A Possible Dynamic Mechanism of the Atmospheric 30-60 Day Period Oscillation in the Extratropical Latitude
中高纬大气30—60天低频振荡的一种动力学机制

In this paper, the barotropic local instability of the inhomogeneous basic current is emphasized to study the dynamic mechanism producing the intraseasonal variability in the extratropical latitude. The mean geopotential height fields on 300 hpa for winter and summer of 1980 are adopted as the basic currents respectively. The results indicate that the barotropic local instability rather satisfactorily accounts for the atmospheric 30-60 day periodic oscillation at the middle and high latitudes. This seems to show that the low frequency oscillation is an intrinsic characteristic of the large scale atmospheric motion interacting with the perturbation rather than the consequence of the external forcing. Moreover the basic currents accommodated to the triggering of the intraseasonal oscillation are discussed. The investigation on the initial value problem reveals that the atmospheric 30-60 day periodic oscillation develops stably on the zonal 3-wave mode, and is independent of the initial disturbance as well as the discrepancy of the basic currents between winter and summer.