Ocean Science www.ocean-sci.net 1812-0784 1812-0792 4 3 2008 10.5194/os-4-215-2008 http://www.ocean-sci.net/4/215/2008/ http://www.ocean-sci.net/4/215/2008/os-4-215-2008.html http://www.ocean-sci.net/4/215/2008/os-4-215-2008.pdf 215 222 2008-09-03 A comparison between vertical motions measured by ADCP and inferred from temperature data H. van Haren hansvh@nioz.nl Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, The Netherlands Combined vertical current (w) and thermistor string data demonstrate that high-, near-buoyancy frequency internal "wave" trains along a pycnocline in a flat-bottom shelf sea consist for 2 periods of a dominant mode-1 non-linear part, while thereafter mainly of linear [mode-2, quadrupled frequency] waves, to first order. In a simple [linear] heat budget the use of unfiltered temperature gradient or its time mean changes results by only 10%. The observations also demonstrate that temperature is not always adequate to estimate vertical motions using the linear 1-D heat equation. In shallow seas, tidal-w estimated from temperature data can be an order of magnitude weaker than directly observed w, and thus do not represent free internal waves. In the ocean, not too far from the main internal wave topography source, tidal motions represent linear waves and are well described by temperature-inferred w. There however, temperature-inferred w and directly observed w differ strongly near the buoyancy frequency, at which w is dominated by non-linear waves, and near [sub]inertial frequencies, at which w is dominated by eddies and gyroscopic waves. Amen, R. and Maxworthy, T.: The gravitational collapse of a mixed region into a linearly stratified fluid, J. Fluid Mech., 96, 65–80, 1980. Davis, R. E. and Acrivos, A.: Solitary internal waves in deep water, J. Fluid Mech., 29, 593–607, 1967. Hallock, Z. R. and Field, R. L.: Internal-wave energy fluxes on the New Jersey Shelf, J. Phys. Oceanogr., 35, 3–12, 2005. Helfrich, K. R.: Internal solitary wave breaking and run-up on a uniform slope, J. Fluid Mech., 243, 133–154, 1992. Helfrich, K. R. and Melville, W. K: On long nonlinear internal waves over slope-shelf topography, J. Fluid Mech., 167, 285–308, 1986. Hosegood, P. and van Haren, H.: Near-bed solibores over the continental slope in the Faeroe-Shetland Channel, Deep-Sea Res. II, 51, 2943–2971, 2004. Howarth, M. J.: The effect of stratification on tidal current profiles, Cont. Shelf Res., 18, 1235–1254, 1998. Inall, M. E., Rippeth, T. P., and Sherwin, T. J.: Impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break, J. Geophys. Res., 105, 8687–8705, 2000. Krauss, W.: Methoden und Ergebnisse der Theoretischen Ozeanographie, II Interne Wellen, Gebrueder Borntraeger, Berlin, 248 pp., 1966. Lee, J. H., Lozovatsky, I., Jang, S.-T., Jang, C. J., Hong, C. S., and Fernando, H. J. S.: Episodes of nonlinear internal waves in the northern East China Sea, Geophys. Res. Lett., 33, L18601, doi:10.1029/2006GL027136, 2006. Pinkel, R.: Observations of the near-surface internal wavefield, J. Phys. Oceanogr., 11, 1248–1257, 1981. van Aken, H. M., van Haren, H., and Maas, L. R. M.: The high-resolution vertical structure of internal tides and near-inertial waves, measured with an ADCP over the continental slope in the Bay of Biscay, Deep-Sea Res. I, 54, 533–556, 2007. van Haren, H. and Millot, C.: Gyroscopic waves in the Mediterranean Sea, Geophys. Res. Lett. 32, L24614, doi:10.1029/2005GL023915, 2005. van Haren, H., Maas, L., Zimmerman, J. T. F., Ridderinkhof, H., and Malschaert, H.: Strong inertial currents and marginal internal wave stability in the central North Sea, Geophys. Res. Lett., 26, 2993–2996, 1999. van Haren, H., Groenewegen, R., Laan, M., and Koster, B.: A fast and accurate thermistor string, J. Atmos. Oceanic Technol., 18, 256–265, 2001.