EGU2020-17014, updated on 04 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-17014
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Observed seasonality in internal wave energy and associated mixing in a temperate shelf sea

Juliane Wihsgott1, Matthew Palmer1, Jonathan Sharples2, and Jo Hopkins1
Juliane Wihsgott et al.
  • 1National Oceanography Centre, Liverpool, United Kingdom of Great Britain and Northern Ireland (jugott@noc.ac.uk)
  • 2School of Environmental Sciences, University of Liverpool, UK

Long-term observations (March’14 − July’15) of ocean density and velocity from the North West European shelf reveal a seasonality in internal wave energy linked to the seasonal cycle of stratification. Further, this seasonality extends to internal mixing associated with internal waves that can be effectively described by the buoyancy frequency (N2), with the strongest mixing associated with strongly stratified summer conditions. To better understand these results a model was used that employed three different, commonly used parameterisations of internal mixing. Each parameterisation produced some degree of seasonality in internal mixing. Contrary to observed results however, all three model scenarios produced a minimum in internal mixing during summer, with enhanced mixing observed during spring and autumn. This failure in each model was attributed to the lack of realistic levels of enhanced baroclinic energy and shear (S2) that is identified in observations to be attributable to internal waves. These observations reveal a close relationship between N2and S2, resulting in a near continuous state of marginal stability; where the gradient Richardson number is maintained at a near critical level. Due to the observed strong dependence of internal wave energy and internal mixing on stratification, a modified version of the MacKinnon and Gregg (2003a) turbulence scaling was employed. This modified parameterisation successfully replicated the observed seasonality in internal mixing. This important result implies that future parameterisations should aim to scale internal mixing on enhanced levels of S2 from internal waves, which are shown here to be suitably predicted by the seasonal cycle of stratification (N2).

How to cite: Wihsgott, J., Palmer, M., Sharples, J., and Hopkins, J.: Observed seasonality in internal wave energy and associated mixing in a temperate shelf sea, EGU General Assembly 2020, Online, 4–8 May 2020, https://doi.org/10.5194/egusphere-egu2020-17014, 2020

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Presentation version 2 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-17014, Paul Pukite, 06 May 2020

    From ypur presentation, are these cycles due to synodic tides?

    • AC1: Reply to CC1, Juliane Wihsgott, 06 May 2020

      Hello, thank you for your question. The plot shows the depth averaged tidal velocities. At the mooring site, the dominant tidal frequencies are M2, S2, N2, K1 and O1. The pattern is caused by the different harmonics moving in and out of phase.I hope this answers your question.

       

      • CC2: Reply to AC1, Paul Pukite, 06 May 2020

        Thanks, I guess I am seeing the envelope which appears to cycle according to be the 14-day lunar tide

        • AC2: Reply to CC2, Juliane Wihsgott, 06 May 2020

          The 14 day variation that you mention is the spring-neap cycle. That is caused by the interaction of the two semi-diurnal constituents, M2 and S2.

Presentation version 1 – uploaded on 05 May 2020 , no comments