The Origin of African Wave Disturbances during Phase III of GATE

Abstract

Spectrum and cross-spectrum analysis are used to determine the source region of the waves observed during the period 23 August–19 September 1974. The spectra are based on time series of 6 h wind observations at 850 and 700 mb for 72 and 55 land stations, respectively, between 50°E and 20°W and 5°S and 25°N. Main findings are as follows: 1) All stations west of 10°E exhibited pronounced spectral peaks at periods of 3–4 days. The peaks were weak or absent at stations cast of 10°E. 2) The main wave amplification took place between 10°E and 0°. A second region of growth appeared near the coast. 3) The cross-spectrum analysis at 700 mb indicated that the initial weak perturbations may have originated as far east as the southern tip of the Red Sea. 4) The region of principal wave growth (0–10°E) was located close to the region of maximum instability of the mid-tropospheric easterly jet stream, as measured by the magnitudes of the vertical and lateral wind shears. 5) Across nearly the full extent of Africa the axis of the jet stream at 700 mb separated a region of poleward flux of westerly momentum from a region of equatorward flux. The associated flux divergence indicated that the easterly jet was a source of kinetic energy for the disturbances. 6) Poleward of 15°N the waves had the vertical tilt characteristic of baroclinic disturbances. No systematic tilt was found south of that latitude where it is known from previous studies that the waves produce a large modulation of rainfall and hence of latent heat release. On the basis of the findings it is proposed that the large wave growth in the region between 0 and 10°E was caused by a combination of two factors: the increased instability of the mid-tropospheric easterly jet in that region, hence the increased release of barotropic and baroclinic energies to the waves; and the better organization of the convection by the enhanced waves allowing condensation heating also to contribute to their growth.