Recent studies of intraseasonal, low frequency variability in midlatitudes of the Northern Hemisphere have strongly suggested that the probability density distributions of the planetary long waves (k = 2 to 4) are significantly bimodal. In contrast, the speed, horizontal and vertical shear of the zonal mean flow possess unimodal probability density distributions. The physical basis for such partitioning has not yet clear. The possibility that the bimodal structures are an artifact of data stratification method or numerical truncation during assimilation has not yet been discounted either. The present study addresses these issues through further dignostic analyses of observational data and numerical model simulations. The Northern Hemisphere data would be examined to delineate the processes involved in transitions between the two modes for the planetary long waves and analysis extended to include the Southern Hemisphere data. Output from selected long integrations of a general circulation model simulations will be analysed to determine the probability density distribution functions of model planetary long waves. The possible influence of bimodality on the predictive skill of global numerical forecasts would be evaluated. This work would contribute to our basic understanding of the dynamical process important in intraseasonal climate variability.
