Abstract

We examine the association between synoptic waves and convection including their variability during the boreal summer over tropical Africa and tropical eastern Atlantic ocean. These waves include off-equatorial African easterly waves (AEWs) and equatorial Kelvin waves. The study is based on a 21-yr period satellite brightness ­temperature (TB) and ECMWF reanalysis data sets.

Spectral analysis shows a significant variance in the 2-6day range across tropical Africa and the Atlantic. Within the regions of deep convection, this time scale accounts for 25-35% of the total variance. Results show similar amplitude in 2-6 day convective variance over western and eastern Africa, while dynamic measures of AEW activity show stronger amplitudes in the west. Weak AEW dynamical activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern Africa, near Darfur (western Sudan) and Ethiopia, has a role in initiating AEWs westwards. The subsequent development and growth of AEWs in West Africa is associated with stronger coherence with convection there. Results also show large year-to-year variability in convection at the 2-6-day time scale, which tends to vary consistently with the mean convection and dynamical measures of AEW activity over West Africa and the Atlantic, but not over central and eastern Africa.

In addition to being important for precursors of AEWs, the Darfur region is also a source of convection that propagates eastward towards Ethiopia. We suggest that these eastward propagating convective features are associated with Kelvin wave activity. Composite analysis shows a convectively coupled Kelvin wave that propagates between the central-eastern Pacific and eastern Africa. This is further illustrated based on a Kelvin wave event that was triggered over eastern Pacific around 29 July 1987 and that reached Africa 8-9 days later. As the Kelvin wave propagates across different regions over Africa, convective activity deepens and rainfall dramatically increases. When the Kelvin wave passed regions, a sharp decrease in rainfall was observed. The same case study also showed AEW initiation or enhancement in association with the enhanced phase of the Kelvin wave.

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