Abstract
Verkley (1987, 1990) constructed modons as a model of the atmospheric blocking phenomenon. They are exact solutions of the barotropic potential vorticity equation on a sphere. In this note, it is shown that the 1987 modon changes continuously into the 1990 modon as the value of -dQ/dΨ within the modon goes to zero, where Q and Ψ are potential vorticity and streamfunction of the 1987 modon, respectively. Thereby it is revealed that the stability properties of the 1990 modon can be studied by means of Arnol'd's invariant of the 1987 modon though Arnol'd's invariant can not be defined for the 1990 modon.
