dimes

The underlying motivation of DIMES is to understand the meridional overturning circulation of the global ocean. This circulation is an important part of the ocean/atmosphere system that transports heat from the equator to the poles. It will play the dominant role on time scales of centuries in the uptake by the ocean of excess CO2 generated by the burning of fossil fuels and by net global deforestation. Thus, it will govern, on the same time scales, the response of the earth’s climate to this excess CO2 and to other perturbations to the radiation balance at the earth’s surface.

The experiment is focused on the region of the Antarctic Circumpolar Current (ACC), which extends from approximately 50¾S to 60¾S, encircling the globe. This region accounts for about 8% of the area of the global ocean. Mixing processes in the ACC are of more importance to the meridional overturning circulation (MOC) than even this large area would suggest. Diapycnal mixing, which plays an important role in controlling the MOC, is almost certainly enhanced in the ACC because of strong flows over rough bathymetry and because of the strength and intermittency of the winds. There is strong evidence that this is the case (REFS). Furthermore, mixing along isopycnals by eddies below the shallow Ekman layer and above the highest topographic features, i.e., above 1500 meters, is virtually the only way to transport heat and mass across the latitudes of the Drake Passage (REFS), and these transports are a major part of the MOC (REFS).

GOALS

The goals of DIMES are to measure the diapycnal diffusivity and mesoscale isopycnal diffusivity at several levels and in several environments in the ACC. The project will focus on the southeast Pacific, the Drake Passage and Scotia Sea, and the southwest Atlantic. Diapycnal diffusivity will be estimated from fine structure and microstructure measurements covering the whole water column, all across these regions, with special attention on features likely to generate strong mixing. Isopycnal diffusivity will be estimated from the trajectories of isopycnal floats released at two depths in the ACC. The diapycnal diffusivity of a passive tracer will be measured with a tracer release experiment on an isopycnal surface near 1500 meters depth, and the tracer patch will also give an independent estimate of mesoscale isopycnal diffusivity.

The progression of the tracer and the floats across the region will set the pace and the evolution of the experiment. The bathymetry beneath the Pacific ACC to the east of 115¾W, where the tracer will be released, is relatively smooth and the eddy energy is relatively weak in that region. The Drake Passage and the Scotia Sea are relatively rough and the eddy field there and north of the Falkland Ridge is intense. Hence, in the first stage of the experiment we will obtain a study of relatively weak mixing in the ACC. Once the tracer and floats pass through the Drake Passage, we will obtain a study of high mixing and transport parameters. Observations will be carried beyond the Scotia Sea into the southwestern Atlantic. The project will enable greatly improved estimates of diapycnal and isopycnal transports for the global ACC band, although work in other regions of the ACC is certainly desirable. We have chosen this particular sector because of the contrast in forcing west and east of the Drake Passage and because it is easier to get U.S. and U.K. ships into this sector of the Southern Ocean for the several cruises that are involved.

DIMES Update (December 2004)

The goals and general strategy of DIMES have not change from previous descriptions of the project. However, the site has changed, the timing has been delayed, and the US and UK have become partners in the experiment.

Site
The region of the experiment has been moved from the Pacific Sector to the Eastern Pacific/Western Atlantic. The release of the tracer and of the first floats would be at 1500 meters depth in the center of the ACC band, around 60 S, 110 W (rather than 180 W) and their advection and dispersion through the eastern Pacific, Scotia Sea, and Southwest Atlantic will be measured. In this way the focus moves from a region of relatively low eddy energy and topographic roughness to a region of relatively high energy and roughness.

Timing
The main U.S. proposal will be submitted in February 2005 to NSF. The main UK proposal will be submitted in mid 2005 Field deployments of sound sources, tracer, and floats will begin in austral summer 2007-08. Surveys of tracer, hydrography, fine and microstructure will be performed in the following three austral summers, focusing on the southeast Pacific in 2008-09, the Scotia Sea in 2009-10 and both the Scotia Sea and southwest Atlantic in 2010-11, as the tracer and floats make there way through the system. Sections across the Drake Passage would begin 2007-08 and continue twice per year until 2010-11.

Scientists Involved
Jim Ledwell (tracer), Sarah Gille, Kevin Speer and Breck Owens (floats), John Toole, Kurt Polzin, and Ray Schmitt (fine and microstructure), and Mat Multrud and Julie McClean (modeling) are the main interested investigators in the U.S., and there will be coordinated proposals from each of the three field groups in February. Some of the interested investigators in the U.K. are Andrew Watson (tracer), and Alberto Garabato-Naveira, Karen Heywood and Mark Inall (hydrography and fine structure).

Work Underway
Field measurements and model studies have been made of sound propagation in the southern ocean in order to help plan sound source deployments for the float program (Speer). The range achievable is greater than was conservatively estimated. Addition of isopycnal capability to floats has been tested in the field (Owens). Model runs of float and tracer dispersion have been made to aid in planning the float and tracer release components (Maltrud and McClean).

A time line for the various components of the experiment is shown in Table 1. In austral summer 2007-08, a cruise would be mounted to deploy sound sources in the southeast Pacific, and to deploy the tracer and floats. A second cruise in 07-08 would be mounted to measure microstructure and fine structure with the High Resolution Profiler (HRP) at Drake Passage and surrounding areas. Twice yearly sampling of tracer in the Drake Passage will begin immediately in 07-08, to provide the boundary condition for the Atlantic part of the tracer experiment.

In 08-09 the tracer will be surveyed in the Pacific, as well as finestructure and microstructure with the HRP. The HRP survey can be combined in this case with the tracer survey because there are no special sites for diapycnal mixing. The sound sources will be recovered on this cruise and will be redeployed east of Drake Passage on a second cruise in 08-09.

The tracer will be sampled in the Scotia Sea in 09-10. An HRP survey will be performed on a separate cruise in this case because the mixing is likely to be concentrated along the rim of the Scotia Basin, while the tracer field will be much more widespread than the features generating high mixing.

In 2010-11, the tracer field in the Atlantic will be sampled on a series of two cruises, since the patch by this time will be quite large. Fine structure and microstructure will be measured with the HRP on one of these cruises to characterize the southeast Atlantic beyond the Scotia Sea.

Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean