UC Berkeley

This work investigates turbulence related to internal waves occurring in shallow, temperature-stratified coastal regions. Motivation for this work comes from two sources: first, a need to extend expensive and logistically taxing turbulent observations to longer time periods using in-situ continuous measurements, and second, a specific need for cross-disciplinary communication about physical-biological interactions important for understanding the global carbon budget. We find that internal waves contribute a significant amount of energy to mixing on the inner shelf of Monterey Bay, as well as in the subsurface thermocline over deep waters in the Southern California Bight.

Chapter one provides context for these two problems. Chapter two derives a scaling for the turbulent kinetic energy dissipation rate (epsilon), an indicator of turbulent activity, based on continuous measurements of velocity; dissipation rate is found to scale with shear, S². This scaling allows us to extend a small number of dissipation rate measurements to describe mixing over time using moored velocity data.

Chapter three explores the concept of water column stability in the presence of internal waves, deriving a scaling factor for determining when a stability threshold is reached. Moored density data and derived isopycnal spacing are used as a proxy for stabilizing and destabilizing effects of passing waves. The change in spacing between isopycnals, h', is found to scale the gradient Richardson number through a factor we define as delRi = (1+h')³. When this factor is smaller than unity, the stability is decreased as a wave passes through; if greater, stability is increased. Regions of low delRi are found to occur within and at the edges of the thermocline as internal waves pass. This factor can be derived using moored density data alone.

Chapter four explores variation in vertical diffusivity of density across the water column, and indicates the necessity of a varying profile within coastal diffusion models. High diffusivity occurs as expected within the surface and bottom boundary layers, but also at the edges of the thermocline.

Chapter five addresses the effect of internal wave-induced turbulence on subsurface phytoplankton layers, focusing on flocculation effects to describe a potential source of large, negatively buoyant aggregates found far below the surface. Turbulent theory is integrated with biological production to suggest a background level of internal wave-induced turbulence may in fact be an important flocculation mechanism. Dissipation within a subsurface mixed layer is sufficient to cause aggregation but not shearing of particles. Internal waves inject energy into the mixed layer slowly but consistently, maintaining aggregation processes. As aggregates settle, they pass through the highly energetic thermocline; the intermittent nature of internal wave-induced shear allows the largest particles to pass through.