Easterly waves over Africa. I: the seasonal cycle and contrasts between wet and dry years

Grist, J.P. (2002) Easterly waves over Africa. I: the seasonal cycle and contrasts between wet and dry years. Monthly Weather Review, 130 (2), 197-211. (doi:10.1175/1520-0493(2002)130<0197:EWOAPI>2.0.CO;2).

Record type: Article

Abstract

The nature of African easterly waves (AEWs) and the easterly wave season over West Africa are examined using the National Centers for Environmental Prediction (NCEP) reanalysis. The study is carried out with two objectives in mind. The first goal is to describe the seasonal cycle of wave activity and to determine if it has a distinctive signature. To achieve this, the temporal evolution of wave period, amplitude, and structure are examined with wavelet analysis. This analysis is carried out at a grid point (15°N, 0°) that is at located where AEWs are well developed. The second goal is to determine differences in the wave characteristics and the wave season between wet and dry years.
Regarding the seasonal cycle, it was found, in accordance with previous research, that AEW activity typically occurs between May and October. Disturbances have a periodicity between 4 and 8 days. At the level (600 mb) of the African easterly jet (AEJ) there was a broad peak in the magnitude of the variance over the months of July, August, and September. This may be due to the increased horizontal shear and barotropic instability, which also peaks at that time. There is a greater contribution of variance from longer periods (6.25–7.5 days) in the later half of the summer. At lower levels, the disturbances appear to be confined to periods of 3.75–5.0 days, with the maximum variance in the mean occurring in July. This may be a response to the change in the magnitude of the vertical shear beneath the AEJ on the same timescale.
It was found that the wave season in wet years tended to be longer and more active. This may be due to the more northerly track of the AEJ in wet years. Wet years were also characterized by stronger waves at 600 mb. These stronger waves may be due to the stronger shear around the AEJ in wet years. At the lower level there were less consistent differences between wet and dry years. This may reflect the fact that although the AEJ is weaker in wet years, westerlies beneath it are often stronger. The net effect being that there are not consistent differences in the vertical shear between wet and dry years.

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