The Influence of Atmospheric Forcing on Turbulent Kinetic Energy Dissipation Rates in Rain-Formed Fresh Lenses in the Upper Meter of the Ocean
[12-Feb-2018] Iyer, S. and Drushka, K.
Presented at the 2018 Ocean Sciences Meeting
Freshwater transport, fluxes, and dissipation in the upper ocean are major components of the global hydrologic cycle, which is critically important to study considering the earth's warming climate. It is known that rainfall can form lenses of relatively fresh water at the ocean surface which persist until they are mixed away by turbulent processes. Previous studies have suggested that stratification suppresses turbulence below and enhances turbulence within fresh lenses until lenses are mixed away by wind or nighttime convection. However, the specific relationships between atmospheric parameters (e.g., wind, rain, heat flux) and turbulent dissipation rates at the sea surface have not been widely studied. The objective of this work is to quantify these relationships, determine how the transfer of turbulent energy controls the evolution of fresh lenses, and assess how these processes may impact the large-scale water cycle.
As part of the recent SPURS-2 experiment, microstructure temperature gradient, temperature, and salinity data were collected at a depth of 35 cm from a ship-towed profiler and indicate that numerous fresh lenses were observed. Assuming that shear and buoyancy production are balanced by turbulent energy dissipation, turbulent kinetic energy dissipation rates within and outside fresh lenses were estimated from underresolved temperature gradient spectra. Dissipation rates were generally on the order of 10-4 to 10-3 m2s-3. The effects of rain rate and wind speed on dissipation rates and the characteristics of turbulent dissipation during the evolution of freshwater lenses were determined. Initial results indicate that, as expected, wind speed correlates with dissipation rate. This suggests that lenses persist during periods of low wind and mix away as wind speeds increase. Understanding the relationships between rain, wind, dissipation, and the evolution of fresh lenses is important for interpreting observations and improving coupled air-sea models.