The Experts below are selected from a list of 2550 Experts worldwide ranked by ideXlab platform
Richard D. Ray - One of the best experts on this subject based on the ideXlab platform.
-
constraints on energy dissipation in the earth s body tide from Satellite tracking and altimetry
2013Co-Authors: Richard D. Ray, R J Eanes, F G LemoineAbstract:The phase lag by which the earth's body tide follows the tidal potential is estimated for the principal lunar semidiurnal tide M(sub 2). The estimate results from combining recent tidal solutions from Satellite tracking data and from Topex/Poseidon Satellite altimeter data. Each data type is sensitive to the body-tide lag: gravitationally for the tracking data, geometrically for the altimetry. Allowance is made for the lunar atmospheric tide. For the tidal potential Love number kappa(sub 2) we obtain a lag epsilon of 0.20 deg +/- 0.05 deg, implying an effective body-tide Q of 280 and body-tide energy dissipation of 110 +/- 25 gigawatts.
-
mapping nonlinear shallow water tides a look at the past and future
Ocean Dynamics, 2006Co-Authors: Ole Baltazar Andersen, Gary D Egbert, Svetlana Y Erofeeva, Richard D. RayAbstract:Overtides and compound tides are generated by nonlinear mechanisms operative primarily in shallow waters. Their presence complicates tidal analysis owing to the multitude of new constituents and their possible frequency overlap with astronomical tides. The science of nonlinear tides was greatly advanced by the pioneering researches of Christian Le Provost who employed analytical theory, physical modeling, and numerical modeling in many extensive studies, especially of the tides of the English Channel. Le Provost’s complementary work with Satellite altimetry motivates our attempts to merge these two interests. After a brief review, we describe initial steps toward the assimilation of altimetry into models of nonlinear tides via generalized inverse methods. A series of barotropic inverse solutions is computed for the M\(_4\) tide over the northwest European Shelf. Future applications of altimetry to regions with fewer in situ measurements will require improved understanding of error covariance models because these control the tradeoffs between fitting hydrodynamics and data, a delicate issue in coastal regions. While M\(_4\) can now be robustly determined along the Topex/Poseidon Satellite ground tracks, many other compound tides face serious aliasing problems.
-
estimates of m2 tidal energy dissipation from topex Poseidon altimeter data
Journal of Geophysical Research, 2001Co-Authors: Gary D Egbert, Richard D. RayAbstract:Most of the tidal energy dissipation in the ocean occurs in shallow seas, as has long been recognized. However, recent work has suggested that a significant fraction of the dissipation, perhaps 1 TW or more, occurs in the deep ocean. This paper builds further evidence for that conclusion. More than 6 years of data from the TOPEX/Poseidon Satellite altimeter are used to map the tidal dissipation rate throughout the world ocean. The dissipation rate is estimated as a balance between the rate of working by tidal forces and the energy flux divergence, computed using currents derived by least squares fitting of the altimeter data and the shallow water equations. Such calculations require dynamical assumptions, in particular about the nature of dissipation. To assess sensitivity of dissipation estimates to input assumptions, a large suite of tidal inversions based on a wide range of drag parameterizations and employing both real and synthetic altimeter data are compared. These experiments and Monte Carlo error fields from a generalized inverse model are used to establish error uncertainties for the dissipation estimates. Owing to the tight constraints on tidal elevation fields provided by the altimeter, area integrals of the energy balance are remarkably insensitive to required dynamical assumptions. Tidal energy dissipation is estimated for all major shallow seas (excluding individual polar seas) and compared with previous model and data-based estimates. Dissipation in the open ocean is significantly enhanced around major bathymetric features, in a manner consistent with simple theories for the generation of baroclinic tides.
-
estimates of internal tide energy fluxes from topex Poseidon altimetry central north pacific
Geophysical Research Letters, 2001Co-Authors: Richard D. Ray, D E CartwrightAbstract:Energy fluxes for first-mode M2 internal tides are deduced throughout the central North Pacific Ocean from Topex/Poseidon Satellite altimeter data. Temporally coherent internal tide signals in the altimetry, combined with climatological hydrographic data, determine the tidal displacements, pressures, and currents at depth, which yield power transmission rates. For a variety of reasons the deduced rates should be considered lower bounds. Internal tides are found to emanate from several large bathymetric structures, especially the Hawaiian Ridge, where the integrated flux amounts to about 6 gigawatts. Internal tides are generated at the Aleutian Trench near 172°W and propagate southwards nearly 2000 km.
-
error spectrum for the global m2 ocean tide
Geophysical Research Letters, 2001Co-Authors: Richard D. Ray, Gary D Egbert, R J Eanes, N K PavlisAbstract:The most accurate determinations of the global ocean tides are currently based on altimeter measurements made by the Topex/Poseidon Satellite. The error spectrum corresponding to the M2 tidal solution is here estimated, primarily by inverse methods and secondarily by simple differencing of several of the best tidal models. The tidal error spectrum is flatter than the tidal signal spectrum, and it exceeds 10% of the signal at spherical harmonic degree 15 and above. The tide errors also exceed the anticipated sensitivity of the upcoming Grace Satellite gravity mission for all degrees below 40, and possibly below 50.
Anny Cazenave - One of the best experts on this subject based on the ideXlab platform.
-
reconstruction of past decades sea level using thermosteric sea level tide gauge Satellite altimetry and ocean reanalysis data
Global and Planetary Change, 2008Co-Authors: Muriel Ergenguye, W Llovel, Anny Cazenave, A Lombard, J Viarre, Jeanfrancois CretauAbstract:Abstract This study investigates past sea level reconstruction (over 1950–2003) based on tide gauge records and EOF spatial patterns from different 2-D fields. In a first step, we test the influence on the reconstructed signal of the 2-D fields temporal coverage. For that purpose we use global grids of thermosteric sea level data, available over 1950–2003. Different time spans (in the range 10–50 yr) for the EOF spatial patterns, and different geographical distributions for the 1-D thermosteric sea level time series (interpolated at specific locations from the 2-D grids), are successively used to reconstruct the 54-year long thermosteric sea level signal. In each case we compare the reconstructed trend map with the reference. The simulation indicates that the longer the time span covered by the spatial EOFs, the closer to the reference the reconstructed thermosteric sea level trends. In a second step, we apply the method to reconstructing 2-D sea level data over 1950–2003, combining sparse tide gauge records available since 1950, with EOF spatial patterns from different sources: (1) thermosteric sea level grids over 1955–2003, (2) sea level grids from Topex/Poseidon Satellite altimetry over 1993–2003, and (3) dynamic height grids from the SODA reanalysis over 1958–2001. The reconstructed global mean sea level trend based on thermosteric EOFs (case 1) is significantly lower than the observed trend, while the interannual/decadal sea level fluctuations are well reproduced. Case 2 (Topex/Poseidon EOFs over 1993–2003) leads to a global mean sea level trend over the 54-year time interval very close to the observed trend. But the spatial trends of the reconstruction over 1950–2003 are significantly different from those obtained with thermosteric EOFs. Case 3 (SODA EOFs over 1958–2001) provides a reconstruction trend map over 1950–2003 that differs significantly from the previous two cases. We discuss possible causes for such differences. For the three cases, on the other hand, reconstructed spatial trends over 1993–2003 agree well with the regional sea level trends observed by Topex/Poseidon.
-
contribution of continental water to sea level variations during the 1997 1998 el nino southern oscillation event comparison between atmospheric model intercomparison project simulations and topex Poseidon Satellite data
Journal of Geophysical Research, 2005Co-Authors: Thanh Ngoduc, K Laval, Jan Polcher, Anny CazenaveAbstract:[1] Satellite altimetry from TOPEX/Poseidon (T/P) is used to estimate the variation of the global sea level. This signal, once corrected for steric effects, reflects water mass exchange with the atmosphere and land reservoirs (mainly ice caps, soils and snowpack). It can thus be used to test the capacity of general circulations models (GCMs) to estimate change in land water storage. In this study, we compare the land hydrology contribution to global mean sea level variations during the major 1997–1998 El Nino–Southern Oscillation event from two data sets: (1) the results of the Organizing Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface scheme, developed at the Institute Pierre Simon Laplace, coupled to the Laboratoire de Meteorologie Dynamique Atmospheric General Circulation Model (LMD AGCM) and (2) the T/P-based estimates. We show that the seasonal variation of the continental water storage is well represented in the model. The drastic amplitude change between the two contrasted years, 1997 and 1998, observed from Satellite altimetry, is also simulated. We analyze the role of each component of simulated water fluxes (precipitation, evaporation, and runoff) in determining the range of annual continental water mass variation and its interannual variability. The difference between the two years, 1997 and 1998, is, for an essential part, due to land precipitation in the 20°N–20°S domain. This analysis emphasizes the important role of tropical regions in interannual variability of climate.
-
ob river discharge from topex Poseidon Satellite altimetry 1992 2002
Remote Sensing of Environment, 2004Co-Authors: A. Kouraev, O. Samain, Elena Zakharova, N M Mognard, Anny CazenaveAbstract:The paper discusses an application of the TOPEX/Poseidon (T/P) altimetry data to estimate the discharge of one of the largest Arctic rivers—the Ob' river. We first discuss the methodology to select and retrieve the altimeter water levels during the various phases of the hydrological regime. Then we establish the relationships between the Satellite-derived water levels and the in situ river discharge measurements at the Salekhard gauging station near the Ob' estuary. The comparison of in situ and Satellite-derived estimations of the Ob' discharge at Salekhard shows that the T/P data can successfully be used for hydrological studies of this river. We address the problems affecting the accuracy of the discharge estimations from altimeter measurements, identify potential solutions and suggest how Satellite altimetry data may benefit hydrological studies of Arctic rivers.
-
present day sea level change observations and causes
Reviews of Geophysics, 2004Co-Authors: Anny Cazenave, R. Steven NeremAbstract:We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale. Wc focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global mean sea level change has been measured by Topex/Poseidon Satellite altimetry. thermal expansion is the dominant contribution to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal contribution is estimated to about 0.5 mm/yr, with a possible accelerated rale of thermosteric rise during the 1990s. Topex/Posei don shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly related to ocean warming.
-
ice cover variability in the caspian and aral seas from active and passive microwave Satellite data
Polar Research, 2003Co-Authors: A. Kouraev, Anny Cazenave, Fabrice Papa, P I Buharizin, Jeanfrancois Cretaux, Julia Dozortseva, Frederique RemyAbstract:The paper discusses time and space variations of ice extent in the Caspian and Aral seas during the last decade (1992–2002). It uses synergy of data from active (radar altimeter) and passive (radiometer) microwave nadirlooking instruments onboard the TOPEX/Poseidon Satellite. The proposed approach is substantiated and validated using both in situ and Satellite imagery data for the Caspian Sea. The results indicate significant spatial and temporal variability of ice conditions, with a significant decrease of both the duration of ice season and ice extent during the last four winters (1998–2002). The TOPEX/Poseidon-derived time series of sea ice extent are very valuable in view of the fragmentary and mostly unpublished data on ice conditions on the Caspian and Aral seas since the mid-1980s.
Sylvain Mangiarotti - One of the best experts on this subject based on the ideXlab platform.
-
surface pressure and wind stress effects on sea level change estimations from topex Poseidon Satellite altimetry in the mediterranean sea
Journal of Atmospheric and Oceanic Technology, 2008Co-Authors: Sylvain Mangiarotti, Florent LyardAbstract:Abstract Using the classical inverse barometer (IB) correction and the Modele d’Onde de Gravite a 2 Dimensions (MOG2D) barotropic model in the Mediterranean Sea during the 1993–2002 period, it is shown that surface pressure and wind stress forcing significantly contribute to sea level elevation variations observed with Ocean Topography Experiment (TOPEX)/Poseidon (T/P) Satellite altimetry. The barotropic model allows the authors to estimate the high-frequency atmospheric ocean response that is aliased into the altimetric sea level. Applying the model barotropic correction allows them to reduce the T/P standard deviation by a mean of 21% over the whole basin, whereas the classical IB correction reduces the standard deviation by only 16%. The trend in sea level rise is also strongly affected due to the aliasing effect, especially when short periods are considered. On a 3-yr period, the correction associated with either of these two models can reach 10–12 mm yr−1. Applying the barotropic model correction rat...
-
coastal sea level trends from topex Poseidon Satellite altimetry and tide gauge data in the mediterranean sea during the 1990s
Geophysical Journal International, 2007Co-Authors: Sylvain MangiarottiAbstract:A joint analysis of TOPEX-Poseidon Satellite altimetry and tide gauge data is used to estimate sea level trends along the Mediterranean Sea coast from 1993 January to 2002 February. A generalized least squares (GLS) method allows us to take into account the space-time distribution of the data and their respective error budgets. Nearly optimal parameters of the covariance functions involved in the GLS method are estimated at different scales of analysis, using a trial and error method. Combined data time series are then reconstructed at 153 locations along the coast. The basin-wide averaged linear trend for the coastal area shows a sea level rise of 4.54 ± 0.30 mm yr −1 compared to 4.28 ± 0.30 mm yr −1 for the open ocean only, i.e. a 0.26 ± 0.16 mm yr −1 difference (the error is smaller because the systematic error does not take part in the error budget) which is quite significant. At local scales, the coastal linear trends ranges from −8.5 to +13.5 mm yr −1 which is much narrower than ranges found by other open sea studies ([−24.0 to −10.0; +16.0 to +29.0] mm yr −1 ). However, a detailed error budget shows that the largest estimation errors come from the spatial extrapolation of the sea level signal required when tide gauge and altimeter data are merged. The second error source is the instrumental noise and the random noise generated by the littoral contamination on altimeter measurements and from the crustal motions that are contaminating the tide gauge signal. At local scale, it is estimated that 85 percent of the error on linear trends does not exceed 2 mm yr −1 .
-
recent sea level change in the mediterranean sea revealed by topex Poseidon Satellite altimetry
Geophysical Research Letters, 2001Co-Authors: Anny Cazenave, Cecile Cabanes, K Dominh, Sylvain MangiarottiAbstract:Using altimetry data of the Topex/Poseidon Satellite available since early 1993, we show that the eastern Mediterranean sea level has been continuously rising during 1993–1999, at a rate up to 20 mm/yr southeast of Crete. Sea level rise is also observed in the Algerian-Provencal basin as well as in the Tyrrhenian and Adriatic seas. The north Ionian sea, on the other hand, shows an opposite trend, i.e., a sea level drop during the past seven years. Sea surface temperature trends are strongly correlated to sea level trends, indicating that at least part of the observed sea level change has a thermal origin. The recent Mediterranean sea level rise observed by Topex/Poseidon may be related to the warming trends reported from hydrographic cruises in the intermediate and deep waters of the eastern basin since the early 1990s, and of the western basin since the 1960s.
John A Church - One of the best experts on this subject based on the ideXlab platform.
-
sea level rise at tropical pacific and indian ocean islands
Global and Planetary Change, 2006Co-Authors: Neil J White, John A Church, J R HunterAbstract:Historical and projected sea-levels for islands in the tropical Pacific and Indian oceans are a subject of considerable interest and some controversy. The large variability (e.g. El Nino) signals and the shortness of many of the individual tide-gauge records contribute to uncertainty of historical rates of sea-level rise. Here, we determine rates of sea-level rise from tide gauges in the region. We also examine sea-level data from the TOPEX/Poseidon Satellite altimeter and from a reconstruction of sea level in order to put the sparse (in space and time) tide-gauge data into context. For 1993 to 2001, all the data show large rates of sea-level rise over the western Pacific and eastern Indian Ocean (approaching 30 mm yr −1 ) and sea-level falls in the eastern Pacific and western Indian Ocean (approaching −10 mm yr −1 ). Over the region 40°S to 40°N, 30°E to 120°W, the average rise is about 4 mm yr −1 . For 1950 to 2001, the average sea-level rise (relative to land) from the six longest tide-gauge records is 1.4 mm yr −1 . After correcting for glacial isostatic adjustment and atmospheric pressure effects, this rate is 2.0 mm yr −1 , close to estimates of the global average and regional average rate of rise. The long tide-gauge records in the equatorial Pacific indicate that the variance of monthly averaged sea-level after 1970 is about twice that before 1970. We find no evidence for the fall in sea level at the Maldives as postulated by Morner et al. (2004). Our best estimate of relative sea-level rise at Funafuti, Tuvalu is 2±1 mm yr −1 over the period 1950 to 2001. The analysis clearly indicates that sea-level in this region is rising. We expect that the continued and increasing rate of sea-level rise and any resulting increase in the frequency or intensity of extreme sea-level events will cause serious problems for the inhabitants of some of these islands during the 21st century.
-
estimates of the regional distribution of sea level rise over the 1950 2000 period
Journal of Climate, 2004Co-Authors: John A Church, Neil J White, R Coleman, Kurt Lambeck, Jerry X MitrovicaAbstract:Abstract TOPEX/Poseidon Satellite altimeter data are used to estimate global empirical orthogonal functions that are then combined with historical tide gauge data to estimate monthly distributions of large-scale sea level variability and change over the period 1950–2000. The reconstruction is an attempt to narrow the current broad range of sea level rise estimates, to identify any pattern of regional sea level rise, and to determine any variation in the rate of sea level rise over the 51-yr period. The computed rate of global-averaged sea level rise from the reconstructed monthly time series is 1.8 ± 0.3 mm yr−1. With the decadal variability in the computed global mean sea level, it is not possible to detect a significant increase in the rate of sea level rise over the period 1950–2000. A regional pattern of sea level rise is identified. The maximum sea level rise is in the eastern off-equatorial Pacific and there is a minimum along the equator, in the western Pacific, and in the eastern Indian Ocean. A g...
-
a southern hemisphere verification for the topex Poseidon Satellite altimeter mission
Journal of Geophysical Research, 1994Co-Authors: Neil J White, John A Church, R Coleman, Peter Morgan, S J WalkerAbstract:As a check on the total system accuracy of the TOPEX/Poseidon Satellite, a southern hemisphere verification study was undertaken in Bass Strait (41°S) immediately adjacent to the Southern Ocean. An acoustic tide gauge and two pressure gauges were installed on the southern side of Bass Strait near where descending pass 88 crosses the Tasmanian coast. The tide gauge benchmark height was estimated relative to the laser tracking station at Orroral Valley and the very long baseline interferometry station near Hobart using Global Positioning System surveying techniques. In the TOPEX/Poseidon geophysical data records, flags on the altimeter data began to be set 15–20 km from land and little useful altimeter data were available within about 13 km of the coast. The acoustic tide gauge heights were corrected for changes in tidal phase and amplitude between the coast and the offshore subSatellite point using results from a numerical model. The altimeter correction for atmospheric water vapor calculated from the TOPEX microwave radiometer data and the insitu precipitable water data agreed to 2 cm RMS, with no significant bias. The RMS variability between the insitu sea level estimates and the TOPEX/Poseidon estimates was less than 3.5 cm. As part of this variability is caused by inaccuracies in the estimates of the sea level variations between the coast and the subSatellite point, the result indicates that the precision of the TOPEX/Poseidon sea level height measurement system, including the time dependent orbit errors, the ionospheric corrections and the wet and dry tropospheric path delays, is less than 3.5 cm over Bass Strait, considerably below the original system specifications of 13 cm. Our estimate of the altimeter bias (including geographically correlated orbit error) is −25±10 cm, consistent with the results at the NASA (Harvest Platform) and Centre National d'Etudes Spatiales (Lampedusa) calibration sites. However, accurate estimation of this total altimeter bias is confounded by the lack of altimeter data close to land and in particular, limitations in the available local geoid data. The altimeter bias shows no significant drift at this site.
Neil J White - One of the best experts on this subject based on the ideXlab platform.
-
sea level rise at tropical pacific and indian ocean islands
Global and Planetary Change, 2006Co-Authors: Neil J White, John A Church, J R HunterAbstract:Historical and projected sea-levels for islands in the tropical Pacific and Indian oceans are a subject of considerable interest and some controversy. The large variability (e.g. El Nino) signals and the shortness of many of the individual tide-gauge records contribute to uncertainty of historical rates of sea-level rise. Here, we determine rates of sea-level rise from tide gauges in the region. We also examine sea-level data from the TOPEX/Poseidon Satellite altimeter and from a reconstruction of sea level in order to put the sparse (in space and time) tide-gauge data into context. For 1993 to 2001, all the data show large rates of sea-level rise over the western Pacific and eastern Indian Ocean (approaching 30 mm yr −1 ) and sea-level falls in the eastern Pacific and western Indian Ocean (approaching −10 mm yr −1 ). Over the region 40°S to 40°N, 30°E to 120°W, the average rise is about 4 mm yr −1 . For 1950 to 2001, the average sea-level rise (relative to land) from the six longest tide-gauge records is 1.4 mm yr −1 . After correcting for glacial isostatic adjustment and atmospheric pressure effects, this rate is 2.0 mm yr −1 , close to estimates of the global average and regional average rate of rise. The long tide-gauge records in the equatorial Pacific indicate that the variance of monthly averaged sea-level after 1970 is about twice that before 1970. We find no evidence for the fall in sea level at the Maldives as postulated by Morner et al. (2004). Our best estimate of relative sea-level rise at Funafuti, Tuvalu is 2±1 mm yr −1 over the period 1950 to 2001. The analysis clearly indicates that sea-level in this region is rising. We expect that the continued and increasing rate of sea-level rise and any resulting increase in the frequency or intensity of extreme sea-level events will cause serious problems for the inhabitants of some of these islands during the 21st century.
-
estimates of the regional distribution of sea level rise over the 1950 2000 period
Journal of Climate, 2004Co-Authors: John A Church, Neil J White, R Coleman, Kurt Lambeck, Jerry X MitrovicaAbstract:Abstract TOPEX/Poseidon Satellite altimeter data are used to estimate global empirical orthogonal functions that are then combined with historical tide gauge data to estimate monthly distributions of large-scale sea level variability and change over the period 1950–2000. The reconstruction is an attempt to narrow the current broad range of sea level rise estimates, to identify any pattern of regional sea level rise, and to determine any variation in the rate of sea level rise over the 51-yr period. The computed rate of global-averaged sea level rise from the reconstructed monthly time series is 1.8 ± 0.3 mm yr−1. With the decadal variability in the computed global mean sea level, it is not possible to detect a significant increase in the rate of sea level rise over the period 1950–2000. A regional pattern of sea level rise is identified. The maximum sea level rise is in the eastern off-equatorial Pacific and there is a minimum along the equator, in the western Pacific, and in the eastern Indian Ocean. A g...
-
a southern hemisphere verification for the topex Poseidon Satellite altimeter mission
Journal of Geophysical Research, 1994Co-Authors: Neil J White, John A Church, R Coleman, Peter Morgan, S J WalkerAbstract:As a check on the total system accuracy of the TOPEX/Poseidon Satellite, a southern hemisphere verification study was undertaken in Bass Strait (41°S) immediately adjacent to the Southern Ocean. An acoustic tide gauge and two pressure gauges were installed on the southern side of Bass Strait near where descending pass 88 crosses the Tasmanian coast. The tide gauge benchmark height was estimated relative to the laser tracking station at Orroral Valley and the very long baseline interferometry station near Hobart using Global Positioning System surveying techniques. In the TOPEX/Poseidon geophysical data records, flags on the altimeter data began to be set 15–20 km from land and little useful altimeter data were available within about 13 km of the coast. The acoustic tide gauge heights were corrected for changes in tidal phase and amplitude between the coast and the offshore subSatellite point using results from a numerical model. The altimeter correction for atmospheric water vapor calculated from the TOPEX microwave radiometer data and the insitu precipitable water data agreed to 2 cm RMS, with no significant bias. The RMS variability between the insitu sea level estimates and the TOPEX/Poseidon estimates was less than 3.5 cm. As part of this variability is caused by inaccuracies in the estimates of the sea level variations between the coast and the subSatellite point, the result indicates that the precision of the TOPEX/Poseidon sea level height measurement system, including the time dependent orbit errors, the ionospheric corrections and the wet and dry tropospheric path delays, is less than 3.5 cm over Bass Strait, considerably below the original system specifications of 13 cm. Our estimate of the altimeter bias (including geographically correlated orbit error) is −25±10 cm, consistent with the results at the NASA (Harvest Platform) and Centre National d'Etudes Spatiales (Lampedusa) calibration sites. However, accurate estimation of this total altimeter bias is confounded by the lack of altimeter data close to land and in particular, limitations in the available local geoid data. The altimeter bias shows no significant drift at this site.