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.
The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) is a multi-institutional US/UK project aimed at assessing mixing processes in the Southern Ocean. At Scripps Institution of Oceanography, DIMES-related research focused on evaluating how water mixes along constant-density (or isopycnal) surfaces within the Southern Ocean. Isopycnal mixing is important, because it determines how heat is transfered laterally within the ocean. No major currents carry water southward across the Antarctic Circumpolar Current, so isopycnal mixing is controlled largely by time-varying eddies. The DIMES field program placed about 180 acoustically-tracked floats in the ocean in order to study isopycnal mixing processes. As part of our analysis work at Scripps, we put "numerical floats" into a a computer simulation of the ocean (a "numerical model") in order to track currents within the simulated ocean. Idealized models with a flat sea floor had suggested a clear vertical structure of ocean mixing. In contrast, our results suggested that in the presence of complicated sea-floor ridges and troughs, the vertical structure of isopycnal mixing was not likely to have the same vertical structure as in the idealized case. This provided a useful framework for evaluating the in situ observations. We also used satellite measurements of sea surface height together with temperature and salinity data collected by autonomous Argo profilers to determine the basic temperature, salinity, and density structure of the Southern Ocean. This provides critical information needed to help interpret the data collect by the floats that were deployed as part of DIMES. The diapycnal component of DIMES has worked to assess vertical mixing within the Southern Ocean. Vertical mixing measurements are difficult to obtain and require specialized microstructure probes. A number of cheap approximations have been suggested for determining vertical mixing using less expensive observing techniques. The Southern Ocean is a particularly challenging location for this, because the ocean density varies little with depth. We assessed two of these cheap methods and found that in cases where velocity data were not available, cheap methods did not reliably reproduce microstructure measurements. Our collaborators included researchers at Woods Hole Oceanographic Institution, Florida State University, the University of Washington, the University of Southampton, the National Oceanography Centre Southampton, the University of East Anglia, the British Antarctic Survey, the University of Exeter, the Scottish Association of Marine Science, and the University of Oslo. They have carried out complementary analyses that together with our results show the impact of the DIMES program. Broader impacts: At Scripps Institution of Oceanography, the project contributed to the training of two graduate students and two post-doctoral researchers and provided early career support for a number of computer programmers/data managers. A variety of outreach materials were produced, and these are available from the project web site: http://dimes.ucsd.edu
