Freshwater Lenses, Double Diffusion, and Cabbeling in the Near-Surface Layer of the Ocean

Convective rains in tropical regions produce lenses of freshened water in the near surface layer of the ocean. These lenses are localized in space and typically involve both salinity and temperature anomalies. Dynamics of freshwater lenses can be linked to the formation of the barrier layer and fronts, thus influencing large scale processes and contributing to the salinity field in the Aquarius and SMOS satellite footprints. Due to significant density anomalies, resulting in strong horizontal pressure gradients, the lateral spreading of freshwater lenses resembles gravity currents in a lock exchange process. The gravity current head develops Kelvin-Helmholtz billows, which results in rapid mixing of the freshwater lens at its edges. The freshwater lens can completely mix with the environment or achieve a compensated state when temperature and salinity anomalies compensate each other in the density field. At this stage, the horizontal pressure gradients diminish and lateral spreading ceases. The compensated lenses are then subject to erosion by double diffusion and cabbeling. As a result, the temperature and salinity anomalies in the upper layer of the ocean produced by convective rains are usually no longer prominent after a relatively short time period, on the time scale of several hours to several days. We have simulated the evolution of freshwater lenses, including double-diffusion and cabbeling, with a 3D computational fluid dynamics model (CFD). In the process of spreading, freshwater lenses become progressively thinner, starting from the lens edges. Limited near-surface data from several field experiments in the tropical ocean were used for validation of numerical simulations. In order to link the Aquarius/SMOS salinity retrievals with in situ sea surface salinity during rain, we have also simulated in CFD the process of rain drop penetration through the air-water interface and resulting vertical salinity profile in the upper few centimeters of the ocean.