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J. Dorandeu
and
P. Y. Le Traon

Abstract

The authors used meteorological pressure fields from the European Centre for Medium-Range Weather Forecasts to calculate a mean global pressure to serve as a reference for an improved inverse barometer correction of altimeter data. These global pressure fields, available every 6 h on a ½ degree grid, enabled the extraction of the dominant mean pressure signals. Then, the effect of an improved inverse barometer correction on TOPEX/Poseidon mean sea level variation was estimated. Different low-pass smoothings of global mean pressure were used with cutoff frequencies ranging from (40 to 2 days)−1. Best results were obtained with the (2 days)−1 cutoff frequency, which was then used for an improved inverse barometer correction. The improved correction reduces the standard deviation of mean sea level variations (relative to an annual cycle and slope) by more than 20% when compared with standard inverse barometer correction and no correction at all. It also slightly reduces the variance of sea surface height differences at crossover points. The impact of the improved correction on the mean sea level annual cycle and slope is also not negligible.

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J. Dorandeu
,
M. Ablain
, and
P-Y. Le Traon

Abstract

A new technique is developed and tested to correct for cross-track geoid gradients in altimeter data. The proposed method is based on direct estimations of geoid variations around nominal tracks and on knowledge of ocean signal variability. Apart from measurement errors, ocean variability is demonstrated to be the major source of error in cross-track geoid estimations using altimeter measurements. The method thus uses the outputs of multimission ocean signal mapping procedures to improve the estimation of geoid features. A detailed error analysis shows that such a technique allows reduction of the estimation error by a factor of 2. Therefore, the method is applied taking advantage of the unprecedented TOPEX/Poseidon mission length. It provides a gain of 50%, in terms of sea level anomaly (SLA) variance reduction, in the cross-track geoid gradient correction used in collocating the repeat-cycle data. It also improves the estimation of altimetric mean profiles. From this study, local mean sea surface estimates can be inferred and applied to present and future altimetric missions, since they can be easily updated using more data. New altimetric missions like Jason-1 and Envisat, with the same ground track as the former TOPEX/Poseidon and European Remote Sensing Satellite (ERS) missions, make the method even more relevant.

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P. Y. Le Traon
,
Y. Faugère
,
F. Hernandez
,
J. Dorandeu
,
F. Mertz
, and
M. Ablain

Abstract

Merging Geosat Follow-On (GFO) with TOPEX/Poseidon (TP) and ERS-2 altimeter data has the potential to improve the mapping of sea level and ocean circulation variations. This can be achieved, however, only if measurement errors and inconsistencies between the different missions are sufficiently reduced. In this paper, it is shown how to get consistent sea surface heights from the three missions using the most precise mission (TP) as a reference. A new technique is then used to estimate a GFO mean profile. This allows consistent sea level anomalies (SLAs) to be extracted from GFO, TP, and ERS-2. SLA data are then merged with a mapping technique that takes into account noise and residual long wavelength errors. Thanks to these techniques, it is shown that GFO can be combined with TP and ERS-2 and that the combination provides a significant improvement in the description of the mesoscale circulation.

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