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Titel |
A model data comparison of different classes of LSW and interannual to decadal variability in a FESOM model setup |
VerfasserIn |
Patrick Scholz, Gerrit Lohmann, Monica Ionita, Dagmar Kieke, Monika Rhein |
Konferenz |
EGU General Assembly 2013
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250082925
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Zusammenfassung |
The climate in the Atlantic region is essentially influenced by the Atlantic meridional
overturning circulation (AMOC) which carries warm waters into northern latitudes and
returns cold deep water southward across the equator. In the Labrador Sea basin a major
component of the cold limb of the Atlantic meridional overturning circulation (AMOC) is
formed. The intermediate water mass that is part of this deep convection process is the
Labrador Sea Water (LSW) which can be separated into two different classes: the deep LSW
(dLSW) and the less dense upper LSW (uLSW). Both LSW modes are formed by convection,
accompanied by a strong surface cooling during winter conditions, which leads to an increase
in the near-surface density and to an unstable stratification and a homogenization of the water
column.
In this study we simulated the deep-water formation in the Labrador Sea using the
Finite-Element Sea-Ice Ocean Model (FESOM) in a global model setup with regional
focus on the Labrador Sea and Greenland Sea. We evaluated the capability of the
model setup to reproduce a realistic deep water formation in the Labrador Sea by
analyzing the modeled Labrador Sea hydrography and we compared the modeled
and observational derived dLSW and uLSW layer thicknesses for the time interval
1958-2007.
It is shown that the model is able to reproduce different phases in the temporal evolution
of the potential density, temperature and salinity, which are known in observational
data.
Based on composite maps of the thermal and haline contributions to the surface density
flux we can prove that the central Labrador Sea in the model is dominated by the thermal
contributions of the surface density flux, while the haline contributions are limited to the
branch of the Labrador Sea Boundary Current system, where they are dominated from the
haline contributions of sea ice melting and formation. Our model results feature a shielding of
the central Labrador Sea from the haline contributions by the Labrador Sea Boundary Current
system.
Furthermore we investigated modes of interannual to decadal variability for the period
1958-2004 and attributed the general variability in the model to the atmospheric forcing
and to internal modes of the ocean system. Based on a North Atlantic Deep Water
(NADW) index defined for a normal and random forced FESOM run, where the
interannual to decadal atmospheric variability in the random forced run is replaced
by white noise, we identify modes of interannual to quasi-decadal variability of
7yr and 14yr, respectively. The origin of the 14yr variability is attributed to the
atmospheric forcing, while the 7yr variability is linked to internal modes of the
ocean.
To further isolate the horizontal, but also the vertical variability in the model, we apply a
principal oscillation pattern analysis in a three dimensional context. Two exceptional stable
interannual modes are captured by the POP analysis and their variability is attributed to a
propagating Rossby wave structure. |
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