Meetings: Documents

Observations of Turbulence Within Rain-Formed Fresh Lenses as Part of SPURS-2
[14-Dec-2018] Iler, S. and Drushka, K.
Presented at the 2018 AGU Fall Meeting
Freshwater transport, surface fluxes, and turbulent mixing in the upper ocean are components of the global hydrologic cycle, which is critically important to study in the context of Earth's warming. Rainfall can form lenses of relatively fresh water at the ocean surface which persist until they are mixed away by turbulent processes or advection. 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 wind, rain, and dissipation rates at the sea surface, and the turbulent conditions necessary to mix away fresh lenses, have not been widely studied. The objective of this work is to analyze these relationships and determine the effects of turbulence on the evolution of fresh lenses.
As part of the recent SPURS-2 experiment, microstructure temperature and conductivity, temperature, and salinity data were collected in the top meter from the ship-towed Surface Salinity Profiler (SSP), and captured numerous fresh lenses. During rainy conditions, turbulent kinetic energy dissipation rates at 35 cm depth varied between roughly 10-7 and 10-3 m2s-3 and were highly dependent on atmospheric and oceanic conditions. The data indicate that both the persistence of fresh lenses and turbulence are influenced by wind speed, with high wind speeds associated with elevated turbulent dissipation. Specifically, lenses persisted during periods of low wind and mixed away when wind speeds increased. Dissipation rates were generally consistent with theoretical levels expected from wind and wave breaking, and diurnal warming events were associated with low turbulent dissipation rates. The momentum input from rain does not appear to have a large effect on turbulent dissipation rates at 35 cm depth. These findings could be potentially useful in improving coupled air-sea model parameterizations.