Having considered the evidence of cycles and the possible mechanisms for the occurrence of natural variability of the climate and the sources of external periodic behaviour, the final exercise is to identify how a physical model or other explanation can link these various observations. But this objective requires first a review of just what features of the climate should be covered by any models. This in itself is not easy. As has now become evident, neither the evidence of cycles nor the possible causes of such behaviour present a simple picture. So it is important to clarify this objective from the outset.

Clearly, the evidence of cycles is not unequivocal. What can be said is that there is a strong need to find an explanation of the QBO. But, as has become apparent, this is not a sharply defined periodicity; moreover, while the stratospheric phenomenon is well defined, its equivalent in the troposphere is an altogether more shadowy creature. So, in the stratosphere we are looking for a physical explanation of a strikingly regular and substantial feature of the equatorial regions.