Negative energy perturbations may be linked to the instability of fronts and gravity currents, for instance when the latter have uniform potential vorticity (in thef-plane). The equatorial Gaussian jet ũ(y) = ũ(0) exp (−y1/L2), in a one-layer, reduced-gravity, β-plane model may also be unstable to negative energy perturbations, particularly when the meridional gradient of potential vorticity does not change sign.