Hidden symmetries, trivial conservation laws and Casimir invariants in geophysical fluid dynamics

Abstract

From a manifestly invariant Lagrangian density based on Clebsch fields and suitable for geophysical fluid dynamics, non-trivial conservation laws and their associated symmetries are described in arbitrary coordinates via Noether’s first theorem. Potential vorticity conservation is however shown to be a trivial law of the second kind with no relevance to Noether’s first theorem. A canonical Hamiltonian formulation is obtained in which Dirac constraints explicitly include the possibly time-dependent metric tensor. It is shown that all Dirac constraints are primary and of the second class, which implies that no infinite-dimensional symmetry transformations of Clebsch fields exist and that Noether’s second theorem does not apply to the governing equations. Therefore, the considered Lagrangian density does not admit a symmetry associated with potential vorticity conservation via Noether’s two theorems. Finally, the corresponding non-canonical Hamiltonian structure with time-dependent strong constraints is derived using tensor components for arbitrary coordinates. The existence of Casimir invariants is linked to trivial conservation laws of the second kind and to symmetries that become hidden after a transformation away from canonical dynamical fields.