Rain-Generated Ocean Surface Turbulence and Salinity Stratification
[12-Feb-2018] Asher, W., Drushka, K., Thompson, E.J., Jessup, A.T., Clark, D.
Presented at the 2018 Ocean Sciences Meeting
Rain on the sea surface can produce a stable near-surface layer of fresher water with a lifetime of O(1-10) hours and a depth of O(1) meter. The surface salinity decrease is a function of wind speed and rain rate, with surface turbulence likely being an important factor in determining the formation and evolution of a fresh lens. Although it is known that both rain and wind generate surface turbulence, the relative importance of rain- and wind-generated turbulence in governing fresh lenses is an open question due to lack of field measurements of surface turbulence during rain. Direct measurement of surface turbulence and vertical profiles of salinity during rain are needed to understand whether the turbulence generated by rain in the upper few centimeters has an impact on fresh lenses generated by rain.
During the 2016 and 2017 SPURS-2 field experiments in the eastern tropical Pacific Ocean, the controlled flux technique (CFT) was used to infer surface turbulence before, during, and after precipitation. CFT uses an infrared imager to track the temperature decay of the patch of water on the ocean surface that has been heated a few degrees Celsius using a carbon dioxide laser. Using a surface renewal model, the rate of the temperature decay can be related to the turbulence dissipation rate at the ocean surface. These turbulence measurements are analyzed within the context of collocated and contemporaneous vertical profiles of salinity and temperature in the upper meter of the ocean surface, which were measured using a surface salinity profiler. In this paper, the CFT dissipation estimates will be correlated with rain rate and wind speed to estimate when rain provides a significant source of turbulence in the upper ocean. Dissipation rates will also be correlated with the presence of salinity/temperature gradients in the upper meter. This will provide a better understanding of the mechanisms that lead to the generation and evolution of fresh lenses generated by rain.