Turbulent mixing by nearshore internal bores

Stanford University, Stanford CA

Investigators

Abstract

Intellectual Merit: The overall understanding of the larger-scale physics of the inner shelf has advanced notably in the past few years, but there has been little work on the shallow nearshore regions where stratified inner shelf waters interact with the shore. Limited observations to date, including work in Monterey Bay and Mamala Bay, suggest that transient mixing and stratification events, presumably associated with the shoaling and breaking of internal wave fields present offshore, dramatically alter the physical environment in this ecologically important part of the ocean. In this project, detailed observations of periodic turbulent bores and fronts will be conducted in 10 to 30 m water in southern Monterey Bay using a set of Acoustic Doppler Velocimeters (ADV) and fast Conductivity/Temperature sensors to describe the behavior of nearshore turbulent mixing and stratification events. The ADVs will be mounted on an 8m high tower and cabled to shore, to measure turbulence variables including Reynolds stresses, turbulence dissipation and buoyancy fluxes. This tower will be supplemented by an array of Acoustic Doppler Current Profilers and thermistor chains centered on it to provide a detailed spatial and temporal view of these flows, including diagnosing their dynamics. An Automated Underwater Vehicle will be used to map larger-scale variations in temperature and salinity. The project is designed to accomplish two related goals that could substantially advance current understanding of flows in the nearshore: (1) Show how these bore-like features interact with topography and other (e.g. tidal) flows, and (2) Examine the turbulence produced during this interaction in light of existing dynamical frameworks of boundary layer and stratified shear turbulence. Broader Impacts: The local dynamics of the bore/relaxation front pattern that has been often reported in the literature can be the dominant source of variability in an ecologically important class of physical environments. This is significant for several reasons: (1) The bores/fronts appears to be an important mechanism by which deeper offshore waters are supplied to nearshore environments in shelf regions of coastal upwelling systems. Because of the interaction between these bottom derived waters and surface process affected waters in nearshore ecosystems, this is important for understanding how climate change will affect these ecosystems since these waters are low in dissolved oxygen and have high acidity. (2) These bores/fronts are thought to be an important mechanism for the exchange of organisms between the nearshore and the outermost regions of the inner shelf. (3) Vertical turbulent mixing is critical to many ecological processes such as benthic grazing and primary production by phytoplankton. The project's investigators will be experimenting with how to best use coastal ocean observing systems and, through their participation in Stanford's Center for Ocean Solutions, how to connect their use with the data and information needs of policy makers as well as those of other marine ecologists and geochemists. From an educational standpoint, the proposed work will be labor intensive and thus will engage a number of Stanford graduate and undergraduate students. Besides the two students directly involved in the project, at least four other graduate students will likely be involved in the field work, including two women Ph.D. students working with the investigators. As part of this project, a graduate field methods class that will be built around the May 2013 experiment will be taught. Finally, the investigators will also develop a microdocumentary (see http://microdocs.org/) on the importance of internal bores to nutrient fluxes, and nearshore ecosystem production. In conjunction with this video, they will visit several local area high schools in largely underrepresented districts to make short (30 min) presentations on oceanography, ocean health, and ocean career opportunities during which the microdocumentary will be presented to the classroom.