4 open review comments to “Future observations for monitoring global ocean heat content”
The authors may find my own review of this topic to be of help (or not?)
A number of relevant papers are on my web site, but see especially:
Trenberth, K. E., and J. T. Fasullo, 2009: Changes in the flow of energy through the climate system. Meteorologische Zeitschrift, in press. [PDF]
Trenberth, K. E., 2009: An imperative for adapting to climate change: Tracking Earth’s global energy. Current Opinion in Environmental Sustainability, submitted. [PDF]
The background to looking at change is given in
Trenberth, K. E., J. T. Fasullo, and J. Kiehl, 2009: Earth’s global energy budget. Bull. Amer. Meteor. Soc., 90, No. 3, 311-324, doi: 10.1175/2008BAMS2634.1. [PDF]
Trenberth, K. E., and J. Fasullo, 2008: An observational estimate of ocean energy divergence. J. Phys. Oceanogr., 38, 984-999. [PDF]
This white paper provides an excellent summary of why measurements of ocean heat content are important and the substantial progress that has been made in measuring upper-ocean heat content since OceanObs’99 through the gradual development of the Argo array of profiling floats. The paper also discusses a number of issues that remain, including systematic errors in XBT and Argo measurements, the need for greater spatial and temporal resolution, and the lack of data coverage in the deep (>2000 m) and ice-covered oceans.
It seems unlikely that it will prove possible to address all of these issues without more fully considering the application of other types of measurements, including satellite altimetry and acoustic tomography, to the determination of ocean heat content, using data assimilation methods and ocean models to combine the disparate data types into dynamically-consistent estimates of the ocean state. Simply increasing the number of Argo floats to obtain greater spatial and temporal resolution seems a bit problematic, for example.
The white paper mentions the role of satellite altimeter data in improving spatial resolution and in identifying and correcting systematic errors in XBT and Argo data, but does not mention satellite altimetry in Section 6, which summarizes the future observations needed for monitoring global ocean heat content. We believe that this topic needs more development and greater emphasis, particularly when discussing the future.
The white paper currently does not mention acoustic remote sensing methods. These methods have a number of attractive properties (Dushaw et al., 2002). They are inherently spatially averaging, which suppresses the effects of mesoscale variability and directly provides precise measures of large-scale temperature. They provide measures of depth-integrated temperature that extend into the deep (> 2000 m) ocean. They provide high temporal resolution. They can be made without risk of calibration drift, as they depend only on the accurate measurement of time. They can be used in ice-covered regions.
A recent paper (Dushaw et al., 2009) presents decade-long time series from acoustic thermometry measurements in the North Pacific and compares the measured travel times with travel times derived from four independent estimates of the North Pacific, including an objective analysis of the upper-ocean temperature field derived from satellite altimetry and in situ profiles (Willis et al., 2004). Comparisons of the measured and calculated travel times provide stringent tests of the large-scale temperature variability in the various estimates of the state of the North Pacific. The differences are sometimes substantial, indicating that acoustic thermometry provides significant additional information. Given the unique and complementary nature of these results, we would hope that some discussion of the potential role of acoustic thermometry in future measurements of ocean heat content would be included in the white paper.
Peter F. Worcester, Scripps Institution of Oceanography, Univ. of California, San Diego
Timothy F. Duda, Woods Hole Oceanographic Institution
Brian D. Dushaw, Applied Physics Laboratory, Univ. of Washington
Dushaw, B. D., G. Bold, C.-S. Chiu, J. A. Colosi, B. D. Cornuelle, Y. Desaubies, M. A. Dzieciuch, A. M. G. Forbes, F. Gaillard, A. Gavrilov, J. Gould, B. M. Howe, M. Lawrence, J. F. Lynch, D. Menemenlis, J. A. Mercer, P. Mikhalevsky, W. H. Munk, I. Nakano, F. Schott, U. Send, R. C. Spindel, T. Terre, P. F. Worcester, and C. Wunsch (2002), 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, pp. 391–418, Bureau of Meteorology, Melbourne, Australia.
Dushaw, B. D., P. F. Worcester, W. H. Munk, R. C. Spindel, J. A. Mercer, B. M. Howe, K. Metzger, Jr., 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., 114, C07021, doi: 10.1029/2008JC005124.
Willis, J. K., D. Roemmich, and B. D. Cornuelle (2004), Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales, J. Geophys. Res., 109, C12036, doi: 10.1029/2003JC002260.
Thanks for the comments Peter et al.
I will certainly incorporate your suggestions in the next revision. I think the acoustic thermometry could be particularly useful for providing independent estimates of subsurface temperature changes, especially as we try to pin-down in-situ data biases.