| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | September 15, 2012 |
| Latest Amendment Date: | September 15, 2012 |
| Award Number: | 1231803 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Eric C. Itsweire
OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2012 |
| End Date: | August 31, 2017 (Estimated) |
| Total Intended Award Amount: | $640,124.00 |
| Total Awarded Amount to Date: | $640,124.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
874 TRADITIONS WAY TALLAHASSEE FL US 32306-0001 (850)644-5260 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
77 Chieftain Way, # 18 Keen Bldg Tallahassee FL US 32306-4360 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | PHYSICAL OCEANOGRAPHY |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Climate-scale ocean models unanimously stress the key regulatory function played by the oceanic overturning circulation in the Earth's climate and biogeochemical cycles over decadal and longer time scales. Yet in their quest to resolve many topical climate problems, the models' credibility is challenged by their extreme sensitivity to the representation of mixing processes in the Southern Ocean. This peculiarity of model behavior reflects the unique role of mixing in mediating the vertical and horizontal transports of water masses in the Antarctic Circumpolar Current (ACC), which shape the overturning circulation through their respective impacts on the overturning rate and inter-ocean exchange. The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) has collected a wealth of data to quantify mixing, including the spreading of a chemical tracer and of 150 floats at two different depths. The goal of this proposal is to analyze the tracer and float data to estimate the vertical and horizontal variations in lateral eddy mixing. The principal investigators' previous work has shown that tracer and floats are, by themselves, insufficient to accurately constrain these variations. Therefore the data will be analyzed together with surrogate tracers and floats advected with a numerical simulation of the regions under study.
Intellectual Merit: This project tackles the formidable problem of quantifying the role of small-scale eddies in the large-scale circulation. Because these eddies are so challenging to observe and model, it is difficult to quantify their large-scale effect. Indeed the ocean uptake of heat and carbon over the next century in climate models can be very different in models that parameterize mesoscale eddies compared to models that explicitly resolve them. This implies that present parameterizations, which either ignore or improperly represent the horizontal and vertical variations in mesoscale eddy mixing, may not have the skill to make accurate climate projections. The goal of this project is to ground new understanding of eddy mixing in data and use these results to develop new parameterizations for climate models. The connection between data and models is key to ensure that the DIMES experiment will have a long term legacy in the climate community.
Broader Impacts: This project addresses the important problem of quantifying mixing by eddies in the ocean and their role in the climate system. Furthermore the work should be seen in the wider context of studies of eddy-mean flow interaction in turbulent flows. The ideas have importance for the general circulation of the atmosphere and of planetary atmospheres, particularly those of the gas giants such as Jupiter. Indeed, parametrically, the ocean is closely akin to Jupiter, with an eddy scale that is hundreds of times smaller than that of the large-scale circulation. Finally, there is a strong educational component through the training of three graduate students and two post-docs, and the development of new curricula to introduce students in the MIT/WHOI Joint Program to the role of the Southern Ocean in the climate system.
The DIMES project is a process study sponsored by the U.S. CLIVAR (climate variability and predictability) program.
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PROJECT OUTCOMES REPORT
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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.
Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES)
The global ocean circulation is often divided into a nearly horizontal, or approximately isopycnal circulation, meaning the water keeps its density, and an overturning component that links sinking in the polar regions to the rest of the World Ocean. The polar extremes of dense water formation create water masses that spread and fill the global ocean, but this spreading depends on the topography of ocean basins and the stirring by energetic ocean “weather” systems or eddies. The cold ancient deep water formed in the northern Pacific Ocean, called Pacific Deep Water, flows south and eventually spreads along the northern flank of the Antarctic Circumpolar Current (ACC). At the same time, the strong winds of the Southern Ocean – the Roaring Forties and Screaming Fifties blow the surface layer of the ocean north toward the ACC. This Upper Cell of the global meridional overturning circulation is documented in many ways through the work of oceanographers who have measured the ocean for decades. But only in recent years has it been possible to deploy the kind of instrumentation needed to observe the interaction of these circulation elements in detail, over the enormous distances of the ACC.
One view of the ACC is that of a large-scale, broad mean eastward flow, with a transport of about 140 million cubic meters per second. The flow structure of the ACC is in marked contrast to the Gulf Stream, for example, where a single primary jet exists. The ACC is made of multiple fronts or jets that form naturally from the complex turbulent motion – at scales that make the ocean filled with turbulence to a much greater degree than the atmosphere. These jets can be locked to seafloor bottom topography, and nearly stationary, or more freely evolving typically in regions with less topographic control.
Although the importance of the ACC to the closure of the meridional overturning circulation has been inferred for some time, direct measurements of the strength and nature of this process have been lacking. An observational campaign, the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), was undertaken in 2009–14 to quantify the magnitude of eddy stirring and mixing. The length of the experiment covered three years, the time it took for our floats to drift across the great distances of the ACC. Our results from the deployment of RAFOS floats (subsurface drifters tracked by a moored acoustic network) in the southeast Pacific Ocean and Scotia Sea sectors of the ACC show an average mixing equivalent to a spreading rate of 100’s of kilometers in a year. Strong variations in this mixing occur both along and across the ACC. In fact, the mixing across the ACC is dramatically blocked by the presence of the jets, so that the floats we deployed spread mainly along the ACC from the Pacific Ocean into the Atlantic Ocean. From the data emerging from this experiment we can quantify ocean mixing from the surface to the top of the ocean bathymetry, and using our results to extrapolate other available data, to the entire circumpolar ocean.
Last Modified: 10/09/2017
Modified by: Kevin Speer
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