Dynamics of the Near-Surface Layer of the Equatorial Ocean under Anomalous Rainfall

The wet phase of the Madden-Julian oscillation is characterized by anomalous convective rainfalls, which produce lenses of freshened water on the ocean surface. These lenses are associated with relatively large, localized in space, density anomalies in the near-surface layer of the ocean. The associated internal feedbacks modify the regime of air-sea interaction due to strong salinity stratification developing in the near-surface layer of the ocean. Freshwater lenses have a tendency to propagate latterly as gravity currents. Gravity currents inherently involve three-dimensional dynamics. As a type of organized structure, gravity currents in the upper layer of the ocean may also interact with, and be shaped by, the ambient oceanic environment and atmospheric conditions. Among the important factors determining the freshwater lens dynamics are the background stratification, wind stress, wind/wave mixing and spatially coherent organized motions in the near-surface layer of the ocean. Under certain conditions, a resonant interaction between a propagating freshwater lens and internal waves in the underlying thermocline or halocline may develop, whereas interaction with wind stress may produce an asymmetry in the freshwater lens and associated mixing. These two types of interactions working in concert may explain fragmentation of the lens edges in a series of sharp frontal interfaces. In this work, we have conducted a series of 3D high-resolution numerical experiments using computational fluid dynamics tools. The numerical model was designed to elucidate the relationship between vertical mixing and horizontal advection of salinity under various environmental conditions (Soloviev et al., 2015). Available near-surface data from field experiments served as guidance for numerical simulations. The results of this study indicate that in the process of lateral spreading, the freshwater lens tends to thin beginning from its edges and continuing toward the core of the lens (see an example in Figure 1). The time scale of the freshwater lens evolution typically ranges from several hours to several days depending on the freshwater influx and the area affected by the convective rain. A very intense subsequent wind/wave mixing is observed within a relatively narrow area (on the order of one hundred meters) at the upwind edge of the lens. At this edge, more dense water from the lens surrounding is forced by wind stress to move over a less dens water inside the lens in the process resembling Stommel's overturning gate. At the core of the lens and its downwind edge, the vertical mixing, nevertheless, is dominated by shear-induced turbulence due to the lateral spreading of this lens. This study demonstrates that freshwater lenses are an essential component of air-sea interaction in areas of strong and localized freshwater influx. Soloviev, A.V., S. Matt, and A. Fujimura (2015). Three-Dimensional Dynamics of Freshwater Lenses in the Ocean's Near-Surface Layer. Oceanography, 28(1), 142–149.