Merrick C. Haller

Aerial image sequences of an ebbing river plume propagating into a tidal cross‐flow demonstrate the presence of along‐front instabilities at multiple scales, which appear to propagate along‐front in the direction of the shear flow. We hypothesize the instabilities are due to cross‐front shear in the along‐front velocity field. Image analysis quantifies the horizontal shear across the plume front as well as the length scales and propagation speeds of the instabilities. In situ measurements from cross‐front ship transects are also used to examine shear and the width of the shear layer. A non‐hydrostatic numerical simulation of an ebb plume in an idealized tidal cross‐flow was conducted using comparable flow parameters and confirms the presence of shear instabilities. An additional numerical simulation without a tidal‐cross flow demonstrates that shear instabilities are not present; rather, lobe‐and‐cleft structures on the nose of the gravity current emerge. According to the scaling of White and Helfrich (2013), utilizing both the observed and modeled parameters, it is shown that shear instabilities are likely in the observed and modeled cross‐flow scenario, and confirms that they are unlikely to emerge in the no cross‐flow model case.