On intermonthly oscillations in the atmosphere, part II

A. WIIN NIELSEN

Abstract

The investigation explores the possibility that the observed intermonthly variations in the atmosphere may be described by a nonlinear low-order model containing nine components. The model is formulated on a f-plane and contains only wave-wave interactions. It is based on the equivalent barotropic atmosphere formulated in such a way that it contains both heating and dissipations. The wave numbers vary from 1 to 3 in the zonal and the meridional direction. Most of the integrations of the model assume that the oscillations are a planetary phenomenon, and they have therefore been performed with a maximum wavelength of 28000 km. The results show that the observed periods (30-35, 45 and 70 days) may be reproduced by the model. Other periods are also found, but they are mainly of small amplitudes. The model permits a determination of the heating pattern necessary to produce the observed oscillations. It is required that the heating level is above the time-averaged heating in the atmosphere, and the heating is such that it displays a strong meridional heating gradient. The investigation is a continuation of an earlier attempt to model intermonthly oscillations using a very simple three-component model based on the first equation of motion with non-linear advection and momentum forcing and dissipation. The new model permits a more physical description of the phenomena.

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