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| Titel |
Upper ocean mixing processes and their impact on the mixed layer heat balance during the onset of the Atlantic Cold Tongue. |
| VerfasserIn |
M. Dengler, P. Brandt, M. J. McPhaden, S. Thomsen, G. Krahmann, T. Fischer, P. Freitag, R. Hummels |
| 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 |
250071119
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| Zusammenfassung |
| An extensive measurement program within the Atlantic Cold Tongue (ACT) region was
carried out during the ACT onset in boreal summer 2011. During two consecutive cruises
shipboard microstructure profiles, conductivity-temperature-depth-O2 (CTD-O2) profiles and
shipboard velocity profiles were collected between mid-May and mid-July. The shipboard
measurements were complemented by a Glider swarm experiment during which 5400
CTD-O2 profiles were collected along specified transects within the ACT region.
One of those Gliders was equipped with a MicroRider turbulence package and
collected a 5-week microstructure time series of about hourly-resolution in the center
of the cold tongue on the equator at 10°W. The MicroRider/Glider package was
circling a PIRATA mooring from which additionally high-resolution acoustic Doppler
current profiles are available for this time period to allow analysis of the background
conditions. In this contribution we use a subset from the above data to detail mixing
processes in the upper stratified ocean and describe the background conditions
favoring enhanced mixing. From end of May to mid–July, sea surface temperature
decreased from 26°C to below 22°C at 10°W. During the whole period of autonomous
microstructure observations, strong bursts of turbulence were observed extending from the
mixed layer into the upper thermocline. These bursts lasted for 3-5 hours and were
found to penetrate to about 30m below the base of the mixed layer. They were
observed to occur predominately during night-time while during day-time they were
less frequent. Dissipation rates of turbulent kinetic energy (ε) during these bursts
were above 3x10-6Wkg-1 in the upper stratified water column and turbulent eddy
diffusivities (KÏ) often reached 1x10-3m2s-1. The data set suggests that strength and
frequency of occurrence of the turbulent bursts is modulated by the presents of
Tropical Instability Waves which additionally enhance background shear at the
equator. The presents of internal waves having frequencies close to the buoyancy
frequency during enhanced mixing events will be discussed. From the first 6 days of
microstructure data, a diapycnal heat flux divergence from the mixed layer into the
upper stratified ocean of 80Wm-2 was inferred. Other contributions to the mixed
layer heat balance will be examined to evaluate their relevance during ACT onset. |
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