Abstract
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)
Highlights
The Ligurian and Tyrrhenian Seas are areas of considerable oceanographic interest in the western Mediterranean region
The XBT sections (Fig. 4) and the Along-Track Scanning Radiometer (ATSR) Sea Surface Temperature (SST) imagery (Fig. 5) confirm that three main semipermanent oceanographic features are present in the area: two cold features correspond to the Ligurian and Bonifacio Gyres, and a warmer patch of water, related to the Modified Atlantic Water (MAW) and Levantine Intermediate Water (LIW) influx, near the coast of Sicily
The purpose of this study is to determine the degree to which measures of sea surface height derived from T/P and XBT data sets, collected along an approximately coincident track in the Ligurian-Tyrrhenian region, are comparable, in an attempt to complement the description of the major observed features and to identify important variability on seasonal time scales
Summary
The Ligurian and Tyrrhenian Seas are areas of considerable oceanographic interest in the western Mediterranean region. As part of the Volunteer Observing System (VOS) program of the Mediterranean Forecasting System Pilot Project (MFSPP), repeated XBT measurements gave an opportunity to monitor seasonal, and eventually interannual, changes in upper ocean thermal structure along a shipping route from Genova to Palermo, crossing significant circulation features, namely the eastern boundary of the Ligurian Gyre, the Bonifacio Gyre, and the Atlantic water inflow along the Sicily coast. An important characteristic of this shipping route is that it is approximately coincident with a TOPEX/Poseidon (hereinafter T/P) ground-track (track 0044, see Fig. 1) This gives a chance of a comparison with respect to space and time between the sea surface heights measured by the T/P satellite altimeter and “synthetic” steric heights from a data set consisting of the XBT measurements and temperature/salinity climatology profiles. Conclusions are drawn concerning their integration, in order to obtain more information about the seasonal variability of the main oceanographic features in the Ligurian-Tyrrhenian area
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