The Energetics of African Wave Disturbances as observed During Phase III of GATE

Abstract

Fields of the meteorological variables in composite wave disturbance constructed for the region from IOOE to 31°W and 1°S to 26°N and for land and ocean subregions are used to diagnose energy transformations in African waves. The composites are based on data contained in the GATE Quick Look Data Set for the period 23 August to 19 September, 1974. The measurements indicate that for the region as a whole the kinetic energy of the waves is maintained almost equally by conversions from zonal kinetic energy and eddy available potential energy. Eddy available potential energy is supplied by the zonal available potential energy at a comparable rate. From the measured conversion rates it is estimated that in the absence of friction the kinetic energy of the waves would double in about 3 days. Measurements for the subregions show that the conversion from zonal to eddy kinetic energy is stronger over the limited oceanic region considered than over the land, while conversely, the conversion of eddy available potential energy to eddy kinetic energy is stronger over the land than over the ocean. The conversion of zonal to eddy available potential energy differs little between the two regions. From these findings, and budgetary considerations, it is inferred that latent beat release in organized convection plays an important role in the wave growth and maintenance in west Africa but not over the adjacent ocean. This conclusion, however, must be regarded as tentative. The distributions of the various energy conversion processes in meridional cross section are considered. The conversions of zonal kinetic and available potential energies to their corresponding eddy energies are characterized by concentrated regions of high values closely associated with the mid-tropospheric easterly jet stream. The conversion of eddy available potential energy to eddy kinetic energy exhibits a complex pattern in which the net conversion is a, small residual. Consequently this conversion cannot be regarded as being determined with the same high degree of reliability as the other two. However, major features of the pattern can be explained on physical grounds.