EASTWARD MOVEMENT OF MIDLATITUDE LOW FREQUENCY WAVES AND THEIR INTERACTION WITH SYSTEM DOWNSTREAM*

Within the framework of forcing/dissipation KDV dynamics, the effect is numerically studied of successive east-travelling troughs on quasi-stationary high's ridges downstream. Results show that two types of low-frequency transiency are available, depending on the intensities of down-drifting troughs. One is that transiency occurs in strength, i.e., the ridge undergoes transient change in intensity with unchanged position; the other is that transiency takes place in pattern, viz., the breakdown of the quasistationary character of the ridge's posi-tion, meaning that the ridge is replaced by a trough, followed by multiple transformation between ridge and trough.