Topological selection in stratified fluids: an example from air–water systems

Topologically non-trivial configurations of stratified fluid domains are shown to generate selection mechanisms for conserved quantities. This is illustrated within the special case of a two-fluid system when the density of one of the fluids limits to zero, such as in the case of air and water. An explicit example is provided, demonstrating how the connection properties of the air domain affect total horizontal momentum conservation, despite the apparent translational invariance of the system. The correspondence between this symmetry and the selection process is also studied within the framework of variational principles for stratified ideal fluids.