Nonmodal Barotropic Dynamics of the Intraseasonal Disturbances

Abstract

The intraseasonal disturbances and their relation to the corresponding time-mean flows at 500 mb of nine boreal winters me investigated. The barotropic normal mode instability properties of the seasonal mean flow are found not to be indicative of the intensity of the intraseasonal variability. An application of the dynamical empirical orthogonal function (DEOF) analysis to two distinctly different seasons reveals that only a small fraction of the observed intraseasonal variance is identifiable with the unstable normal modes as individual entities. Each of the few leading DEOFs mainly consists of a slightly modified stable normal mode. The nonmodal growth of the observed intraseasonal disturbances is analyzed in the context of optimal modes with respect to the corresponding seasonal mean flows for different optimization times. The amplifying 1-day optimal structures are much more in number and have much greater growth rate than the unstable normal modes. Several narrow bands of such optimal modes are particularly prevalent. The most pronounced band has a characteristic time of about 20 days and has a localized structure over the Atlantic. The daily variation of the kinetic energy associated with all the 1-day amplifying optimal structures supports the notion that the nonmodal growth is important for the intraseasonal disturbances in winter. The evolution of all the amplifying optimal modes for a longer optimization time such as τ = 4 days over a period of τ does not describe the variation of the observed disturbances nearly as well. The synoptic-scale eddy forcing of the intraseasonal disturbances contributes significantly to the intraseasonal variability in the winter hemisphere.