Near-inertial wave propagation between stratified and homogeneous layers: Conclusions and References

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22 May 2024

Authors:

(1) Hans van Haren, NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, the Netherlands.

Abstract and Intro

Data

Observations

Simulating Transition

Discussion

Conclusions and References

6 Conclusions

Acknowledgments I thank the crews of the R/V Thethys II and Le Suroît for the sea operations, G. Rougier and C. Millot for preparing the ‘GYROSCOP-2’ mooring, and T. Gerkema for providing the model results. I gratefully acknowledge support from the Netherlands organisation for the advancement of scientific research, NWO, and Centre National de la Recherche Scientifique, CNRS, under the (alas no longer existing) French Dutch scientific collaboration.

References

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Fig. 1 Site and hydrography. a) Western-Mediteranean Sea with observational site *. Depth contours every 500 m for [500, 2500] m and 2750 m. b) Typical local density anomaly profiles referenced to a pressure of 2000 dbar observed using shipborne CTD in April 2005 (blue; stopped at z = -2500 m) and February 2006 (red) during mooring deployment and recovery cruises, respectively. For reference, particular density gradient slopes are given (see text). To the left, the mooring is given schematically, with current meter (CM) depths (same colours as in Figs 2a,b, 3a,b; light-blue: only Tdata) and range of upward looking ADCP (yellow). The local seafloor is at the x-axis.

Fig. 2 Time-series for the entire 10-month mooring period (a,b) and for the 5.5-month period of ADCP-data (c,d). Yeardays in 2006 are +365. a) Current amplitude at ADCP's bin 5 (z = -2025 m) and CM (-1990 m) using the instruments’ colour coding in Fig. 1b. The horizontal bar indicates the period of Fig. 6. The ADCP record ends before a change in the time series' characteristics (days 260-270, indicated by thick black bar) that defines two different periods. b) T-time-series from the same instruments as in a), but at z = -2090 m for the ADCP. T-data are calibrated using CTD-profiles at recovery. During the 10-month period, the local homogeneous-adiabatic T raised by 0.013C, which is approximately the T-sensors' resolution. c) Mooring-line tilt measured by the ADCP and reflecting water-flow drag. d) Vertical current-component (black) and error velocity (light-blue) measured at ADCP’s bin 5 (z = -2025 m) and smoothed using a 3- h running mean. The horizontal black line indicates the period of Fig. 6.

Figure 4. Spectra from ADCP-data at z = -2025 m. Horizontal kinetic energy (black), vertical current (red) and error velocity (light-blue). Vertical lines as in Fig. 3.

Figure 7. Simulation results of vertical current amplitudes |w| for internal wave ‘beams’ in a vertical-horizontal plane with the seafloor at z = -2000 m. The upper two panels show a transition (dashed line) between a weakly stratified layer above a homogeneous one. The lower two panels show a transition between a stratified layer below a homogeneous one. The left two panels are for super-inertial motions, the right two for sub-inertial motions. The left panels show amplitude enhancement and trapping in the homogeneous N=0 layer, while the right panels show enhancement and trapping in the stratified layer. Note the two different angles to the horizontal of up- and down-going rays, which is typical for the non-traditional approach (Gerkema et al. 2008).

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