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Research Spending & Results

Award Detail

Awardee:WOODS HOLE OCEANOGRAPHIC INSTITUTION
Doing Business As Name:Woods Hole Oceanographic Institution
PD/PI:
  • John M Toole
  • (508) 289-2531
  • jtoole@whoi.edu
Award Date:07/09/2007
Estimated Total Award Amount: $ 2,431,219
Funds Obligated to Date: $ 2,431,219
  • FY 2010=$634,025
  • FY 2008=$286,625
  • FY 2007=$108,729
  • FY 2011=$710,106
  • FY 2009=$691,734
Start Date:07/01/2007
End Date:06/30/2013
Transaction Type:Grant
Agency:NSF
Awarding Agency Code:4900
Funding Agency Code:4900
CFDA Number:47.050
Primary Program Source:040100 NSF RESEARCH & RELATED ACTIVIT
Award Title or Description:Collaborative Research: DIMES, Diapycnal and Isopycnal Mixing in the Southern Ocean
Federal Award ID Number:0622630
DUNS ID:001766682
Parent DUNS ID:001766682
Program:PHYSICAL OCEANOGRAPHY

Awardee Location

Street:183 OYSTER POND ROAD
City:WOODS HOLE
State:MA
ZIP:02543-1041
County:Woods Hole
Country:US
Awardee Cong. District:09

Primary Place of Performance

Organization Name:Woods Hole Oceanographic Institution
Street:183 OYSTER POND ROAD
City:WOODS HOLE
State:MA
ZIP:02543-1041
County:Woods Hole
Country:US
Cong. District:09

Abstract at Time of Award

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.

Publications Produced as a Result of this Research

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Ledwell, J.R., L.C. St. Laurent, J.B. Girton, and J.M. Toole "Diapycnal Mixing in the Antarctic Circumpolar Current" Journal of Physical Oceanography, v., 2011, p.. doi:10.1175/2010JPO4557.1 

Sheen, K. L., J. A. Brearley, J. R. Ledwell, M. P. Meredith, A. C. Naveira Garabato, D. A. Smeed, L. St. Laurent, A. M. Thurnherr, J. M. Toole, S. Waterman, and A. J. Watson "Rates and mechanisms of turbulent dissipation and mixing in the Southern Ocean: Results from the DIMES experiment" Journal of Geophysical Research, v.118, 2013, p.2774. doi:10.1002/jgrc.20217 

L. St. Laurent, A. C. Naveira Garabato, J. R. Ledwell, A. M. Thurnherr, J. M. Toole, A. J. Watson "Turbulence and Diapycnal Mixing in Drake Passage" Journal of Physical Oceanography, v.42, 2012, p.2143. doi:10.1175/JPO-D-12-027.1 

Ledwell, J.R., L.C. St. Laurent, J.B. Girton, and J.M. Toole "Diapycnal Mixing in the Antarctic Circumpolar Current" Journal of Physical Oceanography, v.41, 2011, p.241. doi:10.1175/2010JPO4557.1 


Project Outcomes Report

Disclaimer

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

The two polar oceans on the earth, the Arctic Ocean and the Southern Ocean, are unique in having sub-regions in which no land boundaries block flow along latitude lines for a full 360 degrees of longitude.  In the latter, this domain is home to the Antarctic Circumpolar Current (ACC), one of the major circulation features on the globe.  Significant watermass transformations forced by air-sea/ice-sea exchange in the Southern Ocean produce major bottom and intermediate waters that are traceable as they spread throughout much of the world's oceans.  Owing to the lack of zonal boundaries in the Southern Ocean, the dynamics governing the meridional exchange of these newly-modified waters northward, and their parent waters southward across the ACC are dominated by temporally-/spatially- variable eddy motions having horizontal scales of order 10 km and time scales of days to weeks. 

On space of order 1-m or smaller, turbulent mixing processes exchange water properties vertically and potentially short-circuit some of the eddy-driven meridional exchange in the Southern Ocean with internal waves providing a dynamical connection between these disparate scales of motion, extracting energy from the eddy motions and supplying it to the small-scale turbulence.  Importantly, models used to assess climate states of the earth are unable to resolve eddy or smaller-scale motions; the effects of these motions must be parameterized.  But to do so, one must first develop physical understanding of the processes involved and their influences on larger-scale motions. 

 Building understanding of lateral eddy stirring processes and vertical turbulent mixing processes in the ocean that will, in turn, hopefully lead to climate model improvements constitute the main goals of the international Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES).  (The isopycnal direction is defined to lie along density surfaces, which in the ocean are very nearly horizontal, thus the "I" piece of DIMES is chiefly concerned with quasi-horizontal eddy stirring.  The "D" in DIMES, denoting the diapycnal - across density surfaces - direction is concerned with "nearly-vertical" turbulent mixing.)  Focal point for the DIMES field program is tracking the spread of a chemical tracer injected into the Southeast Pacific Ocean around 1500 m depth in the ACC as it spreads laterally and vertically while being carried east by the current.  The spreading rates yield time-average estimates of the eddy stirring and turbulent mixing rates. 

Research funded by the present grant, in concert with a companion program led by Dr. Louis St. Laurent and field work conducted by U.K. colleagues, focused on the turbulent mixing processes active in the Southern Ocean.  Our approach was to join the tracer survey cruises and repeatedly deploy profiling instrumentation to sample the turbulent fluctuations responsible for diapycnal mixing, as well as sensors to document the motions that supply the energy to support the turbulence.  Notably, the profiler sampling extended from the ocean surface to well below the level of the tracer, thus providing a broader view of the turbulent mixing distribution in the Southern Ocean than obtained by analysis of the tracer spreading.   In total, grant OCE-0622630 supported turbulence sampling on four cruises to the Southern Ocean.      

DIMES was initiated the Southeast Pacific where the sea floor is relatively flat and the expectation was that the internal wave field in the region and the turbulent mixing it supports were at background intensity.  Observations taken during the "US-2" cruise in early 2010 (approximately one year after the tracer was injected into the ocean) confirmed these hypotheses.  In contrast, data taken at the end...

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