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On the Relationship between African Easterly Waves and the African Easterly Jet

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  • 1 LTHE, Université de Grenoble, Grenoble, France
  • | 2 LEGOS, Université de Toulouse, Toulouse, France
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Abstract

This idealized modeling study investigates how convectively triggered African easterly waves (AEWs) are influenced by the intraseasonal variability of the African easterly jet (AEJ). A set of 10-day averaged zonally varying basic states is constructed with the NCEP-2 reanalysis (1979–2006). A primitive equation model is used to simulate linear AEWs on each of these basic states using the same idealized convective heating localized over the Darfur mountains as an initial trigger. It is shown that the transient response depends strongly on the basic state. With the same trigger, many configurations of the AEJ fail to produce a wave disturbance, while others produce strong easterly wave structures. Necessary conditions for the development of strong waves can be characterized by a strong jet, a strong vertical shear, or a strong and extended potential vorticity reversal. In strong-wave cases the jet is extended to the south and west, and the jet core is aligned with the maximum of surface westerlies, maximizing the vertical shear. The pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the AEJ revealed by an empirical orthogonal function (EOF) analysis of the set of basic states.

Corresponding author address: Stephanie Leroux, LTHE, BP 53, 38 041 Grenoble CEDEX 09, France. Email: stephanie.leroux@hmg.inpg.fr

Abstract

This idealized modeling study investigates how convectively triggered African easterly waves (AEWs) are influenced by the intraseasonal variability of the African easterly jet (AEJ). A set of 10-day averaged zonally varying basic states is constructed with the NCEP-2 reanalysis (1979–2006). A primitive equation model is used to simulate linear AEWs on each of these basic states using the same idealized convective heating localized over the Darfur mountains as an initial trigger. It is shown that the transient response depends strongly on the basic state. With the same trigger, many configurations of the AEJ fail to produce a wave disturbance, while others produce strong easterly wave structures. Necessary conditions for the development of strong waves can be characterized by a strong jet, a strong vertical shear, or a strong and extended potential vorticity reversal. In strong-wave cases the jet is extended to the south and west, and the jet core is aligned with the maximum of surface westerlies, maximizing the vertical shear. The pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the AEJ revealed by an empirical orthogonal function (EOF) analysis of the set of basic states.

Corresponding author address: Stephanie Leroux, LTHE, BP 53, 38 041 Grenoble CEDEX 09, France. Email: stephanie.leroux@hmg.inpg.fr

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