A study of turbulence and fine scale temperature variability of the ocean thermal boundary layer under breaking surface waves
Date
2018-08-02
Authors
Gemmrich, Johannes Richard
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Although turbulence near the ocean surface is of great significance to the air-sea exchange of heat, gas and momentum it is a poorly understood phenomenon especially at high wind speeds when vertical transfer processes tend to be greatest. This work evaluates ocean surface turbulence at high sea states by exploiting heat as a naturally occurring passive tracer. To this end, a freely drifting instrument with a mechanically driven temperature profiler, fixed depth thermistors and conductivity cells was used to monitor the fine scale temperature structure and breaking wave activity. These open ocean measurements form the basis for a comprehensive account of the near surface turbulence field. Temperature profiles reveal a rich fine structure which, when combined with independent air-sea heat flux measurements reveal the presence of a surface layer of wave enhanced turbulence, modulated by subsurface advection associated with Langmuir circulation. The concept of wave enhanced turbulence, previously based on observations in fetch limited environments, is here extended to open ocean storm conditions.
Generation of turbulence depends on the scale and frequency of breaking events. Our observations, which span a wide range of conditions from a coastal strait to the open ocean, show that wind speed or wave age are inadequate predictors of the occurrence frequency of wave breaking, motivating a scaling based on energy input. The decay of turbulence following wave breaking proceeds more rapidly than for isotropic turbulence, permitting generation of a thermal boundary layer a few centimetres thick, which accounts for brief temperature fluctuations observed beneath breaking waves. Advection due to Langmuir circulation also leaves its signature on the near surface temperature field. Both advection and enhanced diffusion are reconciled in a two-dimensional model of the upper ocean boundary layer, providing a framework for studying Langmuir circulation and upper ocean turbulence in terms of the measured temperature structure. The depth integrated dissipation derived from a model analysis of the data closely matches the energy input into the wave field, identifying breaking waves as the major source of turbulent kinetic energy.
Description
Keywords
Ocean waves, Turbulence, Ocean temperature