A theoretical and synoptic study of western disturbances

Summary The theoretical part of the paper deals with the stability characteristics of the observed mean zonal current over India and neighbourhood in winter. The analysis shows that the mean zonal current is unstable with respect to a small perturbation superimposed on it.The synoptic part deals with a case study of a western disturbance which developed into a depression. The development of the western disturbance is similar to the development of a baroclinically unstable disturbance. Power spectral analysis of the meridional component of wind shows a periodicity of 810 days for western disturbances. The estimated wavelength of the western disturbance is in good agreement with the theoretically determined most unstable wavelength.