The existence of the intraseasonal abrupt and the intraseasonal gradual northward motions of the western Pacific subtropical high has been known. However, the explanation for these phenomena has not been found. In this article we suggest that they are associated with the given external forcing. Utilizing the highly truncated spectral model of barotropic atmosphere, in which the two different truncated trigonometric function sets describing the intraseasonal abrupt and the intraseasonal gradual change of the western Pacific subtropical high are respectively retrieved with observational data, we have shown that since the external thermal forcing waveforms lead to the positive anomaly around the Philippines and the east part of the Tibet Plateau, the atmospheric circulation gives rise to corresponding responded waveforms in which there are wave—wave and wave—mean flow interactions. The existence of these interactions leads to the mean flow jump from one stable equilibrium to another equilibrium when the thermal forcing exceeds certain critical value. This evolution closely parallels that of the intraseasonal northward jump of the western Pacific subtropical high during summer. On the contrary, since the external thermal forcing around the Philippines and the east part of the Tibet Plateau being of negative anomaly, there are very weak wave—wave and wave—mean flow interactions, which brings about single equilibrium during the intraseasonal evolution process of the western Pacific subtropical high, among the responded waveforms of atmospheric circulation. In such case, the western Pacific subtropical high northward shifting is not obvious. The topography can affect the two kinds of intraseasonal evolution of the western Pacific subtropical high during summer. If topographic parameters, of which the components are different for the two kinds of intraseasonal evolution, are larger, the intraseasonal northward shifting range of the western Pacific subtropical high is less. We introduce the spatial Fourier analysis method in the classical highly truncated spectral method. The highly truncated trigonometric functions are objectively selected with the observational data and the spatial Fourier analysis method. It is seen that the method has two merits. One is that the selected function has clearer physical significance; the other is that the defect of artificially selecting the highly truncated trigonometric functions has been overcome. We believe that this approach can lead to better understanding of short-term climate change.
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