Observability of fine-scale ocean dynamics in the northwestern Mediterranean Sea
Technological advances in the recent satellite altimeter missions of Jason-2, SARAL/AltiKa and CryoSat-2 have improved their signal-to-noise ratio, allowing us to observe finer-scale ocean processes with along-track data. Here, we analyse the noise levels and observable ocean scales in the northwest...
Main Authors: | , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-01-01
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Series: | Ocean Science |
Online Access: | http://www.ocean-sci.net/13/13/2017/os-13-13-2017.pdf |
Summary: | Technological advances in the recent satellite altimeter missions of Jason-2,
SARAL/AltiKa and CryoSat-2 have improved their signal-to-noise ratio,
allowing us to observe finer-scale ocean processes with along-track data.
Here, we analyse the noise levels and observable ocean scales in the
northwestern Mediterranean Sea, using spectral analyses of along-track sea
surface height from the three missions. Jason-2 has a higher mean noise level
with strong seasonal variations, with higher noise in winter due to the
rougher sea state. SARAL/AltiKa has the lowest noise, again with strong
seasonal variations. CryoSat-2 is in synthetic aperture radar (SAR) mode in
the Mediterranean Sea but with lower-resolution ocean corrections; its
statistical noise level is moderate with little seasonal variation. These
noise levels impact on the ocean scales we can observe. In winter, when the
mixed layers are deepest and the submesoscale is energetic, all of the
altimeter missions can observe wavelengths down to 40–50 km (individual
feature diameters of 20–25 km). In summer when the submesoscales are
weaker, SARAL can detect ocean scales down to 35 km wavelength, whereas the
higher noise from Jason-2 and CryoSat-2 blocks the observation of scales less
than 50–55 km wavelength.
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This statistical analysis is completed by individual case studies, where
filtered along-track altimeter data are compared with co-located glider and
high-frequency (HF) radar data. The glider comparisons work well for larger ocean structures,
but observations of the smaller, rapidly moving dynamics are difficult to
co-locate in space and time (gliders cover 200 km in a few days, altimetry
in 30 s). HF radar surface currents at Toulon measure the meandering
Northern Current, and their good temporal sampling shows promising results in
comparison to co-located SARAL altimetric currents. Techniques to separate
the geostrophic component from the wind-driven ageostrophic flow need further
development in this coastal band. |
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ISSN: | 1812-0784 1812-0792 |