Summary:
In January 2016, Dr. Kurt Polzin at the Woods Hole Oceanographic Institution was awarded an RFP-V grant totaling $3,117,495 to lead the GoMRI project entitled Understanding How the Complex Topography of the Deepwater Gulf of Mexico Influences Water-column Mixing Processes and the Vertical and Horizontal Distribution of Oil and Gas after a Blowout which consisted of 4 collaborative institutions and approximately 6 research team members (including students).
Specifically, it was expected that linkages between the vertical distribution of turbulent mixing, the characteristics of the regional bathymetry and the nature of physical forcing phenomena of the northern Gulf of Mexico would be established including the Loop Current, Loop Current Eddies, bottom intensified Topographic Rossby Waves, internal waves, internal tides and surface and near bottom trapped inertial oscillations.
Quantification of the turbulent field would support vastly improved forecast capabilities of present and planned numerical models.
Research Highlights
As of December 31, 2019, this project’s research resulted in 3 peer-reviewed publications, 25 scientific presentations and 14 datasets being submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are/will be made available to the public. The project also engaged 1 PhD student over its award period. Significant outcomes of this project’s research are highlighted below.
On a scientific level the biggest highlight is the contrast between the proposal and the field measurements. The proposal was written from the perspective that there would be a quasi-stationary response to quasi-stationary flow over the steep and rough topography of the northern Gulf of Mexico. The assumption was that eddy flow over the bottom would give rise to boundary layers and near-boundary flows that had the time scale of the eddy field imprinted upon them. Instead, based upon our moored time series observations, the
response at the local diurnal (once a day) / inertial (once a pendulum day) time scale is fundamental.
The physical situation is complicated by the fact that there are six or seven non- dimensional parameters that are linked to ‘system behavior’ and there is simply a lack of understanding about the relative importance of these parameters. However, the reason for the time dependence assuredly involves dynamic instabilities of the boundary layer. The moored and turbulence observation document the space-time scales of this process.
In its downwelling configuration (which happens when flow is in the sense of Kelvin wave propagation, i.e. with shallow topography to the right in the northern hemisphere), the bottom boundary becomes a sink for Ertel potential vorticity and this can lead to unconditionally unstable conditions at inertial lime scales. This has decided implications for the ventilation of the bottom boundary / near boundary region and thus tracer concentrations. The instabilities that drive this are directly linked to the inertial frequency, which provides the rational for the noted time dependent behavior in the observations from both ship and moorings.
The way this plays out for the modeling community is that in order to get the instabilities right one needs to get the structure of the boundary layer right, and even ultra-high
regional models lack this resolution.1 The turbulence and high frequency moored data
enable us to have a better understanding of the boundary layer structure in the northern Gulf of Mexico.
This insight has been and is being communicated in oral presentations. It sorely needs to be published in the mainstream literature.
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1 Bracco et al., Transport, Fate and Impacts of the Deep Plume of Petroleum Hydrocarbons Formed during the Macondo Blowout. submitted.
Proposal Abstract - RFP-V PI Kurt Polzin
Project Research Overview (2016):
An overview of the proposed research activities from the GoMRI 2016 Meeting in Tampa.
Direct link to the Research Overview presentation.