In:
Ocean Science, Copernicus GmbH, Vol. 19, No. 3 ( 2023-06-26), p. 903-921
Abstract:
Abstract. Over the last 3 decades, satellite altimetry has observed sea surface
height variations, providing a regular monitoring of the surface ocean
circulation. Altimetry measurements have an intrinsic signal-to-noise ratio
that limits the spatial scales of the currents that can be captured. However,
the recent progress made on both altimetry sensors and data processing allows
us to observe smaller geophysical signals, offering new perspectives in
coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter
missions with three different technologies to capture the Northern Current
(northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic
aperture radar altimeter, launched in 2016). Therefore, we use a
high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the
model is very close to the observations of high-frequency radars and gliders
in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on
average, with a mean core velocity of 0.39 m s−1. Its signature in terms of sea
level consists of a drop whose mean value at 6.14∘ E is 6.9 cm,
extending over 20 km. These variations show a clear seasonal pattern, but
high-frequency signals are also present most of the time. In comparison, in
1 Hz altimetry data, the mean sea level drop associated with the Northern
Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with
SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of
corresponding sea level variability, Jason-2 and SARAL altimetry estimates
are larger than the model reference (+1.3 and +1 cm respectively),
whereas Sentinel-3A shows closer values (−0.4 cm). When we derive
geostrophic surface currents from the satellite sea level variations
without any data filtering, in comparison to the model, the standard
deviations of the velocity values are also very different from one mission to the
other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for
SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea
level data with different cutoff wavelengths, the best agreement between the
model and the altimetry distributions of velocity values are obtained with a
60, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and
Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to
illustrate how the advances in the performances of altimeters and in the
data processing improve the observation of coastal currents but also to
quantify the corresponding gain.
Type of Medium:
Online Resource
ISSN:
1812-0792
DOI:
10.5194/os-19-903-2023
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2023
detail.hit.zdb_id:
2183769-7
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