The Structure and Properties of African Wave Disturbances as Observed During Phase III of GATE

Abstract

A compositing method is used to determine the average structure and properties of eight wave disturbances observed over west Africa and the eastern Atlantic during the period 23 August-19 September, 1974, a period marked by well-developed and regular wave activity. The disturbance centers propagated westward in the zone of cyclonic shear to the south of the 700 mb easterly jet, located at 16–17°N. The mean wave- length was about 25M km and the mean period 3.5 days. The mean zonal current satisfied the necessary condition for barotropic instability. The composite disturbance was most intense at 650 mb, being cold core below and warm core above. Two circulation centers were evident at the surface, one located below the upper center and the other displaced 10° to the north at about the latitude of the monsoon trough. When separate composites were constructed for land and ocean stations, the dual centers were found to be primarily a land phenomenon. Distinctive features of the high-level (200 inb) circulation were a strong region of divergence located just ahead of the disturbance center and pronounced regions of anticyclonic and cyclonic vorticity situated several hundred kilometers to the north and south, respectively. Maximum low-level convergence and upward vertical motion were found in the region ahead and slightly south of the center. This was also the region of greatest convective cloud cover and largest precipitation amount. Some minor differences are noted between wave behavior over land and sea. Over the ocean wavelengths were shorter, vorticities were greater at all levels, especially at the surface, and the horizontal wave axis was more tilted at levels close to the core of the mid-tropospheric jet stream. In association with the greater tilt, the northward momentum flux and transformation of zonal kinetic energy to eddy kinetic energy were stronger.