3D Dynamics of the Freshwater Lenses Produced by Convective Rain and River Runoff in the Near-Surface Layer of the Ocean

Stratiform rains produce the freshwater anomalies that are relatively homogeneous in horizontal direction. 1D simulations and parameterizations have a good chance of success in this case. In contrast to stratiform rains, convective rains, and/or river runoff produce localized freshwater lenses. These lenses create significant horizontal density anomalies and, consequently, significant horizontal pressure gradients in the near-surface layer of the ocean. As a result, freshwater lenses can spread and propagate as gravity currents. We investigate the 3D dynamics of freshwater lenses using computational fluid dynamics (CFD) tools and field measurements in the Equatorial Pacific (TOCS) and the Gulf of Mexico (CARTHE). The CFD model reveals the so-called ‘head’ at the edge of the spreading freshwater 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 results in the convection triggering 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 coherent structures at the frontal edge of the spreading freshwater lens, further intensifying mixing. These coherent structures resemble the 3D pattern of water motion pattern 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 results are consistent with observations of freshwater lenses in the Equatorial Pacific and in the Gulf of Mexico. The practical applications include pollution propagation in coastal waters (e.g., oil spills), open ocean dynamics (e.g., Madden-Julian Oscillation), and interpretation of sea surface salinity satellite measurements (e.g., Aquarius, SMOS, SMAP).