We report on a set of turbulent flow experiments of the Taylor type in which the fluid is contained between a rotating inner circular cylinder and a fixed concentric outer cylinder, focusing our attention on very large Taylor number values, i.e. \[ 10^3 \leqslant T/T_c \leqslant 10^5, \] where Tc is the critical value of the Taylor number T for onset of Taylor vortices. At such large values of T, the turbulent vortex flow structure is similar to the one observed when T – Tc is small and this structure is apparently insensitive to further increases in T. These flows are characterized by two widely separated length scales: the scale of the gap width which characterizes the Taylor vortex flow and a much smaller scale which is made visible by streaks in the form of a ‘herring-bone’-like pattern visible at the walls. These are conjectured to be Görtler vortices which arise as a result of centrifugal instability in the wall boundary layers. Ideas of marginal instability by which we postulate that both the Taylor and Görtler vortex structures are marginally unstable on their own scale seem to provide good quantitative agreement between predicted and observed Görtler vortex spacings.

A simple mathematical model is constructed to describe the regime of flow, extending over a wide range of values of Taylor number, in which turbulent Taylor–Couette flow in the annular region between two coaxial circular cylinders is characterized by the coexistence of steady coherent motion on two widely separated scales. These scales of motion, corresponding to the gap width of the annular region and to a boundary-layer thickness, are each identified as the consequence of a centrifugal instability, and are described as Taylor vortices and Görtler vortices respectively.The assumption that both scales of motion are near marginal stability gives a closure to a pair of coupled eigenvalue problems, and the results of a linear analysis are shown to be in good agreement with many features of experimental observations.

Differences in the basic state over West Africa between wet and dry years are well documented. This study investigates whether there are also observable differences in the easterly waves between wet and dry years and if these differences might be attributed to the changes in the basic state. Contrasting basic states from the rainy seasons of the four wet years (1958-61) and four dry years (1982-85) were derived from the NCEP reanalysis. The basic states served as input for the linear instability model. The model results indicated faster growth rates and greater phase speeds in the wet years. These results were consistent with a wavelet analysis of the 600-mb meridional wind. This analysis showed that waves were stronger and tended to have a greater contribution from the longer periods during the wet years. The differences in the waves appear to be due to the greater horizontal and vertical shear in wet years. The relative importance of these two were assessed using the Charney-Stern necessary condition for instability. It appears that the horizontal shear is more important in causing the differences. Although the baroclinic and barotropic terms were of similar magnitude in dry years, in wet years the barotropic term increased significantly, whereas the baroclinic term did not. Implications of the results for the understanding of interannual and interdecadal rainfall variability over West Africa are discussed.