Integrated use of altimeter and in situ data for understanding the water exchanges between the Tyrrhenian and Ligurian Seas

Abstract

Abstract. In this paper we examine the relationship between the seasonal and interannual variability observed in the water flow through the Corsica Channel and the sea level difference between the Tyrrhenian and Ligurian Seas. The steric contribution to the sea level difference, computed from historical hydrological data, is in good agreement with the stable presence of the seasonal signal in the water exchanges. We obtain the maximum steric difference in winter (-16 cm) and the minimum in summer (-2 cm). These values are consistent with the corresponding estimates of water volume transport (0.8 and <0.1 Sv in winter and summer, respectively). Also, TOPEX/Poseidon (T/P) satellite altimetry is shown to be capable of capturing the sea level difference anomaly between the two basins at seasonal and interannual scales. Accuracy of altimeter data in the study region has been checked using measurements from a bottom pressure recorder deployed in Capraia Island (rms difference is found to be -2 cm after application of a 30-day half-amplitude Gaussian filter). Because of the lack of an accurate geoid, the total water transport cannot be adequately monitored by satellite altimetry alone. However, the recovered signal can be directly related to the water transport anomaly through the channel. The resulting T/P signal can be considered as representing, to a great extent, the real steric variation induced by the net sea surface heat

Comparison between XBT data and TOPEX/Poseidon satellite altimetry in the Ligurian-Tyrrhenian area

“…Dynamical interpretation of the differences requires knowledge of whether these are due to a deeper baroclinic variability and/or a barotropic contribution. As shown by Vignudelli et al (2000), the steric contribution resulting from density variation in the upper layers is much larger than that due to the deep layers. By contrast, large-scale features, like the Bonifacio Gyre, clearly may extend beyond the 300 m depth, causing mass redistribution (bottom pressure variability), whose effect is to offset the total sea level.…”

“…The accuracy for the time-varying part of the single measurement is certainly better than this value, since the timeinvariant systematic error component is removed. Moreover, Vignudelli et al (2000), using measurements from a bottom pressure recorder deployed at Capraia Island, found that altimeter data matched the in situ data within 2.8 cm RMS after application of a 30-day half-amplitude Gaussian filter. This error value is not significantly greater than the 2-cm solution obtained near Sao Tome Island (Verstraete and Park, 1995) and indicates that the T/P-derived altimetric heights are accurate enough in the area when used to investigate the seasonal variability.…”

“…The technique used by Vignudelli et al (2000) for the processing of the altimeter data has proven to be effective in the target area. The RMS accuracy for a single-pass altimeter measurement was estimated to be 5 cm by Fu et al (1994).…”

“…The steric heights were calculated assuming 300 m as the reference level, where the seasonal variability becomes minor, as demonstrated by Astraldi and Gasparini (1994) and Vignudelli et al (2000). The computed values are affected by many errors related to the methodology of salinity reconstruction, precision of XBT measurements, and the choice of the no-motion level.…”

“…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. The combination of subsurface information with concurrent satellite altimetry overflights permitted the extension of our knowledge of the altimeter performances in the open part of the basin, complementing the tide gauge intercomparison near Capraia Island made by Vignudelli et al (2000).…”