Collaborative Research: Lateral mixing and dispersion on the inner shelf

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Intellectual MeritThis award is for a suite of comprehensive field observations to elucidate and quantify the mixing processes in the inner shelf and to directly examine the impact of these processes on inner shelf biological distributions. Predictive modeling of coastal flows requires parameterizations of sub-gridscale processes, which, in the horizontal dimension have characteristic length-scales of 10s to 1000s of meters. This study will provide a comprehensive picture of horizontal dispersion on the inner shelf at these scales and its implications for scalar distributions. It is hypothesized that the physical limitations imposed by vertical boundaries and stratification restrict vertical motions and exchange processes leading to strong anisotropies in the flow field and dispersion in the inner shelf. Previous work has shown that mixing in the bottom boundary layer is scale-dependent in the horizontal direction in spite of the limiting effects of the bottom boundary. It is likely that scale-dependency may be different higher in the water column where stratification may create even greater anisotropies and restrictions on vertical transfer rates, and in the surface mixed layer due to the influence of waves, winds, as well as surface heating and cooling. This project will directly measure lateral dispersion rates in the bottom mixed layer, stratified interior, and surface mixed layer on the inner shelf under different forcing conditions in order to quantify any scale-dependent mixing and highlight the similarities and differences in lateral mixing at different depths. Once the dispersion rates and scale dependencies are determined, they will be used to examine how the lateral rates of mixing may correspond with biological thin layer development, maintenance and degradation. Two, two-week long field experiments will be conducted on the inner shelf of northern Monterey Bay to examine these issues. After deploying an array of moored instruments to measure physical, optical and acoustical properties, the research team will release a tracer dye, in separate releases, within the bottom boundary layer, stratified interior, and surface mixed layer during both the early and late portions of the upwelling season in northern California. The dye?s spatial and temporal distribution will be measured using an autonomous underwater vehicle equipped with a fast response fluorometer, as well as a tow body equipped with a fluorometer and ancilliary instrumentation that measure concurrently biological and geochemical properties of interest. The dye measurements will be used to quantify the lateral mixing by fitting the data to scale dependent laws, and comparing and contrasting the corresponding rates of dispersion with different climatological and hydrodynamics forcing conditions (winds, surface waves, internal wave, stratification, and turbulence) during these two measurement periods. Connecting the lateral mixing processes and forcing conditions with measured biological distributions (thin layers) will help determine the role of horizontal mixing in homogenizing and/or redistributing biological scalars.Broader ImpactsThis work will provide valuable new insights into the transport and mixing of scalars in the coastal region, particularly at the 'intermediate' scales of 10s to 1000s of meters. It will have a broad scientific impact, especially on interdisciplinary modeling efforts of the inner shelf where estimates and/or parameterizations of lateral mixing cause significant uncertainty.As coastal populations continue to grow, and pressure on marine ecosystems continues to increase, it will be increasingly important to understand the underlying mechanisms determining transport and/or retention in the nearshore environment. Understanding lateral dispersion in the coastal environment has significant implications not only for the transport/retention of marine larvae, and harmful algal blooms but also for the transport/retention of pollutants. Better understanding of these mixing processes will ultimately be of interest to environmental planners, policy makers and other stakeholders in coastal communities. The proposed research will also support one postdoctoral scholar and two PhD students.