This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
The project will contribute to the effort to better understand and measure ocean mixing around the globe, which is important for understanding the ocean role in climate and improving ocean climate prediction models. Lowered Acoustic Doppler Current Profiler (LADCP) and CTD data will be compared with micro-structure observations to test and improve methods for implementing the Gregg-Henyey method of parameterization. The PI is an expert in the treatment and processing of LADCP data, and believes that widely varying discrepancies in mixing estimates may result from problems in how that data is used. The ultimate goals of the project will be the public distribution of a combine fine- and micro-structure database for estimating ocean mixing; an accurate and reliable implementation of the Gregg-Henyey parameterization; and an improved suite of LADCP processing software.
The PI has a strong record of community service to oceanographers through the development of refined techniques for obtaining and processing LADCP data. These data are used globally and influence many areas of oceanographic research. The project has the potential to further impact the oceanographic and climate communities by making it possible to confidently estimate ocean mixing from routinely available data at sea. The PI has intimate knowledge of the caveats and issues and is uniquely poised to make transformative progress on the topic.
PROJECT MOTIVATION AND GOALS In particular near its boundaries the ocean is highly turbulent and the resulting mixing is important, for example, for closing the global overturning circulation. Obtaining direct measurements of turbulence and mixing levels in the ocean is time consuming and expensive, because very small scales (<1cm, called "microstructure") must be resolved. Since turbulence in the ocean is closely linked to internal-wave activity, it has become popular to estimate turbulence and mixing levels from internal-wave measurements. One particularly popular family of methods is based on velocity measurements on vertical scales of ~50-300m -- the so-called "finestructure" band -- obtained with a type of velocity sampler called a Lowered Acoustic Doppler Current Profiler (LADCP). While theoretical uncertainties of this method are quite well known, the method had never been systematically validated against more direct microstructure measurements. The primary goal of this project was to understand the magnitude and nature of the uncertainties associated with turbulence and mixing estimates derived using several related finestructure parameterization methods based on LADCP data. INTELLECTUAL MERIT I-1. NEW AND NOVEL DATA. In the context of this project more than 500 high-quality LADCP profiles of velocity in the ocean were obtained. Importantly, these are the first ever LADCP-derived profiles that include the vertical velocity component. Analyses of these data have already led to three published papers with two more in review; further analyses are expected. I-2. IMPROVED FINESTRUCTURE PARAMETERIZATION METHOD. Now that the most popular finestructure-parameterization method has been validated, it can be applied with much greater confidence. An important finding of the project is that there are biases in finestructure parameterization methods resulting from seemingly innocuous implementation choices, such as the method used to process the LADCP data. As part of the project a set of "best practices" recommendations for applying the method have been established, which will make future studies much more inter-comparable. I-3. A NEW FINESTRUCTURE PARAMETERIZATION METHOD. In the context of this project an entirely new finestructure parameterization method based on vertical velocities was developed. Application of the new method to the LADCP data collected during this project has revealed that the method is at least as accurate as the methods currently in use, while at the same time being much simpler to apply. Importantly, the new method can be applied to the very large set of archived LADCP data that was collected, for example, during the international WOCE and CLIVAR programs. BROADER IMPACTS B-1. OTHER USES OF THE DATA. The archived LADCP data collected in the context of this proposal are available to anybody for any purpose. It is expected that, at the very least, the data will eventually find their way into climatologies, e.g. of internal wave activity, and be used to validate and constrain numerical circulation models. B-2. IMPROVED QUANTITATIVE KNOWLEDGE OF OCEAN MIXING. Both the validation of existing finestructure-parameterization methods and the development of the new method improve our quantitative knowledge of turbulence and mixing in the ocean. Such quantitative information is important in disciplines beyond physical oceanography. For example, the effects of turbulence and mixing must be parameterized in the circulation models that are used for climate research. In the context of tracer studies, apparent water-mass ages depend on the mixing history of the water parcels involved. In biological oceanography, turbulence levels are important in many different contexts, including for quantifying nutrient fluxes and for providing settlement cues for planktonic larvae. B-3. TRAINING AND PROFESSIONAL DEVELOPMENT. In the context of the project, hands-on training in sea-going oceanography was provided for a PhD student. Additionally, specific training in LADCP data acquisition, processing, and quality control was provided for several students and post-doctoral researchers, but also for more senior colleagues. Several of the trainees have since led independent LADCP acquisition campaigns.
