Case Study of an Intense African Easterly Wave

Abstract

The life cycle of an intense African easterly wave (AEW) over the African continent is examined using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, Meteosat satellite images, and synoptic observations. This system, the strongest AEW of 2000, can be tracked from central North Africa into the eastern Atlantic Ocean, where it is associated with the genesis of Hurricane Alberto. Synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity (PV), allowing exploration at the role of multiple scales in the evolution of this AEW. The authors’ analysis promotes the division of the AEW life cycle into three distinctive phases. (i) Initiation: The AEW development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in Sudan. This convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical North Africa. (ii) Baroclinic growth: A low-level warm anomaly, generated close to the initial convection, interacts with a midtropospheric strip of high PV that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. This interaction is reinforced by the generation of subsynoptic-scale PV anomalies by deep convection that is embedded within the baroclinic AEW structure. (iii) West coast development: Near the West African coast, the baroclinic structure weakens, but convection is maintained. The midtropospheric PV anomalies embedded within the AEW merge with one another and with PV anomalies that are generated by convection over topography ahead of the system. These mergers result in the production of a significant PV feature that leaves the West African coast and rapidly undergoes tropical cyclogenesis.

The impact of moist processes on the African easterly jet–African easterly wave system

“…The Thorncroft and Blackburn approach is extended by allowing nonlinear AEW life cycles to evolve on this jet rather than considering linear normal modes. As recent observations of the PV and potential temperature fields associated with AEWs do exhibit marked nonlinear 'wrapping up' of their contours (Berry and Thorncroft, 2005), nonlinearities may be important.…”

“…This is thought to occur by direct transformation of a cold-core AEW into a warm-core system as shown by Pytharoulis (1999), or, as Berry and Thorncroft (2005) have suggested, by the amplification of an AEW, through merger of embedded vortices with one another and with diabatically generated potential vorticity (PV) anomalies associated with topography near the west coast, to produce a significant PV feature on leaving the West African coast that can rapidly undergo tropical cyclogenesis over the Atlantic. But in order to improve the simulation of the WAM, and thus improve our understanding of the intraseasonal variability of the rainfall, we still need to understand some fundamental truths.…”

“…The spatial and temporal resolution of GCMs is insufficient to resolve convection and the parametrizations are hampered by our limited understanding of tropical land-based convection and particularly its triggering, although this seems to be primarily in regions of elevated orography for West Africa, such as the Dharfur region, the Air and Hoggar mountains and the Fouta-Djallon Plateau (Hodges and Thorncroft, 1997;Drobinski et al, 2005;Berry and Thorncroft, 2005).…”

Disagreements in Low-Level Moisture between (Re)Analyses over Summertime West Africa

“…However, it is also the case that meteorological features that favor the development of MCSs can be responsible for similar northward surges of the monsoon flow. In particular, the passage of AEWs across West Africa (Berry and Thorncroft 2005;Cuesta et al 2010) or the formation of tropical plumes is due to interactions with the subtropical jet stream that enhance southerly flow (Roberts and Knippertz 2014).…”

“…Processes that are known to influence the position of the ITD include interactions with the extratropics Vizy and Cook 2009;Knippertz and Todd 2010;Roehrig et al 2011; Denotes Open Access content. Roberts and Knippertz 2014), changes to the strength of the SHL (Parker et al 2005), African easterly waves (AEWs) on the African easterly jet (AEJ) (Berry and Thorncroft 2005;Couvreux et al 2010;Bain et al 2011), convectively generated cold pools (Flamant et al 2007;Cuesta et al 2010;Marsham et al 2008Marsham et al , 2013bGarcia-Carreras et al 2013;Roberts and Knippertz 2014), and eastward-propagating convectively coupled Kelvin waves (Mounier et al 2007;Mekonnen et al 2008;Mera et al 2014). Increased boundary layer moisture south of the ITD, in regions where the monsoon flow is deep, allows for the production of mesoscale convective systems (MCSs).…”