Combining T/P altimetric data with hydrographic data to estimate the mean dynamic topography of the North Atlantic and improve the geoid

Legrand, Pascal; Mercier, Herlé; Reynaud, Thierry

doi:10.1007/s005850050634

Cited by 7 publications

(15 citation statements)

References 6 publications

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“…The approach is similar to that of LeGrand and Minster [1999], but the present study incorporates several improvements: a refined estimate of geoid height uncertainties produced during The LPO finite difference inverse model has been extensively described in previous papers [Mercier et al, 1993;LeGrand et al, 1998], so only its main characteristics need to be recalled here. One important characteristic is that the model treats ocean dynamic topography as an explicit variable that can be directly constrained by observational estimates of the geoid height and of the mean sea surface height.…”

Section: Methodsmentioning

confidence: 99%

Impact of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates: Volume fluxes in a climatological inverse model of the Atlantic

Legrand

2001

J. Geophys. Res.

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Abstract. The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission,by providing a precise estimate of the marine geoid height, will allow the determination of absolute geostrophic velocities at the surface of the ocean with unprecedented accuracy. The resulting impact on oceanic flux estimates is quantified within a climatological inverse model of the Atlantic in terms of reduction of uncertainties in volume transports. These uncertainty reductions are obtained by replacing the error spectrum of present-day geoid models by the error spectrum expected for the GOCE mission. The impact is large in the Circumpolar Current, with relative uncertainty reductions reaching 50% in the upper layers of the ocean, and 40% in the whole water column. It is also large in regions of sharp oceanic fronts like the Gulf Stream or the Brazil Current, with uncertainty reductions reaching 60% in the upper layers of the ocean. The reduction in transport uncertainties is large enough in absolute terms to have a significant impact on estimates of important climate processes like the rate of overturning in the Atlantic or the exchange of water between the Circumpolar Current and the South Atlantic. The impact of the Gravity Recovery and Climate Experiment (GRACE) mission, estimated within the same inverse model, is on average less than half the impact of GOCE because of the lower precision of this mission at small spatial scales. The fact that uncertainties in the baroclinic component of the velocity field limit the impact of GOCE at depths points to the need for precise in situ observations to complement gravity and altimetric observations.

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Section: Methodsmentioning

confidence: 99%

Impact of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates: Volume fluxes in a climatological inverse model of the Atlantic

Legrand

2001

J. Geophys. Res.

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“…If an increase of model resolution is able to limit those discrepancies (Smith et al 2000), typical errors on actual model MDTs (see Barnier et al 2006) are not compatible with the accuracy required to reference the SLA signal. One the other hand, synthetic solutions (e.g., Mercier 1986;Le Grand et al 1998;Niiler et al 2003) are not straightforward to build and often suffer from a lack of resolution. They are based on a set of assumptions that are, in several respects, not sufficiently accurate for our purposes (geostrophic balance, reference level of no motion...).…”

Section: An Observed Mdt From Grace Gravimetric Datamentioning

confidence: 99%

“…Several methods have been developed to estimate the ocean MDT independently of the knowledge of the small-scale features of the geoid (e.g. Mercier 1986;Le Grand et al 1998, 2003Niiler et al 2003;Rio and Hernandez 2004;Maximenko and Niiler 2005). However, none of the solutions has been fully satisfactory, and significant errors remain on the reference MDT (Rio et al 2006).…”

Section: Introductionmentioning

confidence: 99%

Joint altimetric and in-situ data assimilation using the GRACE mean dynamic topography: a 1993–1998 hindcast experiment in the Tropical Pacific Ocean

et al. 2008

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The altimetric satellite signal is the sum of the geoid and the dynamic topography, but only the latter is relevant to oceanographic applications. Poor knowledge of the geoid has prevented oceanographers from fully exploiting altimetric measurements through its absolute component, and applications have concentrated on ocean variability through analyses of sea level anomalies. Recent geodetic missions like CHAMP, GRACE and the forthcoming GOCE are changing this perspective. In this study, data assimilation is used to reconstruct the Tropical Pacific Ocean circulation during the 1993-1996 period. Multivariate observations are assimilated into a primitive equation ocean model (OPA) using a reduced order Kalman filter (the Singular Evolutive Extended Kalman filter). A 6-year (1993-1998) hindcast experiment is analyzed and validated by comparison with observations. In this experiment, the new capability offered by an observed absolute dynamic topography (built using the GRACE geoid to reference the altimetric data) is used to assimilate, in an efficient way, the in-situ temperature profiles from the TAO/TRITON moorings together with the T/P and ERS1&2 altimetric signal. GRACE data improves compatibility between both observation data sets. The difficulties encountered in this regard in previous studies such as Parent et al. (J Mar Syst 40-41: 381-401, 2003) are now circumvented. This improvement helps provide more efficient data assimilation, as evidenced, by assessing the results against independent data. This leads in particular to significantly more realistic currents and vertical thermal structures.

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“…The latter is obtained at 1°resolution [Le Grand et al, 1998] by a non-linear inverse model of the North Atlantic general circulation constrained by a compilation of 70 years of hydrographic data and altimetric observations over 1993-1996. This approach is based on dynamical considerations but limited by resolution and accuracy of available data.…”

Section: The Dynamic Topography Computationmentioning

confidence: 99%

“…Absolute dynamic topography has only been accessible by the addition of a mean dynamic topography estimate (MDT) to the SLA [see Birol et al, 2004, Figure 4]. A variety of methods have been applied to generate different MDT products used in ocean prediction systems [Le Grand et al, 1998;Rio and Hernandez, 2004]. Birol et al [2004] showed that errors in such MDT estimates have an important impact on assimilation systems.…”

Section: Introductionmentioning

confidence: 99%

Assimilation of satellite altimetry referenced to the new GRACE geoid estimate

Birol

Brankart

Lemoine

et al. 2005

Geophysical Research Letters

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[1] Currently, two satellite gravimetric missions (CHAMP, GRACE) are dedicated to the improvement of our knowledge of the geoid, and one (GOCE) is planned in the near future. This will allow the absolute altimeter ocean height measurements to be exploited, instead of only sea level variations. In this paper, we evaluate the impact of the GRACE mission on ocean data assimilation. The new approach is to directly assimilate the full altimetric signal relative to the first release of a GRACE geoid. The response of an eddy-permitting ocean model of the North Atlantic to the assimilation of this altimetric signal is analysed. The results are compared to that obtained using the usual approach, i.e., the assimilation of the dynamic topography derived from the addition of altimetric sea level anomalies and a mean dynamic topography estimate. Even if the GRACE mission resolution (333 km) is not yet compatible with oceanographic studies at mid latitude, we show that the geoid estimate can already be used with success in basin scale altimetric data assimilation problems.

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Combining T/P altimetric data with hydrographic data to estimate the mean dynamic topography of the North Atlantic and improve the geoid

Cited by 7 publications

References 6 publications

Impact of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates: Volume fluxes in a climatological inverse model of the Atlantic

Impact of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates: Volume fluxes in a climatological inverse model of the Atlantic

Joint altimetric and in-situ data assimilation using the GRACE mean dynamic topography: a 1993–1998 hindcast experiment in the Tropical Pacific Ocean

Assimilation of satellite altimetry referenced to the new GRACE geoid estimate

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