Storms modify baroclinic energy fluxes in a seasonally stratified shelf sea: Inertial‐tidal interaction

Joanne E Hopkins,Mark E Inall,Gordon R Stephenson,J A M Green,Matthew R Palmer

doi:10.1002/2014jc010011

Abstract

AbstractObservations made near the Celtic Sea shelf edge are used to investigate the interaction between wind‐generated near‐inertial oscillations and the semidiurnal internal tide. Linear, baroclinic energy fluxes within the near‐inertial (f) and semidiurnal (M2) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the 2 week deployment period, the semidiurnal tide drove 28–48 W m−1 of energy directly on‐shelf. Little spring‐neap variability could be detected. Horizontal near‐inertial energy fluxes were an order of magnitude weaker, but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25–43% increase in positive on‐shelf energy flux. The phase relationship between f and M2 determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2 day counter‐clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations.

Highlights

The oceans’ rich internal wavefield is an essential link in the energy cascade from large to small-scale motions and is a major source of energy available for vertical mixing
We propose that the dominant physical mechanism behind the importance of this term is a coupling between vertical shear from wind-induced oscillations and vertical velocities associated with the M2 internal tide
We demonstrate how wind-generated near-inertial oscillations can modify baroclinic internal wave energy fluxes in a stratified shelf sea environment

Summary

Introduction

The oceans’ rich internal wavefield is an essential link in the energy cascade from large to small-scale motions and is a major source of energy available for vertical mixing. In the oceans’ interior, dissipation of low-mode internal wave energy provides deep mixing that is crucial in maintaining the Meridional Ocean Circulation (see, e.g., the reviews by Garrett [2003] and Wunsch and Ferrari [2004]). Propagating internal gravity waves, supported by density gradients within the ocean interior, occupy a frequency range bounded below by the local inertial frequency Using measurements made in the field, this paper describes (1) interactions between nearinertial waves and the semidiurnal internal tide and (2) the resulting energy fluxes in the wave band bounded by these frequencies in the Celtic Sea, a seasonally stratified section of the Northwest European

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Discussion

Conclusion