This project funds the acquisition and analysis of fine- and microstructure data in conjunction with the UK funded GEOTRACES A10 section along 40S in the South Atlantic. A combination of vertical profiling and glider platforms will be used. The resulting data will be used to estimate regional diapycnal mixing and isopycnal stirring rates. It is anticipated that there will be significant regional variability in response to variations in tides, eddies, topographic morphology and mean geostrophic currents. Estimates of diapycnal mixing will be aided by radiation balance equations and estimates of isopycnal stirring will be informed by comparison to published scalings.
Intellectual Merit
There is a great deal of uncertainty regarding the details of how oceanic budgets of heat, mass, momentum, energy, potential vorticity or passive tracers are balanced, particularly at the submesoscale and especially with regards to the roles of nonlinearity versus wave radiation and the role of internal waves versus geostrophic motions. The proposed work will address this uncertainty by documenting fine- and microstructure fields along the GEOTRACES A10 section and will quantitatively investigate the relationship to, and dependence upon, variability on larger spatial and temporal scales. Measured diapycnal mixing rates will be compared to radiation balance equation based estimates of turbulent production associated with the regional internal wave climate. Isopycnal stirring rates will be compared to published scalings based upon both weakly nonlinear and turbulent analyses.
Broader Impacts
Results of the proposed research will contribute to the understanding and prediction of global climate change and the carbon cycle. Efforts will be in direct support of the GEOTRACES program as the biogeochemical budgets that are the focus of that study depend upon diapycnal mixing and isopycnal stirring rates. The project will provide direct estimates of these and will assist with the interpretation. It will also assist with the development and implementation of turbulent mixing and isopycnal stirring parameterizations for general circulation models used to predict the ocean climate response. Specifically, the characterization of diapycnal mixing and isopycnal stirring derived from the microstructure observations will be incorporated by collaborators at Oxford University in their general circulation model simulations of tracer transport.
This grant provided support to deploy untethered instruments from a ship in conjunction with a UK funded GEOTRACES cruise crossing the South Atlantic along 40S. This specialized instrumentation is designed to measure the the dissipation of energy and mixing of heat at millimeter scales. We had several physical hypotheses to test: first, that the deep currents in the Argentine Basin are damped through internal wave generation by as the deep currents encounter hills at the ocean bottom, in the same way the Jet Stream generates waves as it crosses the Rocky Mountains. The spatial scales are different, but you sample the mixing and turbulence associated with such waves when you encounter 'clear air turbulence' in a jet. The second hypothesis is that we could quantify the stirring (commingling of water masses) accomplished by mesoscale eddies (weather systems in the ocean) with isolated profiles of our special instruments. Intellectual Merit: The computers that we use to predict climate are limited in the extent of spatial and temporal scales that they can represent. One needs to represent the effects of unrepresented scales on those that are. The task of writing equations that faithfully represent such physical relationships is an iterative process of measuring, analyzing and theorizing that occupies a significant fraction of the field of Physical Oceanography. We obtained 21 profiles using a VMP-5500 (Vertical Microstructure Profiler) and 3 deployments of a Spray glider with a microRider (a self contained package for estimating turbulence piggy backed atop the glider) in conjunction with the 24 GEOTRACES stations. The VMP returned very high quality data which have been processed to final form. They will be distributed to the wider community through the internal wave mixing Climate Process Team and archived at the Woods Hole Data Library. The Principal Investigator has attended a cruise science meeting in Oxford during September of 2012 and liaised with co-PIs of the UK scientific party during a subsequent visit to Southampton in March 2013. A poster presentation drawing on these data is planned for June at a meeting sponsored by the Los Alamos National Lab. Two publications are being prepared to focus on our two hypothesis. We did not obtain clear cut evidence of internal lee wave generation. The process depends upon the presence of topography and large currents at the bottom. Our stations sampled places with hills and large currents, but not both at the same time. The first publication discusses the dissipation data in the context of the statistics of lee wave generation. On the other hand, the Argentine Basin is a crossroads for different water masses and very robust signatures of eddy stirring can be obtained with the microstructure data. Unfortunately, the glider did not fly sufficiently well as to permit quality data return. Consequently the data have not been processed. Broader Impacts: The GEOTRACES program is an international study to document the trace chemistry of the World's Ocean and investigate the spatial temporal distributions of the trace chemistry (micro-nutrients) relative to an entire suite of bio-geo-chemical interactions that relate to the carbon cycle. Our participation in the cruise permits us to provide expert support and guidance in defining the role that physics (eddy stirring and turbulent mixing) plays in setting the space/time patterns of the micro-nutrient distributions.
