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| Titel |
How storms modify baroclinic energy fluxes in a seasonally stratified shelf sea: inertial-tidal interaction |
| VerfasserIn |
Jo Hopkins, Gordon Jr. Stephenson, Mattias Green, Mark Inall, Matthew Palmer |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107434
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| Publikation (Nr.) |
 EGU/EGU2015-7136.pdf |
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| Zusammenfassung |
| The oceans’ rich internal wave field 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 shallow shelf seas, vertical mixing across the thermocline maintains elevated
summer time production, helping continental shelves to make a disproportionally
large contribution to total ocean primary production relative to their surface area.
Temperate shelf seas are therefore a significant CO2 sink and a critical link in the
ocean-earth-atmosphere system. The two most energetic parts of the global internal wave
spectrum are near-inertial waves with frequencies Ï ≈f, and the lunar semi-diurnal
frequency, M2.
Using data from a mooring array, we demonstrate how wind generated near-inertial
oscillations can modify baroclinic internal wave energy fluxes in the Celtic Sea, a seasonally
stratified shelf. Linear fluxes of baroclinic energy are dominated by the semi-diurnal tide that
outside of the complex generation zone drives a modest 28-48 W m-1 directly on-shelf.
Given the complex 3-dimensional nature of the generation and propagation however spatial
variability is high and net flux vectors may differ by 90°or more within an internal tidal
wavelength. Horizontal energy fluxes driven independently by near-inertial motions are an
order of magnitude weaker, but non-linear interaction between the vertical shear of
inertial-oscillations and the vertical velocity associated with the M2 internal tide is a
significant source of energy at the sum of their frequencies (M2+f). The phase relationship
between M2 and f determines whether this non-linear interaction constructively
enhances or destructively dampens the linear tidal component of the flux, a phasing that
introduces a 2-2.3 day counter-clockwise beating to the energy transport. Relative
to the M2 contribution, this beating and increase in flux magnitude explains an
additional 10% of the variability of the full flux time series. Over individual tidal
periods, inertial-tidal interaction resulted in a 50% increase in flux magnitude. Over
the whole 2 week deployment, a 25-43% increase in positive on-shelf energy flux
was observed. Our data set clearly identifies a switch between tidal and inertially
dominated shear and energy flux regimes. These findings are highly relevant in the
much needed development of mixing parameterizations for shelf sea models where
non-linear interactions and the processes driving temporal and spatial variability of
shear, instability and consequently turbulence are of importance. Failure to represent
the inertial-tidal interactions described here will lead to underestimation of the
magnitude and episodic nature of turbulent dissipation and thermocline mixing. |
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