Small-scale turbulence dynamics under sea surface gravity waves

Abstract

The ocean surface is a complex boundary where air-sea fluxes of mass, momentum and energy take place. The processes at this dynamic interface are of great importance in the coupled atmosphere-ocean system. The coupling between winds, surface gravity waves, and currents in the adjacent turbulent boundary layers plays a significant role in the the global energy budget, and hence on synoptic weather system and the global climate. However, the characteristics of upper ocean turbulence very close to the airsea interface still remain insufficiently resolved both theoretically and experimentally. This dissertation aims to improve the gap in our knowledge about air-sea interaction by combining theory, numerical model predictions, and high quality observations under various sea states and wind conditions. An autonomous Microstructure Ocean Turbulence System (MATS) has been designed and constructed in collaboration with Rockland Scientific International, Canada, to collect long time series of turbulent quantities at a fixed level below the wavy air-sea interface. The system allows for measurements of the turbulent dissipation rate using shear probe signals. The motion of the platform is monitored using an inertial measurement unit. Four successful deployments have been carried out, including two periods with storm conditions in late 2011 and early 2012, respectively. Surface gravity waves were also estimated using a high-resolution pressure sensor mounted on the MATS. In addition, a direct covariance flux system was mounted on a moored buoy, and measurements of turbulent heat and momentum fluxes were performed approximately 3.7 m above the sea surface during a field campaign at Martha’s Vineyard Coastal Observatory (MVCO), Massachusetts. This dissertation also aims to improve the parameterizations in numerical modelling studies of upper ocean mixing, by incorporating the effects of wave forcing which are quantified by means of model-observation comparisons. Responsible mechanisms such as breaking and non-breaking waves, and Langmuir Circulation contributions to the enhanced dissipation rate of Turbulent Kinetic Energy (TKE) in the upper ocean are studied theoretically and numerically. A wave toolbox to be incorporated in the General Ocean Turbulence Model (GOTM) has been developed to implement wave effects in an upper ocean vertical mixing model. Moreover, statistics of small scale turbulence below the air-sea interface are investigated under a variety of environmental conditions using model results compared with MATS-measured TKE dissipation rates. The wave-modified model results are further compared with some published empirical parameterizations of the dissipation rate, and some other data from the literature. All model-observation study results support the importance of wave forcing in modulating and modifying upper ocean dynamics and the turbulence structure near the sea surface.