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| Titel |
A complete sketch for fine-structure contamination by internal waves |
| VerfasserIn |
L. Gostiaux, H. van Haren |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062013
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| Zusammenfassung |
| Oceanic fine structure has been studied since the development of CTD and microstructure
profilers allowed to resolve the vertical scales of temperature and salinity in the ocean. In the
context of internal waves, it rapidly appeared that the advection of oceanic fine-structure may
lead to erroneous interpretations of temperature measurements, and much theoretical work
was achieved to distinguish real internal wave signal from the so-called fine-structure
contamination. The pioneering work of Phillips (1971) revealed how the vertical advection of
temperature steps by internal waves contaminates temperature records at fixed depths.
Fine-structure contamination can be recognized in the super-buoyant part of the spectrum as a
typical -2 slope, theoretically predicted for sharp stair cases in the temperature
profile.
However, distinguishing fine-structure contamination from other (turbulent) signals in
real datasets is sometimes difficult. We will show how the use of a large number of highly
accurate temperature sensors allows to completely resolve the fine-structure contamination
sketch. More precisely, the coherence spectrum between vertically separated sensors shows a
characteristic Ï-phase signature above the Brunt-Väisälä frequency N that we can reproduce
using a simple kinematic model.
The dataset used consists of temperature time series (1Hz during 1.5 year) obtained in the
Canary Basin. Over a range of 132.5m, 54 NIOZ High Sampling Rate Thermistors
(NIOZ-HST, 1mK relative accuracy) were moored around 1455m. Coherence between
individual records shows a weak, but significant peak above N for all vertical separations.
Instead of a dominant 0-phase difference over the range of sensors, as observed for internal
waves at frequencies f < Ïă < N, f denoting the inertial frequency, this super-buoyancy
coherence shows Ï-phase difference over a frequency band, that shifts to higher frequencies
as the vertical separation between thermistors diminishes. In the time domain it is observed
that this high frequency coherence mainly occurs when non-linearities in the temperature
gradient, such as steps in the temperature profile, are advected passed the sensors.
A kinematic model of fine structure contamination successfully reproduces these
observation.
Surprisingly, the canonical -2 slope of the temperature spectrum above N is not observed
in the in situ data, which rather slope like -8/3. The -8/3 slope can nevertheless be reproduced
in our kinematic model, in which there is no turbulence, provided the jumps in the
temperature profile are not infinitely thin. |
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