A convectively coupled equatorial Kelvin wave's impact on African easterly wave activity

The role of convectively coupled atmospheric Kelvin waves (CCKWs) on African easterly wave (AEW) variability is explored over tropical Africa during boreal summer. The initiation of the pre-Alberto AEW (2000) is investigated to demonstrate that the initiation occurred during the passage of the convectively active phase of a CCKW. A significant modulation of the African synoptic environment by the convective envelope of the CCKW favors the formation and intensification of AEWs during and after its passage. This environmental modulation is explored using fields of low-level (925-700 hPa) vertical wind shear and 700 hPa zonal wind anomalies.

Each phase of the CCKW modulates the African synoptic environment oppositely. The convectively suppressed phase of the CCKW reduces 925-700 hPa vertical wind shear over Africa, whereas the convectively active phase increases it. Over Africa, the convectively suppressed phase of the CCKW is associated with 700 hPa easterly wind anomalies, while the convectively active phase is associated with westerly wind anomalies. The 700 hPa zonal wind anomalies are strongest equatorward of the African easterly jet (AEJ), which imposes horizontal shear on the AEJ. The suppressed phase of the CCKW reduces the horizontal shear of the AEJ, suggesting that the jet becomes less unstable and vice versa for the convectively active phase.

Anomalous AEW activity develops just after the passage of the convectively active phase of the CCKW over West Africa and during the passage over central-eastern Africa. These two areas of anomalous AEW activity later move back towards the west over the Atlantic and represent a train of AEWs.