  Conserved quantities in geophysical flows play an important role in the
characterisation of geophysical dynamics and aid the development of
structure-preserving numerical methods. A significant family of conserved
quantities is formed by the Casimirs i.e., integral conservation laws that are
in the kernel of the underlying Poisson bracket. The Casimirs hence determine
the geometric structure of the geophysical fluid equations among which the
enstrophy is well known. Often Casimirs are proposed on heuristic grounds and
later verified to be part of the kernel of the Poisson bracket. In this work,
we will explicitly construct Casimirs by rewriting the Poisson bracket in
vorticity-divergence coordinates thereby providing explicit construction of
Casimirs for 2D geophysical flow dynamics.
