DYNAMICAL DECOMPOSITION OF LOW-FREQUENCY TENDENCIES

A nearly complete vorticity equation is used to diagnose the tendency components of the low-frequency variations of the 500-mb streamfunctio n induced by various internal linear-nonlinear interaction processes. With the aid of a special composite technique (''phase-shifting'' meth od) that effectively records the observations in a coordinate system m oving with an identifiable low-frequency pattern, the authors are able to separate the internal interactions that primarily act to make low- frequency waves propagate from those that are mostly responsible for d evelopment/maintenance/decay (''maintenance'' for brevity) of low-freq uency transients. It is found that the low-frequency transients are ma intained primarily by two nonlinear interaction processes: one is the vorticity flux of high-frequency eddies and the other is the interacti on of low-frequency transients and stationary waves. It is also found that an individual propagation tendency component may be much larger t han a maintenance tendency component. In particular, the beta effect a nd the advection of the low-frequency vorticity by the zonally average d climatological wind are the dominant terms among the propagation ten dency components. But there is a great deal of cancellation among the propagation tendency components. As a result, the net magnitude of the tendency components describing propagation is only slightly larger th an those relating to maintenance of low-frequency waves. From a foreca st point of view, both propagation and forcing terms are equally impor tant if an accurate forecast beyond a few days is required.