Linear instability calculations were carried out on a three layer Gulf Stream front model in an attempt to elucidate the interaction of the thermocline layer with surface slopewater shoreward of the front. The basic state is geostrophic balance and constant potential vorticity in the two active layers, but the perturbations are ageostrophic. The flow is found to be unstable to long wave perturbations, the wavelength of the most unstable wave to be of order 10 radii of deformation. The instability is mainly baroclinic, 75-85% of the energy supply to the growing perturbation coming from basic flow potential energy. Calculated wavelengths and growth rates, using parameters typical of the Gulf Stream, are similar to those observed. The eigenfunctions and particle trajectories reveal large cross-frontal excursions in the thermocline layer, and a large, if weak, cyclonic eddy in the surface slopewater in a meander trough
