  We suggest a way of rationalizing an intra-seasonal oscillations (IOs) of the
Earth atmospheric flow as four meteorological relevant triads of interacting
planetary waves, isolated from the system of all the rest planetary waves.
  Our model is independent of the topography (mountains, etc.) and gives a
natural explanation of IOs both in the North and South Hemispheres. Spherical
planetary waves are an example of a wave mesoscopic system obeying discrete
resonances that also appears in other areas of physics.
