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

Abstract

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.

Robust predictors for seasonal Atlantic hurricane activity identified with causal effect networks

“…Besides a direct application of our early April forecast model, we encourage other statistical forecasting groups to investigate whether our newly identified predictors can help to improve their statistical forecast models. Furthermore, the causal links identified here could form the basis for hybrid forecasting techniques where a dynamical forecast ensemble is constrained by selecting only members that adequately reproduce the causal links as demonstrated by Dobrynin et al (2018).…”

A Numerical Study of the Global Formation of Tropical Cyclones

“…For example, TC formation in the North Atlantic basin often shows a strong connection to active tropical waves associated with African Jet (Avila & Pasch, 1992;Landsea, 1993;DeMaria, 1996;Pasch et al, 1998;Molinari et al, 2000;Dieng et al, 2017). In the Northwestern Pacific basin, previous studies showed that tropical cyclogenesis is closely tied to variability in Inter-Tropical Convergence Zone (ITCZ) and monsoon activities (Yanai, 1964;Gray, 1968Gray, , 1982Mark & J., 1993;Harr et al, 1996;Ferreira & Schubert, 1997).…”

Instabilities of low-latitude easterly jets in the presence of moist convection and topography and related cyclogenesis, in a simple atmospheric model

“…It is well-known that the cyclogenesis is most often triggered by depressions due to the African Easterly Waves (AEW) propagating over Atlantic, (e.g. Bracken and Bosart 2000, Brammer and Thorncroft 2015, Dieng et al 2017, and references therein. In turn, it is widely accepted that AEW are related to instabilities of the African Easterly Jet (AEJ), a low-latitude atmospheric easterly jet evolving over the West-African plateau and the Eastern Atlantic Ocean (Burpee 1972, Hsie and Cook 2005, 2008, Wu et al 2012.…”