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| Titel |
Coherence of off-shore steric height and island sea level. |
| VerfasserIn |
J. Williams, C. W. Hughes |
| 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 |
250060350
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| Zusammenfassung |
| What is the relationship between the sea level at a deep-ocean island tide gauge and that
offshore? For every mid-ocean island in the OCCAM 1/12 model we compared sea level
close to the island with the sea-level, steric, and bottom-pressure signals averaged over a ring
of deeper water around the island. Analysis of these time series clearly illustrates the
propagation of barotropic and baroclinic signals to the coast on time scales for which Rossby
waves dominate, but suggests that sea-level signals associated with eddies may not influence
the coastal signal as strongly.
At frequencies higher than the allowed baroclinic Rossby wave frequency range,
the coastal signal is poorly predicted by offshore density, but coastal sea level and
off-shore bottom pressure tend to be well-correlated, indicating a dominance of
barotropic processes. Baroclinic Rossby waves are prohibited at periods shorter than a
latitude-dependent limit — about 1 month for ±5°N, rising to 5 months at ±25°N — and this
dependence shows up clearly. At the lowest frequencies, the off-shore steric signal is
well correlated with the coastal signal. However, there is an intermediate band of
frequencies, in the region of the baroclinic Rossby waves cut-off frequency, for which
off-shore sea level is not strongly related to coastal sea level. The low frequency
cut-off of this band is also latitude dependent and corresponds well to the maximum
eddy duration by latitude seen in satellite altimeter data by Chelton et al. (2011)
— about 3.5 months at ±5°N, rising to 24 months at ±25°N. In this band, we
hypothesise that a mixture of behaviours may occur, perhaps with off-shore and
coastal sea levels being correlated for wave-like behaviour and poorly correlated for
eddies.
In the Southern Ocean, linear baroclinic wave speeds become irrelevant, but a switch
between high frequency barotropic signals and an increasingly important steric contribution
at lower frequencies still occurs in many regions. The steric contribution is consistent with
what would be expected of the observed equivalent-barotropic structure in this
region.
Reference
D. B. Chelton, M. G. Schlax, and R. M. Samelson (2011), Global observations
of nonlinear mesoscale eddies, Progress In Oceanography, 91(2), 167 – 216,
doi:10.1016/j.pocean.2011.01.002. |
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