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| Titel |
Vertical-mode decomposition as a dynamic description of the Atlantic meridional overturning circulation |
| VerfasserIn |
Zoltan Szuts, Jochem Marotzke |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036047
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| Zusammenfassung |
| The RAPID array monitors the vertical density structure across the North Atlantic at 26.5-N
in order to calculate the basin or interior component of the meridional overturning circulation
(MOC). By decomposing density fluctuations onto vertical modal structures whose dynamics
are well understood, we gain a framework for interpreting fluctuations in the overturning
circulation. In addition, this method separates the barotropic and baroclinic components and
allows a ready comparison of satellite altimetry with the first baroclinic mode. We
consider three data sets: moored density or geopotential anomalies (GPA) from
the RAPID temperature and salinity moorings, bottom pressure (BP) measured
at the base of the RAPID moorings, and sea surface height (SSH) from satellite
altimetry. As a result of our hydrographic data coming from moored CTDs, a new
decomposition technique was developed that extracts low-frequency (periods longer than 2
days) modal amplitudes directly from density anomalies. The barotropic mode that
results is combined with BP. GPA from the original RAPID processing is called
“standard," while that which is synthesized using the modal decomposition is called
“reconstructed".
The modal decomposition recovers almost all of the variance in the vertical (r2 > 0.9)
with slightly less effectiveness right at the western boundary (r2 = 0.8). Away
from the boundaries, the reconstructed GPA and its gradient agrees closely with the
standard GPA and with SSH, and almost all of the variance is contained in the first
baroclinic mode. In contrast, at the boundaries the signals are much weaker overall
and a significant amount of the variance is contained outside of the first baroclinic
mode. Despite the fact that the first baroclinic mode extracts the near boundary
transport signal most correlated with SSH, the reconstructed GPA does a poor job of
recovering total transport fluctuations adjacent to the boundaries. An additional
difference at the western boundary is the presence of energetic fluctuations with periods
of 2–20 days. Given that the interior component of the MOC is calculated from
boundary-to-boundary density gradients, our results suggest that the theoretical
assumptions behind modal dynamics do not apply to the interior MOC, and that
understanding boundary effects is critical to understanding the measured MOC signal. |
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