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Some Thoughts on Logistics, Mixing, and Power

J. N. Moum*


I can think of several things that are likely to be true in 2025. Some are definitely not good for the field of oceanography, some offer debatable benefits and, as has always been the case, technical developments that have nothing to do with oceanography will offer the potential to revolutionize it. These raise some questions about our capability to observe and understand future ocean circulation.

We are on track to have fewer scientific research vessels in the near future and beyond. It also looks like we will be concentrating a significant portion of our observational and intellectual resources at a very few locations—observatories. At the same time, climate scientists have been telling us that, under reasonable warming scenarios, there may occur significant changes in ocean circulation. If significant changes occur, will we have the resources available to properly observe them?

I know that the wonderful satellite-based measurements we now all have access to will steer us toward shifts in major current systems, but they only tell us what’s happening at the sea surface. And while the governing equations aren’t going to change and numerical simulations are getting progressively better, they still don’t deal with the subgrid scales very well, and they won’t in 2025, or 2100 for that matter. As one example, mixing parameterizations that appear to work reasonably well in the Denmark Overflow do not work at the equator, even though scales of shear and stratification are pretty much the same. This is probably

*

College of Oceanic and Atmospheric Sciences, Oregon State University



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Some Thoughts on Logistics, Mixing, and Power J. N. Moum* I can think of several things that are likely to be true in 2025. Some are definitely not good for the field of oceanography, some offer debatable benefits and, as has always been the case, technical developments that have nothing to do with oceanography will offer the potential to revolu- tionize it. These raise some questions about our capability to observe and understand future ocean circulation. We are on track to have fewer scientific research vessels in the near future and beyond. It also looks like we will be concentrating a signifi- cant portion of our observational and intellectual resources at a very few locations—observatories. At the same time, climate scientists have been telling us that, under reasonable warming scenarios, there may occur significant changes in ocean circulation. If significant changes occur, will we have the resources available to properly observe them? I know that the wonderful satellite-based measurements we now all have access to will steer us toward shifts in major current systems, but they only tell us what’s happening at the sea surface. And while the governing equations aren’t going to change and numerical simulations are getting progressively better, they still don’t deal with the subgrid scales very well, and they won’t in 2025, or 2100 for that matter. As one example, mixing parameterizations that appear to work reasonably well in the Denmark Overflow do not work at the equator, even though scales of shear and stratification are pretty much the same. This is probably * College of Oceanic and Atmospheric Sciences, Oregon State University 0

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1 J. N. MOUM because the scales at which the parameterizations are applied are simply not appropriate for predicting the mixing. At the equator there is clearly an important intermediary between the mixing scales and the resolved scales in the form of an energetic narrowband internal gravity wave field that we are only just beginning to understand.* In the event of a climate change-induced reorganization of ocean circulation, what happens if (for example) the vertical structure of the Gulf Stream intensifies to equatorial- strength shear and stratification? Does the mixing there need to be re- parameterized? Will we have the resources to properly measure this on the range of time and space scales necessary to determine the subsurface structure and how this influences what we observe from satellite? Or will we be spending our resources servicing fixed observatories elsewhere? On a different note, technical developments over the past decade or so have allowed deployment of relatively inexpensive, high data-rate, battery-powered instrumentation for extended periods. These develop- ments include low-power surface mount electronics (originally developed for the space program), and great improvements in battery capacity and data storage for the digital camera industry. We have found that we can power dual thermistors, pressure sensor, 3-axis accelerometers and compass on a TAO mooring for a year at data rates of 7 Mb/h. As part of our deployments of mixing meters on equatorial moorings, we have had to determine the motions in the frequency band 0.001-100 Hz. Any conventional mooring with a surface float is forced at surface wave fre- quencies. Is it possible to harvest this energy in order to extend deploy- ment lifetimes? Small, very efficient power generators have recently been developed to harvest biomechanical energy with the idea that this can be used to power prosthetic limbs and other portable medical devices. The demon- stration was a generator built into a knee brace used to power an iPod™ (the generator was used only to assist the muscles in decelerating rather than requiring them to be an energy source). This delivered 5 W average, which is at least 20 times greater than needed for our mixing meters. I guess it is possible to adapt small generators to take advantage of the available energy in mooring motions—the energy available is orders of magnitude greater than that from a decelerating knee joint. Perhaps gliders and other AUVs can take advantage of this while at the sea sur- face. This suggests the potential to extend deployments, an important fac- tor for an agency like NOAA, for example, which is attempting to main- tain its vast equatorial mooring arrays with reduced ship availability. Related to this, long-range power transmission is grossly inefficient. * This brings up a separate but fundamental issue that has yet to be decided—does each flow require its own parameterization?

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2 OCEANOGRAPHY IN 2025 Many who have thought about reducing carbon emissions suggest that local power generation will play an important role. Cloud computing cen- ters are being clustered near inexpensive power sources, such as Colum- bia River hydro-electric dams. Interestingly, Google™ has applied for a patent for ship-based computing centers, to take advantage of both the available cooling water as well as the energy available from the “natural motion of the water.” It seems that there is considerable effort from indus- try to eliminate long-range power transmission; maybe small, efficient power generators will be of help to oceanographers. Cabled observato- ries require long-range power transmission. Wouldn’t it be ironic if, once observatories are cabled, it won’t be necessary to transmit power through those cables?