15 Mar 2021

15 Mar 2021

Review status: a revised version of this preprint is currently under review for the journal OS.

The dependency of internal tides on background stratification variability: a case study on the Amazon shelf and the Bay of Biscay

Simon Barbot1, Florent Lyard1, Michel Tchilibou1, and Loren Carrere2 Simon Barbot et al.
  • 1LEGOS, Université de Toulouse, CNES, CNRS, IRD, UPS, Toulouse, France
  • 2CLS, Ramonville-Saint-Agne, France

Abstract. The forthcoming SWOT altimetric mission aims to access the smaller mesoscale oceanic circulation with an unprecedented spatial resolution and swath. The repetitivity of the mission orbit implies that high frequency processes, such as the internal tides (ITs), are under-sampled in time and their full temporal evolution is not observed. They are therefore aliased onto lower frequencies and possibly mixed into the mesoscale signals. As with the barotropic tides, the ITs must be corrected from the altimetric observations in order to access to the smaller mesoscales. But unlike barotropic tides, ITs are not completely stationary and have significant temporal variability due to their interactions with the ocean circulation and the stratification variability. ITs prediction, correction and error calculation requires a precise understanding of the ITs' surface elevation signature and its temporal variability. Stratification changes impact both on the generation and the propagation of ITs. This present study proposes to quantify the impacts of the background stratification variations alone with a classification of the observed stratification and an idealized modelling of the ITs. A single methodology is developed to handle a very broad range of stratification variability. The classification is made using clustering methods and the modelling uses the frequency domain model T-UGO.

The methodology is successfully tested on the Amazon shelf and in the Bay of Biscay. For the Amazon shelf, the pycnocline depth linearly impacts on the amplitudes and wavelengths of the ITs first two modes. An increase of the pycnocline depth increases the total ITs' amplitude but also transfers the energy from the mode 2 to the mode 1. An increase of the pycnocline depth also increases the wavelengths of both modes 1 and 2. In the Bay of Biscay we found no such proxy to describe the changes in ITs' characteristics so a seasonal climatology is explored. The seasonality of the stratification strongly affects the amplitudes of modes 2 and 3 and significantly impacts on the surface elevation of ITs. Whereas the wavelengths of all modes and the amplitude of mode 1 are only weakly affected by the stratification seasonality. The amplitude variability of modes 2 and 3 also modifies the ratio between the modes in presence and makes the horizontal scales of ITs variable. The significance of the ITs wavelength modifications with stratification changes suggests that a more accurate ITs' surface elevation correction for SWOT measurements should take into account this stratification variability.

Simon Barbot et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2021-19', Anonymous Referee #1, 13 Apr 2021
    • AC1: 'Reply on RC1', Simon Barbot, 08 Jun 2021
  • RC2: 'Comment on os-2021-19', Anonymous Referee #2, 26 Apr 2021
    • AC2: 'Reply on RC2', Simon Barbot, 08 Jun 2021

Simon Barbot et al.

Simon Barbot et al.


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Short summary
The internal tides are responsible for surface deformations that could affect the measurement of the forthcoming SWOT altimetric mission. The unidentified internal tides deformations by the satellite could reduce the detection of small eddies in the ocean. This study highlights the variability of the properties of internal tides deformations based on the stratification variability only. A single methodology is successfully applied on two areas driven by different oceanic processes.