In this paper, we study the velocity field at the density interface of a two-layer
stratification system when the flow is forced at the mid-depth of the lower layer
by the source–sink forcing method. It is known that, in a sufficiently strong linear
stratification, the source–sink forcing in certain configurations produces a single-vortex
pattern which corresponds to the lowest eigenmode of the Helmholtz equation
(Kanda & Linden 2001). Two types of forcing configuration are used for the two-layer
experiments: one that leads to a steady single-vortex pattern in a linear stratification,
and one that results in an unsteady irregular state. Strong single-vortex patterns appear
intermittently for the former configurations despite the absence of stratification at
the forcing height. When the single-vortex pattern occurs at the density interface, a
similar flow field extends down to the forcing height. The behaviour is explained as the
coupling of the resonant eigenmode at the interface with the horizontal component of
the forcing jets. The results show that stratification can organise a flow, even though
it is forced by an apparently random three-dimensional forcing.