Long-wave instability of an isolated front

The stability of an isolated one-layer reduced gravity front is examined. It is shown that the system is unstable to long-wave disturbances provided merely that a simple condition on the depth profile is satisfied far from the front. The instability does not require the extremum of potential vorticity needed by quasi-geostrophic theory. The instability releases mean kinetic and mean potential energy from the system, but lacking a second layer cannot truly be termed baroclinic instability.