DIAGNOSTIC STUDY FOR INTRASEASONAL OSCILLATION OF THE MIDDLE-HIGH LATITUDES IN A LOW RESOLUTION GLOBAL SPECTRAL MODEL

In this paper, five-year simulated data from a low-resolution global spectral model with triangular truncalion at wavenumber 10 are analyzed in order to study dynamical features and propagation characteristics of intraseasonal oxillations over the mid-latitudes and the tropical atmosphere. The simulations show that there is the 30-50 day periodic oscillation in the low-resolution spectral model without non-seasonal external forcing,and spatial scale of the intraseasonal oscihations is of the globe.Further analysis finds that propagation characters of intraseasonal oscillations over the mid-latitudes and the tropics are different. The 30-50 day oscillation over the tropics exhibits structure of the velocity potential wave with wavenumber 1 in the latitudinal and the character of the traveling wave eastward at speed of 8 longitudes/day. However, the 30-50 day oscillations in mid-latitude atmosphere exhibit phase and amplitude oscillation of the standing planetary waves and they are related to transform of teleconnection patterns over the mid-latitudes. The energy is not only transferred between the tropics and the middle-high latitudes, but also between different regions over the tropics. Based on the analysis of 5-year band pass filtered data from a 5-layer global spectral model of low-ordet with truncated wavenumber l0,investigation is done of the source of intraseasonal oscillations in the extratropical model atmosphere and its mechanism. Results show that (1) the convective heat transferred eastward along the equator serves as the source of the intraseasonal oxillation both in the tropical and the extratropical atmosphere; (2) the velocity-potential wave of a zonal structure of wavenumber 1 gives rise to oxillation in divergentand convergent wind fields of a dipole-form as seen from the equatorial Indian Ocean to the western Pacificduring its eastward propagation, thus indicating the oscillation in the dipole-form heat soure:e/sink pattertl; (3) the tropical heat-source oscillation is responsible for the variation in phase and intensity of the extratropical stationary wave train, and the interaction between the oscillating low-frequency inertial gravity and stationary Rossby modes that are probably mechanisms for the oscillations ip the middle-high latitudes.