and their relationship to tropical cyclone genesis in the Eastern Atlantic (original) (raw)
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Analysis of African Easterly Wave Structures and Their Role in Influencing Tropical Cyclogenesis
Monthly Weather Review, 2010
Composite structures of African easterly waves (AEWs) that develop into named tropical cyclones in the Atlantic are compared and contrasted with nondeveloping 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 nondeveloping 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 nondeveloping...
Atmosphere, 2021
This study revisited the association of African easterly waves (AEWs) to Atlantic tropical cyclone (TC) development using weather states (WSs) from the International Satellite Cloud Climatology Project, National Hurricane Center best track hurricane data (HURDAT2), and reanalysis products. The WS data are used as a proxy for two different types of deep convection. This study covers July–October 1984–2009. Statistical analysis based on HURDAT2 and objectively tracked AEWs has shown that a small fraction (~20%) of the AEWs that propagate from Africa serve as TC precursors. About 80% of the AEWs from the continent were non-developing. As in the past work, our study showed an important difference between developing and non-developing AEWs. Composites based on developing AEWs revealed well-organized large scale deep convection (one type, composed of mesoscale systems and thick anvil clouds) is tightly coupled to the AEW trough, while scattered, less well-organized deep convection (second...
caps.ou.edu
As African (Tropical) Easterly Waves (AEW) form in eastern to central Africa, convective storms propagate across Northern Africa. Tropical cyclone genesis is a tough question; however, under stronger tropical wave circumstances, hurricanes are more likely to develop. During summer of 2006, two field experiments called African Monsoon Multidisciplinary Analyses (AMMA) and NASA AMMA gathered valuable tropical upper-air data that is available to further study AEW. The observations permit the distinctions of the stronger AEW to the weaker ones. Meridional wind anomaly plot shows strong signals of AEWs' propagation. The plot will be analyzed to determine the exact date of trough axis passage. Using the dates, Infrared images will be analyzed to determine the differences of developing and non-developing AEWs. This study confirms some of previous studies' findings on structures of AEW. This study shows that the stronger meridional wind relates to the Cape Verde Storms. All the lesser meridional wind waves did not develop. Satellite imagery shows that the differences of convective cloud fields are not easily noticeable between developing and non-developing waves.
African Easterly Waves and Cyclonic Activity over the Eastern Atlantic: Composite and Case Studies
International Journal of Geophysics, 2011
This study aims to understand the main differences over the African continent and the Eastern Atlantic Ocean between African Easterly Waves (AEWs) associated with Atlantic cyclones (developing AEWs) and non-developing AEWs. A statistical study showed that most of the named cyclones generated near the West African coast have a long lifecycle and all are associated with intense AEWs. Using NCEP/NCAR reanalyses, a composite study of the characteristics of developing AEWs is carried out and compared to those of non-developing AEWs. Developing AEWs exhibit the greatest baroclinic and barotropic conversions which are known to be the main processes involved in AEWs growth suggesting that these AEWs are stronger than the non-developing ones. Moreover, the developing AEWs are characterized by the existence of a relatively more unstable environment over West Africa and the Atlantic Ocean. A case study using rawinsonde data showed that the developing AEW is associated with dynamic and thermodynamic conditions conducive for deep convection and subsequent cyclogenesis compared to the non-developing AEW case.
African Easterly Wave Variability and Its Relationship to Atlantic Tropical Cyclone Activity
Journal of Climate, 2001
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 600 and 850 mb confirm the complicated easterly wave structures present over the African continent. The rainy zone equatorward of 15ЊN is dominated by 600-mb activity, and the much drier Saharan region poleward of 15ЊN is more 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 15ЊN 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 (10ЊN, 10ЊW), 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 10Њ and 15ЊN, 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 postreanalysis 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.
Atmospheric Research, 2011
The most common synoptic-scale disturbances related to cyclogenesis over the tropical north Atlantic Ocean are African easterly waves (AEWs) that originated from the northern African continent. However, most of these waves do not evolve in tropical depressions, storms, or hurricanes. The reasons why only few AEWs develop and the necessary conditions for cyclogenetic evolution are still the subject of intense debate. Tropical cyclogenesis occurring near the Cape Verde Islands in the eastern tropical Atlantic is investigated here with five seasons (July-August-September of 2004-2008) of European Centre for Medium-Range Weather Forecasts analyses, Meteosat-9 images, and National Hurricane Center (National Oceanic and Atmospheric Administration, National Centers for Environmental Prediction) "best track" archives. The nine named storms that first reached tropical depression intensity east of 30°W, and two among six which developed between 30 and 40°W, during these five years evolved from intense AEW troughs, associated with lowlevel cyclonic circulation, weak mid-level anticyclonic Saharan flow to the east, and deep convection near the center of cyclonic vorticity. The cyclogenetic evolution of three AEW troughs, which verified these conditions but failed to develop into named storms, was probably inhibited by unusually dry environment and strong vertical wind shear. The fate of other AEW troughs, which did not satisfy the necessary conditions, is also discussed.
Monthly Weather Review, 2012
This paper explores a three-way interaction between an African easterly wave (AEW), the diurnal cycle of convection over the Guinea Highlands (GHs), and a convectively coupled atmospheric equatorial Kelvin wave (CCKW). These interactions resulted in the genesis of Tropical Storm Debby over the eastern tropical Atlantic during late August 2006. The diurnal cycle of convection downstream of the GHs during the month of August is explored. Convection associated with the coherent diurnal cycle is observed off the coast of West Africa during the morning. Later, convection initiates over and downstream of the GHs during the afternoon. These convective features were pronounced during the passage of the pre-Debby AEW. The superposition between the convectively active phase of a strong CCKW and the pre-Debby AEW occurred shortly after merging with the diurnally varying convection downstream of the GHs. The CCKW–AEW interaction preceded tropical cyclogenesis by 18 h. The CCKW provided a favora...
The Low-Level Structure of African Easterly Waves in 1995
Monthly Weather Review, 1999
The existence of African easterly waves (AEWs) north of the African easterly jet (AEJ) core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the U.K. Meteorological Office global model analysis from the hurricane season of 1995. At Bamako (12.5ЊN, 8.0ЊW) the AEWs were characterized mainly by maximum amplitudes at the level of the AEJ (around 700 mb), whereas at Dakar (14.7ЊN, 17.5ЊW) the waves were characterized by maxima between 850 and 950 mb. The low-level waves to the north of the AEJ arise in association with baroclinic interactions between the negative meridional potential vorticity (PV) gradients in the jet core and the positive low-level gradient of potential temperature, , enhanced by the presence of low-static-stability air north of the AEJ. These waves follow the positive meridional gradients over northern Africa in contrast to the jet-level AEWs that follow the meridional PV gradients at the level of the AEJ. Cross-correlation analysis shows that there is strong coherence between the low-level AEWs and the well-known cold core AEWs that propagate south of the jet, confirming that AEWs are associated with a combined barotropic-baroclinic instability mechanism.
Tropical Atlantic Hurricanes, Easterly Waves, and West African Mesoscale Convective Systems
Advances in Meteorology, 2010
The relationship between tropical Atlantic hurricanes (Hs), atmospheric easterly waves (AEWs), and West African mesoscale convective systems (MCSs) is investigated. It points out atmospheric conditions over West Africa before hurricane formation. The analysis was performed for two periods, June-November in 2004 and 2005, during which 12 hurricanes (seven in 2004, five in 2005) were selected. Using the AEW signature in the 700 hPa vorticity, a backward trajectory was performed to the African coast, starting from the date and position of each hurricane, when and where it was catalogued as a tropical depression. At this step, using the Meteosat-7 satellite dataset, we selected all the MCSs around this time and region, and tracked them from their initiation until their dissipation. This procedure allowed us to relate each of the selected Hs with AEWs and a succession of MCSs that occurred a few times over West Africa before initiation of the hurricane. Finally, a dipole in sea surface temperature (SST) was observed with a positive SST anomaly within the region of H generation and a negative SST anomaly within the Gulf of Guinea. This SST anomaly dipole could contribute to enhance the continental convergence associated with the monsoon that impacts on the West African MCSs formation.