2 open review comments to “Progressing towards global sustained deep ocean observations”
This well-written white paper clearly makes the case for (a) the importance of measuring the abyssal ocean, particularly in several critical areas and (b) the difficulty of making such measurements. It seems to me there are two approaches that are deserving of further development in the white paper. The first is the notion of making fairly direct inferences of abyssal variability using combinations of depth-integrating measurements and near-surface measurements. The second is that given the difficulty and sparseness of the abyssal measurements, it is fairly important to leverage whatever data/information can be obtained using general circulation models and data assimilation. The two approaches are not unrelated, of course; data assimilation allows one to combine disparate data types to form a single consistent estimate of the ocean state.
Three examples of depth-integrating measurement types from which abyssal variablity can be inferred, if near-surface information is available, are satellite altimetry, acoustic tomography, and the horizontal electric field (HEF) measurements mentioned briefly in the paper. Through steric height, altimetry measures depth-integrated heat content, which when combined with near-surface heat content estimated through, e.g., Argo floats, should provide a measure of abyssal heat content. This measure is complicated by the effects of mass redistribution and salinity, of course. Tomography provides a more direct measure of depth-integrated temperature through sound speed (salinity is a negligible contributor), as well as barotropic (depth-integrated) current when reciprocal transmissions are employed. Here too, this measurement can be used to directly infer abyssal heat content or current variability, if complementary near-surface measurements are available. Tomographic measurements also have the property of being naturally integrating horizontally so that small-scale noise is greatly reduced. The HEF measurements are a similar measurement of depth-integrated current, but at a particular location. Given the difficulty of making the abyssal measurements, considering this sort of “remote sensing” of the abyssal ocean may be essential. While the HPIE instrumentation mentioned in this paper is new and under development, it is worth noting that the HEF technique is also decades old and well established.
The abyssal ocean will likely always be undersampled with measurements at a variety of perhaps not optimal places and with a variety of different measuring technologies. Synthesizing the various data types into a coherent, self-consistent estimate of the ocean state requires the data assimilation approach. Without such estimates, one runs the risk of unjustified conclusions if they are based on unrepresentative data (e.g., aliased in time, space, choice of location, etc.) Further, as the authors point out, the abyss and the near-surface are coupled - through modeling, some resolution of deep-ocean variability is achieved by shallow-ocean constraints. I suspect the authors are aware of general circulation models, but it seems essential to at least touch on the essential role of modeling in the paper.
Dushaw, B. D., G. Bold, C.-S. Chui, J. Colosi, B. Cornuelle, Y. Desaubies, M. Dzieciuch, A. Forbes, F. Gaillard, J. Gould, B. Howe, M. Lawrence, J. Lynch, D. Menemenlis, J. Mercer, P. Mikhaelvsky, W. Munk, I. Nakano, F. Schott, U. Send, R. Spindel, T. Terre, P. Worcester, and C. Wunsch, 2001. “Observing the ocean in the 2000’s: A strategy for the role of acoustic tomography in ocean climate observation” in Observing the Oceans in the 21st Century, edited by C. J. Koblinsky and N. R. Smith (GODAE Project Office and Bureau of Meteorology, Melbourne), pp. 391-418.
Dushaw, B. D., P. F. Worcester, W. H. Munk, R. C. Spindel, J. A. Mercer, B. M. Howe, K. Metzger, T. G. Birdsall, R. K. Andrew, M. A. Dzieciuch, B.D. Cornuelle, and D. Menemenlis, 2009. A decade of acoustic thermometry in the North Pacific Ocean, J. Geophys. Res., doi:10.1029/2008JC005124, in press.
Thank you very much Brian for your comments.
They are very appropriate and I included them in the revised version