Freshwater Lenses in the Near-Surface Layer of the Ocean Laterally Spreading as Gravity Currents

Convective rains and/or river runoff produce localized freshwater lenses in the near-surface layer of the ocean that have significant density anomalies and thus horizontal pressure gradients. As a result, these lenses can spread and propagate as gravity currents, interacting with wind stress. We have modeled freshwater lens dynamics in the near-surface layer of the ocean using computational fluid dynamics (CFD) tools. We are able to reproduce generic features of freshwater lens spreading and interaction with wind using a 3D CFD model developed with ANSYS Fluent software. The model set up included an initial 0.5 psu salinity and 0.8° C temperature anomaly with a 50 m radius. Wind stress corresponding to U ₁₀ = 8 m/s was applied to the water surface. The freshwater lens spread laterally as a gravity current, producing a typical gravity current "head" with some asymmetry of the lens edges due to the effect of the wind. Interestingly, coherent structures developed at the frontal edge of the spreading freshwater lens, apparently intensifying mixing. These structures resemble a complex pattern of three-dimensional water flow motions in the leading edge of the gravity current and trailing fluid, as previously reported by Özgökmen et al. (2004) and Soloviev et al. (2015). The results of the CFD model have been compared with measurements conducted as part of the Gulf of Mexico Research Initiative Consortium for Advanced Research on Transport of Hydrocarbons in the Environment. These results have a number of practical applications including pollution propagation in coastal waters (e.g., oil spills), open ocean dynamics (e.g., Madden-Julian Oscillation), and interpretation of Aquarius and SMOS sea surface salinity satellite measurements.