Comparison of TOPEX/POSEIDON altimetry and in situ sea level data at Sao Tome Island, Gulf of Guinea

Abstract

In situ sea level data from a shallow pressure tide gauge (mean depth 3.21 m) at Sao Tome Island, Gulf of Guinea, and altimeter data from TOPEX/POSEIDON (T/P) were analyzed and compared. The Texas model, a T/P‐derived tide model, is used for the tidal correction of altimeter data. Sea level anomaly time series from both data sets were low‐pass filtered by using a Gaussian recurrent interpolation scheme, suppressing high‐frequency fluctuations of periods much less than 2 months. According to the 1992–1993 Sao Tome tide gauge data, a clear seasonal signal was observed both in 1992 and in 1993, presenting two sea level maxima in February and October and two minima in June and December. These sea level variations are consistent with the known seasonal mean sea level cycle of the area, most of which can be explained by steric sea level changes due to the seasonal cycle of upper 500‐m water column properties in the Gulf of Guinea. On the basis of the 60‐day low‐passed tide gauge time series, significant year to year variations were observed as well, with a 6.3‐cm sea level difference in February (+8.1 cm in 1992 versus +1.8 cm in 1993) and a 4.4‐cm difference in October (+4.6 cm in 1992 versus +9.0 cm in 1993). Minimum sea levels, occurring just prior to the upwelling season (July–August), were almost the same for both years (−9 to −10 cm) but lasted much longer in 1992 (2 months) than in 1993 (2 weeks). During the 15‐month period of concomitant observations, from October 1992 to December 1993, these seasonal and year to year sea level variations are remarkably well reproduced by the T/P sea level time series, with an rms difference of 2.2 cm and a correlation coefficient of 0.88. This result in the equatorial Atlantic is consistent with other intercomparisons in the Pacific and Indian Oceans. This excellent recovery of the oceanic signal is undoubtedly the result of unprecedented high precision of T/P measurements and the reliable T/P‐derived tide model that is used in the present study.

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

“…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.…”

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

“…A stronger filtering, by applying a 30-day Gaussian filter (with a cutoff period at 15 days) to both the altimeter SLAs and the pressure recorder series, reduces the rms difference to 2.8 cm, with r = 0.88 (0.85-0.9l at 95% s.l.). This error value is not significantly greater than the 2-cm solution obtained near Sao Tome Island by Verstraete and Park [1995]…”

Tidal corrections for TOPEX altimetry in the Coral Sea and Great Barrier Reef Lagoon: Comparisons with long‐term tide gauge records

“…Actual calibration studies [ Born et al , 1994; Christensen et al , 1994; Ménard et al , 1994; White et al , 1996; Exertier et al , 2000] seek to compare absolute levels from TOPEX at, or carefully extrapolated to, the calibration site, and have demonstrated RMS differences ranging from to about 1.5 to 3.5 cm. Studies using non‐coincident coastal and island tide gauges, pressure gauges or inverted echosounders, and dynamic heights computed from temperature sensors [ Katz et al , 1995; Mitchum , 1994; Picaut et al , 1995; Verstraete and Park , 1995; Woodworth et al , 1996; Murphy et al , 1996] generally achieve RMS differences ranging between about 4 and 12 cm, depending upon location, with correlations ranging from 0.4 to 0.9, but typically 0.6 to 0.7. The results we report here are consistent with these ranges, but may be less conservative (larger than would otherwise be the case) because we have optimized lags and locations to account for phase propagation.…”