Observation and Modeling of Low-salinity Lenses off the Louisiana Coast

River runoff on the Louisiana Coast produce relatively shallow, low-salinity lenses. These lenses create significant horizontal density gradients in the upper ocean and can spread as gravity currents. In this paper, we analyze the field and synthetic aperture radar (SAR) satellite measurements conducted off the Louisiana Coast during LASER and SPLASH field campaigns. We investigate dynamics of low-salinity lenses using computational fluid dynamics (CFD) tools. The CFD model reveals the so-called ‘head’ at the edge of the spreading lens, which is a distinctive feature of gravity currents. The upwind edge of the lens is destabilized by the wind-driven flow of higher salinity/density water over the lens’s edge, which triggers the Kelvin-Helmholtz (KH) instability in the form of billows. The KH billows initiate strong mixing and large entrainment fluxes at the upwind edge of the lens. The downwind edge of the lens is stabilized by horizontal advection of the lower salinity (less dense) water. The model also develops spiraling coherent structures at the frontal edge of the spreading low-salinity lens, further intensifying mixing. These coherent structures resemble the 3D pattern of water motion in the leading edge of the gravity current and trailing fluid as reported by Özgökmen et al. (2004) and Soloviev et al. (2015). The model reproduces the main features of in situ and SAR observations of low-salinity lenses off the Louisiana Coast. The results of this work help to understand the effect of river runoff on oil spill propagation during the Deepwater Horizon disaster.