Characteristics of North African Easterly Waves During the Summers of 1968 and 1969

Abstract

This study uses both spectral and compositing analyses to determine the structure of North African easterly waves during the summers of 1968 and 1969. The average period and wavelength computed by the power-spectrum method are 4.5 days and 3800 km and for the compositing analysis 3.8 days and 3100 km. Although the period and wavelength of individual waves vary considerably, the spectral results are in qualitative agreement with the compositing results except in extreme northern Africa where eastward-moving disturbances of the same period affect the spectral computations. The maximum surface amplitudes of u, v, p and Td associated with the waves occur about 20N near the mean location of the east-west oriented region of minimum surface pressure. East of the Greenwich meridian the amplitude decreases and the wave motion is barely detectable with surface data near 25E. The compositing results show that the waves influence latitudes between 5 and 3ON and that wave features tilt from southwest to northeast. Both rainfall amount and thunderstorm activity are modulated by the passage of the waves with the rainfall maxima about 50% greater than the corresponding minima. Upper air calculations show that the vertical structure of the waves varies with the mean zonal wind in such a way that the meridional wind patterns tilt in the opposite direction to the mean zonal shear.