Abstract:
In the framework of stable free modes, a criterion is derived for the localisation in latitude of the intensified sector of the mode, i.e. northwards, southwards, or equally distributed in the north and south. The criterion is based on the sign taken by the inverse of the total vorticity at the middle latitude of the reference basin, and resorts to the necessarily westward orientation of an unforced current according to one of Greenspan's theorems. Some solutions, also of non-linear modes, elucidate the result.