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 study of mixing and stirring in the Antarctic Circumpolar Current (ACC), one of the world ocean's great currents, that goes all around Antarctica from west to east driven by the strong westerly winds in that region. Transport of heat and constituents, especially carbon dioxide, across that current between the waters off Antarctica and the rest of the world ocean is an important facet of the way in which the ocean governs the climate of the earth. The nature and intensity of those transports depend on mixing across surfaces of constant density (diapycnal mixing) and eddy stirring along those surfaces (isopycnal mixing). DIMES is a multifaceted, multi-investigator project, jointly performed by the United States and the United Kingdom, with field work starting in 2009 and continuing until 2014. The grant reported on here was for a major part of a tracer release experiment which was part of DIMES. Ledwell, one of the PIs on this grant, is also one of the instigators and leaders of the overall DIMES project. A tracer, 76 kilograms of trifluoromethyl sulfur pentafluoride (CF3SF5), was released on a surface of constant density early in 2009, 2200 kilometers west of Drake Passage in the middle of the ACC, i.e., between the Polar Front and the Subantarctic Front. The depth of the isopycnal surface was approximately 1500 meters, but the surface shoals toward the south and deepens toward the north because of the balance between the current and the Coriolis acceration. Spreading of the tracer to other isopycnal surfaces with time gives a direct measure of diapycnal mixing. The cross isopycnal (nearly vertical) distribution of the tracer tends to be very close to a normal curve, and the growth of the width of this curve gives the diffusivity. Having conducted several surveys between 2009 and 2012, following the tracer as it moves east through Drake Passage and the Scotia Sea, we have determined the diffusivity to be quite small (0.13 centimeters squared per second) over the relatively smooth bottom in the southeast Pacific, but more than ten times larger over the rough bathymetry in Drake Passage. The small value in the Pacific sector affirms current assessments that the diffusivity in the interior of the open ocean is quite small - a "mixing time" for the global ocean would be about 10,000 years if it were mixed by this diffusivity. We know from radiocarbon and oxygen measurements that the mixing time of the global ocean is on the order of 1000 years, and so processes other than diapcynal mixing in the interior must be ventilating the global ocean. Such a result leads to a simplification, in a way, in theories and models of ocean circulation - the flow is constrained to be very nearly along surfaces of constant density in the ocean interior. Low mixing, however, also places the demand on numerical models of ocean circulation that they avoid spurious mixing due to approximations in the numerics. The larger value for diapycnal mixing in Drake Passage illustrates the point that is becoming ever clearer, that most ocean mixing occurs in limited regions, and especially in regions of rough topography. The leading hypothesis regarding the enhanced mixing in Drake Passage is that it is driven by the turbulent breakdown of lee waves generated by flows over submarine ridges and hills, similar to mountain lee waves seen in the atmosphere. The tracer, while spreading just 100 meters or so in the vertical direction, has spread hundreds of kilometers along and across the ACC. The spreading across the ACC is of particular interest because it will help with estimates of the rate of transport of mass, heat, and carbon dioxide, as well as momentum across the ACC. We are finding that at the level of the tracer there does seem to be a bit of a barrier to transport across the Polar Front. Otherwise, the order of magnitude of eddy transport seems to confirm expectations based on less direct measurements. Broader impacts of our research involve the relation of our work to climate studies. Circulation in the region of the ACC is an important component of the overturning circulation of the global ocean. That circulation has been implicated in studies of past climate changes over the history of the earth, and is an important part of the system that will govern future changes in climate caused by human activities, astronomical forcing or geological events. Our reserach activities have also proven to be provide excellent experience, both in the field and in the lab, to advanced students from the U.S., Argentina, Chile, and the United Kingdom, and have been a successful example of international cooperation.
