The overturning circulation of the ocean plays a governing role in the earth's climate because of the enormous capacity of the ocean to hold heat and carbon dioxide. The Southern Ocean, which surrounds Antarctica, plays a disproportionate role in this overturning circulation because this is one of the main areas where deep waters rise to the surface to exchange heat and carbon dioxide with the atmosphere. Although the Antarctic Circumpolar Current (ACC) system brings deep water to the surface, dynamical constraints inhibit meridional exchanges. Ocean eddies are believed to play a dominant role in transporting water south across the ACC above deep ridges, feeding water driven northward by the intense winds. The extent to which this Isopycnal circulation is "short-circuited" by mixing across density layers is important to climate models but is unknown.
Intellectual Merit: Conceptual models of global meridional overturning and numerical predictions for future climate are strongly sensitive to the methods used to represent mixingalong and across the Antarctic Circumpolar Current (ACC), where isopycnals are steeply tilted. Neither diapycnal nor isopycnal mixing has been measured in the Southern Ocean in a systematic way. The goals of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) are to measure eddy mixing along density surfaces in the subsurface ocean (isopycnal mixing), and across those density layers (diapycnal mixing), and to determine how those processes depend on the larger scale dynamics of the ocean, so that they can be properly represented in numerical models of ocean circulation and of climate. To reveal these processes at work in the ACC, a chemical tracer and 75 floats that follow the water along isopycnal surfaces will be released in the ACC near 1300 m depth, 60 S, and 110 W, early in 2008. Floats that measure fine-structure T, S, and velocity within and above the tracer cloud will be released at the same time. The floats and tracer will be carried by the ACC over the relatively smooth bottom of the SE Pacific, spreading both across and along the current as they travel. After a year, the leading edge of the tracer will just start to pass over the ridges of Drake Passage into the Scotia Sea. Another 75 isopycnal floats will be released near the center of the tracer patch at this time. Trajectories of the floats, measured acoustically with an array of sound sources, will be used to study and to measure isopycnal dispersion. Spreading of the tracer will give integrated measures of both isopycnal and diapycnal dispersion. The eddy field, and its vertical structure, will be studied with sea surface height measured by satellite altimeters, and with hydrographic profiles taken from research vessels and from autonomous instruments drifting with the tracer. Turbulent dissipation, from which diapycnal mixing can be estimated, will be measured with ship-based free-falling profilers to study the spatial and temporal scales of the mixing and to examine suspected hot spots of mixing. Shear driving this mixing will be measured with the free-falling profilers and with special floats drifting with the tracer and floats that profile between the surface and the tracer layer.
Broader Impact: DIMES will deploy a variety of instruments including microstructure and finestructure profilers and and isopycnal-following autonomous floats, some for the first time in Southern Ocean. The mixing results will be made available to aid in improving representations of mixing in climate models. In addition, profiling DIMES floats will augment the Argo database for the Southern Ocean. The project will involve a postdoctoral investigator, graduate students at Florida State University and Scripps Institution of Oceanography and will offer research opportunities to one to two undergraduates per year.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.
Under this grant, which was a collaborative grant with Florida State University, a total of 180 isopycnal RAFOS floats were deployed in the Southern Ocean as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Most of the floats were launched in triplets so that relative dispersion statistics could be estimated. The roughly two thirds of the floats were launched on the 26.9 σθ (~ 1300 m depth) and the remainder on the 26.6 σθ (~ 500 m depth). An array of RAFOS sound sources was deployed to provide positions for the floats. Five SOLO profiling floats were also deployed to monitor the performance of the sound sources. Due to an unexpected decrease in the range for the acoustic signals from the sound sources tracking of the floats were more difficult than expected. Nevertheless, preliminary data processing has been completed. A basic description of the results has been completed. This includes: A description of the circulation at 1300 m depth, including the influence of the fronts and standing eddies Calculation of the potential vorticity balance in the standing eddies, which shows that the changes relative vorticity are due to changes in planetary vorticity Estimates of isopycnal mixing show that Relative dispersion, is due to shear dispersion at large scales, an inverse cascade and non-local dispersion at shorter scales Scotia sea mixing is higher by a factor of almost 3 A manuscript describing these results, with FSU graduate student Dhruv Balwada as the lead author, will be submitted in Fall 2014.
