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[1] Infrared imaging provides a new way to detect internal waves under conditions where techniques that rely on backscatter from the sea surface may not be effective and it provides a new means to investigate the spatial variability associated with internal waves. This is illustrated with imagery collected in a bay under light winds using an airborne infrared camera. The internal waves appear as groups of dark and bright bands, corresponding to surface temperature fluctuations of about 0.05°C. A signal of this size is shown to be plausible based on straining of the water's surface thermal boundary layer ('cool skin') by internal waves having a strain rate of the order of 10 À2 s À1 . In addition, fine-scale temperature structure was detected that we speculate may indicate instabilities induced by the internal waves.

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This paper deals with frontogenesis in the presence of ageostrophic vertical current shears and horizontal density gradients. The problem has broad application to the situation encountered in tidal fronts and current system meanders, but specific focus here is on Gulf Stream meander crests and filaments that advance onto the continental shelf just north of Cape Hatteras. These occur typically every few days as Gulf Stream meanders progress northeastward through the South Atlantic Bight and past Cape Hatteras. We model the submesoscale evolution of the interface between the continental shelf water and these Gulf Stream features while they are on the continental shelf. We assume the region to be characterized by an initial condition consisting of a horizontal density transition region and an ageostrophic, surface‐intensified horizontal flow. The ensuing frontogensis process is modeled numerically with an f plane calculation employing the full nonlinear equations in the depth/cross‐front plane; flow is assumed out of this plane (along the front), but no variation of the flow in this direction is allowed. A pseudospectral model is employed using trigonometric functions in the horizontal and Chebyshev polynomials in the vertical. Many different scenarios are investigated by changing the width, shape, and relative positions of the density transition and velocity jet. In the majority of cases a propagating hydraulic jump is formed. Simultaneously, the initial surface jet evolves to a subsurface‐intensified jet while it weakens and ultimately changes directions. The presence of this strong velocity jet can substantially enhance the rate of jump formation or completely inhibit frontogenesis. Supporting analytical calculations are used to show that the presence of vertical ageostrophic shear can augment or oppose the usual frontogenesis mechanism present when the collapsing horizontal density gradient is acted on by the resulting convergent surface current. The outcome of the shear/density gradient interaction depends upon the position of each field with respect to the other. In the vicinity of the nose of the hydraulic jump for the cases investigated, the density is seen to have a qualitatively similar dependence upon the stream function in the translating frame, irrespective of the initial condition from which it evolved.