An inquiry on the nature of CISK. Part I

It is shown that a puzzling conspicuous feature in Charney and Eliassen's (1964) result represents essentially an unstable internal gravity wave distorted by the use of the balance condition. This mode arises from the part of the heating representation associated with the vertical velocity per se at the interior level. This mode is fundamentally identical with the most unstable mode in a conventional Wave-CISK model. Such modes would not arise if the heating is parameterized in general solely in terms of the rotational component of the flow. The connection between this proposed treatment and the one using an additional frequency dependent heating parameter will be discussed.