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| Titel |
Bubble momentum plume as a mechanism for an early breakdown of the seasonal stratification in the northern North Sea |
| VerfasserIn |
Janine Nauw, Peter Linke, Ira Leifer |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094709
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| Publikation (Nr.) |
 EGU/EGU2014-10137.pdf |
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| Zusammenfassung |
| The presence of a seasonal thermocline likely plays a key role in restraining methane released
from a seabed source in the deeper water column, thereby inhibiting exchange to the
atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g.
upwelling and may thereby be partially responsible for an early breakdown of the seasonal
thermocline.
Measurements at site 22/4b, located at (57°55’N, 1°38’E) in the UK Central North Sea,
200 km east of the Scottish mainland, where gas is still being released since a blow out in
1990, have been used to identify the generation of the seasonal thermocline, the depth of the
upper mixed layer and its breakdown in autumn. Data derived from two landers, containing
an Acoustic Doppler Current Profiler (ADCP) and a Conductivity Temperature
Depth (CTD) sensor, were used to determine the mixed layer depth (MLD) and the
breakdown of the thermocline. Mixing of upper layer fluid into the lower layer
has been inferred from large amplitude variations in the near-bottom temperature
observations.
The ADCPs estimate velocities profiles in four beam directions using Doppler shifted
frequency from acoustic pings sent out and received by four different transducers in a specific
configuration. Besides that, the intensity of the backscattered sound per transducer is also
recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal
cycle, but in bubble free periods, the MLD can be estimated using the acoustic backscatter
signal as local maxima. Results show that the thermocline broke down between
mid-October and early November, several weeks earlier than the breakdown of the
thermocline in nearby/comparable areas, likely induced by bubble-induced downwelling at
the site. The early breakdown of the thermocline was accompanied by multiple
occurrence of a strong jetlike structure, associated with the seasonal tidal mixing front. |
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