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Ann. Geophys., 21, 123-135, 2003
www.ann-geophys.net/21/123/2003/
© European Geosciences Union 2003
Comparison between XBT data and TOPEX/Poseidon satellite altimetry in the Ligurian-Tyrrhenian area
S. Vignudelli1, P. Cipollini2, F. Reseghetti3, G. Fusco3, G. P. Gasparini4, and G. M. R. Manzella3
1CNR – Istituto Elaborazione della Informazione, 56 100 Pisa, Italy
2James Rennell Division and Laboratory for Satellite Oceanography, Southampton Oceanography Centre, SO14 3ZH, Southampton, UK
3ENEA – CRAM, P.O. Box 224, 19 100 La Spezia, Italy
4CNR – Istituto per lo studio dell’Oceanografia Fisica, 19 036 Lerici, La Spezia, Italy
Abstract. From September 1999 to
December 2000, eXpendable Bathy-Thermograph (XBT) profiles were collected along
the Genova-Palermo shipping route in the framework of the Mediterranean
Forecasting System Pilot Project (MFSPP). The route is virtually coincident
with track 0044 of the TOPEX/Poseidon satellite altimeter, crossing the
Ligurian and Tyrrhenian basins in an approximate N–S direction. This allows a
direct comparison between XBT and altimetry, whose findings are presented in
this paper. XBT sections reveal the presence of the major features of the
regional circulation, namely the eastern boundary of the Ligurian gyre, the
Bonifacio gyre and the Modified Atlantic Water inflow along the Sicily coast.
Twenty-two comparisons of steric heights derived from the XBT data set with
concurrent realizations of single-pass altimetric heights are made. The overall
correlation is around 0.55 with an RMS difference of less than 3 cm. In the
Tyrrhenian Sea the spectra are remarkably similar in shape, but in general the
altimetric heights contain more energy. This difference is explained in terms
of oceanographic signals, which are captured with a different intensity by the
satellite altimeter and XBTs, as well as computational errors. On scales larger
than 100 km, the data sets are also significantly coherent, with increasing
coherence values at longer wavelengths. The XBTs were dropped every 18–20 km
along the track: as a consequence, the spacing scale was unable to resolve
adequately the internal radius of deformation (< 20 km). Furthermore, few
XBT drops were carried out in the Ligurian Sea, due to the limited north-south
extent of this basin, so the comparison is problematic there. On the contrary,
the major features observed in the XBT data in the Tyrrhenian Sea are also
detected by TOPEX/Poseidon. The manuscript is completed by a discussion on how
to integrate the two data sets, in order to extract additional information. In
particular, the results emphasize their complementariety in providing a
dynamically complete description of the observed structures.
Key words. Oceanography: general
(descriptive and regional oceanography) Oceanography: physical (sea level
variations; instruments and techniques)
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