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Award Detail

Doing Business As Name:University of Washington
  • James B Girton
  • (206) 543-8467
Award Date:07/09/2007
Estimated Total Award Amount: $ 825,720
Funds Obligated to Date: $ 825,720
  • FY 2011=$46,795
  • FY 2007=$238,451
  • FY 2008=$305,870
  • FY 2009=$145,243
  • FY 2010=$89,361
Start Date:07/01/2007
End Date:06/30/2012
Transaction Type:Grant
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:0623177
DUNS ID:605799469
Parent DUNS ID:042803536

Awardee Location

Street:4333 Brooklyn Ave NE
Awardee Cong. District:07

Primary Place of Performance

Organization Name:University of Washington
Street:4333 Brooklyn Ave NE
Cong. District:07

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.41, 2011, p.241.

Project Outcomes Report


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.


1 Introduction

1.1 The Southern Ocean

The Antarctic Circumpolar Current (ACC) is a continuous eastward current that circles the continent of Antarctica. Collectively, the oceanic regions through which it flows (the southern portions of the Pacific, Atlantic, and Indian Oceans) are known as the “Southern Ocean”. Because it connects the major oceans, the ACC is a hub of the large-scale overturning circulation that ventilates the deep ocean and transports heat between the equator and the poles. For this reason, and the role it plays in regulating the flow of heat to the Antarctic continental margin, the Southern Ocean is an important part of many computer simulations of climate change. Understanding the processes at work in the ACC, then, is critical to improving predictions of the ocean’s response to climate change, including greenhouse gas forcing.


1.2 Diapycnal Mixing Processes

Many of the processes least able to be simulated by computer models of the ocean involve “small-scale” (i.e., not well-resolved by the model’s discrete grid) fluid motions ranging from 100 km eddies down to centimeter-scale turbulence. The part of these motions that stirs and ultimately mixes fluids of different densities (i. e., primarily vertical turbulent motions) is known as “diapycnal” (or across density surfaces). Much of this mixing occurs near the surface and bottom of the ocean, where direct wind forcing or drag on bottom topography is able to generate turbulence. In the ocean interior, mixing is much reduced and water is able to move for long distances with very little modification. However, because of the long residence time of water in the ocean interior (up to many hundreds of years), the mixing that does occur there can have far-reaching consequences.

For the most part, it is believed that mid-ocean mixing occurs through the breaking of internal waves, which propagate both vertically and horizontally throughout the ocean’s stratified interior. The unique aspects of the ACC, including strong winds and cold air temperatures, a strong current that extends to the bottom of the ocean, and complex bottom topography, suggest that information about “typical” deep ocean

conditions may not be applicable.



The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean, was conceived to measure the processes and rates at which fluid is mixed across and along density surfaces. The experiment has been underway since early 2009, with major components including (a) the release of a patch of chemical tracer (CF3SF5) that can be detected in minute quantities over the span of many years; (b) ship surveys of hydrography (temperature, salinity, currents) and microstructure (turbulen...

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