Response of the Equatorial Ocean to Convective and Stratiform Rains During Wind Bursts

In this work, we are interested in the upper ocean response to the freshwater and wind forcing during the Madden-Julian Oscillation. We have conducted a series of three-dimensional numerical experiments using computational fluid dynamics tools to elucidate the relationship between vertical mixing and horizontal advection of salinity under various environmental conditions including convective and stratiform rains. The numeric results are discussed in the context of the TOGA COARE and DYNAMO field data. Freshwater inflow due to convective rains produces freshwater lenses that are localized in space and time. Anomalously strong horizontal density and pressure gradients developing in the near-surface layer of the ocean result in lateral spreading of these freshwater lenses in the form of gravity currents. Interaction of the gravity current with wind stress results in lens asymmetry. The upwind edge of the lens is destabilized by the wind-driven overflow of higher salinity/density water over the lens’s edge, which results in the convection triggering Kelvin-Helmholtz instability in the form of billows. The Kelvin-Helmholtz billows produce strong local mixing and large entrainment fluxes at the upwind edge of the lens. In contrast, the downwind edge of the lens is stabilized by horizontal advection of the lower salinity (less dense) water. At the same time, the vertical mixing in the core of the lens is mainly driven by the vertical shear due to horizontal spreading of the lens. The impact of the wind stress appears to be less important in the core of the freshwater lens.. Response to convective rain inherently involves three-dimensional dynamics. A freshwater inflow to the upper ocean due to a stratiform rain produces freshwater anomalies, which are relatively homogeneous in space. One-dimensional simulation and parameterizations still have a chance of success in the case of the freshwater anomalies produced by stratiform rains.