Mean Dynamic Ocean Topography Research Articles

Epoch-Based Height Reference System for Sea Level Rise Impact Assessment on the Coast of Peninsular Malaysia

The Peninsular Malaysia Geodetic Vertical Datum 2000 (PMGVD2000) inherited several deficiencies due to offsets between local datums used, levelling error propagations, land subsidence, sea level rise, and sea level slopes along the southern half of the Malacca Strait on the west coast and the South China Sea in the east coast of the Peninsular relative to the Port Klang (PTK) datum point. To cater for a more reliable elevation-based assessment of both sea level rise and coastal flooding exposure, a new epoch-based height reference system PMGVD2022 has been developed. We have undertaken the processing of more than 30 years of sea level data from twelve tide gauge (TG) stations along the Peninsular Malaysia coast for the determination of the relative mean sea level (RMSL) at epoch 2022.0 with their respective trends and incorporates the quantification of the local vertical land motion (VLM) impact. PMGVD2022 is based on a new gravimetric geoid (PMGeoid2022) fitted to the RMSL at PTK. The orthometric height is realised through the GNSS levelling concept H = hGNSS–Nfit_PTK–NRMDT, where NRMDT is a constant offset due to the relative mean dynamic ocean topography (RMDT) between the fitted geoid at PTK and the local MSL datums along the Peninsular Malaysia coast. PMGVD2022 will become a single height reference system with absolute accuracies of better than ±3 cm and ±10 cm across most of the land/coastal area and the continental shelf of Peninsular Malaysia, respectively.

A Spectrally Consistent Globally Defined Geodetic Mean Dynamic Ocean Topography

AbstractThe space‐borne geodetic temporal mean dynamic topography (MDT) is obtained from the difference of altimetric mean sea surface (MSS) h and the geoid height N. With the geostrophic surface currents obtained from its gradient, the MDT is an essential parameter when describing the ocean dynamics. Spectral consistency of h and N is crucial to minimize MDT errors. Frequently, h is globalized to allow for a spherical harmonic analysis, and small scales beyond maximum degree and order (d/o) resolved in the geoid are cut off. However, common globalization causes ocean‐land steps in h−N and spectral inconsistencies of N and h over land. To overcome both issues, a methodology is proposed based on globalization of the MDT. A Laplacian smoother with the coastal MDT values as boundary condition is applied, resulting in a smooth surface over land and a continuous ocean‐land transition. The new methodology strongly reduces Gibbs effects and the need to work with high‐resolution MDTs to minimize them. Reduction of resolution is tested to reduce MDT uncertainties caused by the commission error expected to increase with decreasing scale. Applying drifter data and a high‐resolution hydrodynamic ocean model, it is shown that for the Gulf Stream and the Kuroshio, geodetic MDTs applying recent combined geoid models contain physical information up to at least d/o 420 (48‐km spatial scale). For oceanic regions with strong geoid gradients, a higher‐resolution MDT might be needed to prevent Gibbs effects caused by remaining inconsistencies between the geoid and the MSS.

A new ocean mean dynamic topography model, derived from a combination of gravity, altimetry and drifter velocity data

Abstract We present a new mean dynamic ocean topography product based on the improved geoid model, derived in the Optimal Geoid for Modelling Ocean Circulation (OGMOC) project supported by the European Space Agency. This geoid model is based on the Gravity Recovery and Climate Experiment (GRACE) and the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite missions combined with the newer DTU15GRA altimetric surface gravity and it was optimized to eliminate artificial features common for other geoid models such as striations and orange skin. Subsequently, the model has been augmented using the EIGEN-6C4 coefficients to harmonic degree 2160. The new geodetic mean dynamic ocean topography model DTU17MDT has been derived using this geoid model and the DTU15MSS mean sea surface, with the quasi-gaussian filter optimized to best fit velocities of oceanographic drifting buoys. The new model improves the resolution of important details of the ocean circulation. Finally, the drifter data were integrated into the model to refine its spatial resolution. The new topography product DTU17cMDT, combining the wealth of information from satellite gravity and altimetry and drifters, is rich with more features of the ocean surface circulation. In particular, as shown in the case of the Gulf Stream off the east coast of Florida, DTU17cMDT improves description of the western boundary currents.

Using satellite altimetry leveling to assess the marine geoid

Based on the concept of Global Position System (GPS)/leveling, the satellite altimetry leveling (SAL) is first proposed to evaluate the marine geoid. SAL is derived by the difference among the mean sea surface (MSS), mean dynamic ocean topography (MDT), and leveling origin. In this study, (1) the original satellite altimetry data are processed to infer the vertical deflection and gravity anomaly, (2) the Chinese coastal marine geoids (CMG) are determined by using the different methods (including Molodensky, least square collocation, Stokes formula, and two-dimensional fast Fourier transformation (FFT) with the vertical deflection and gravity anomaly data), (3) CMG are evaluated by using the results from above different methods, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) gravity potential model (GGPM), and SAL. The results show that (1) CMG from the Molodensky method has the highest precision by using vertical deflection data, (2) the accuracy of CMG indicate good consistency between the SAL and GGPM, (3) SAL can be used as a new method for assessing marine geoid.

Tracking of geodetic height of sea level in the west of Java Sea, Indonesia: A preliminary assessment

This paper assesses the agreement between observed heights of sea level from Global Navigation Satellite System (GNSS) and a global model of Mean Sea Surface (MSS). The assessment of the agreement is carried out according to the direct comparison between the height of MSS model and the geodetic height of actual sea level. Here, MSS is generated according to Gravity Recovery And Climate Experiment (GRACE) Gravity Model (GGM) and Mean Dynamic Ocean Topography (MDOT). The tracking of geodetic heights of actual sea level are done by Wide Area Differential (WA D) and Real Time Precise Point Positioning (RTPPP) Global Navigation Satellite System (GNSS) along an approximately 180 Nm SW-NE transect of away-return ship track in the west of the Java Sea, Indonesia. It is found that the overall agreement between geodetic height of sea level and MSS observed by WA DGNSS is 7.5 cm (away tracking), while those observed by RTPPP GNSS is 39.5 cm (away tracking) and 36.0 cm (return tracking). This work recommends selection of the best-fit tide model and careful examination on the dynamics of antenna offset due to vessel attitude.

Determination of the mean dynamic ocean topography model through combining multi-source gravity data and DTU15 MSS around China's coast

Mean dynamic ocean topography (or MDT) is closely related to ocean circulation and global climate change. It has important scientific significance and application value for the development and utilization of marine resources in China's coastal areas. Based on the terrain gravity, marine gravity, and SRTM 3 s data, an algorithm to reduce the problem of gravity data gaps between land and sea is proposed. A consistent land-sea gravity model is established based on point-mass fusion method. Then geoid model, which accuracy was estimated to be 8.5 cm through the verification of 348 GNSS/level data from the coastal provinces, of China's coastal areas was calculated through remove-restore technique. Connecting the above geoid model with DTU15 MSS model to establish a MDT model in China's coastal areas using the direct method in space domain. The effect of gravity field model, dominant factors of sea surface topography, and low pass filter are analyzed. Taking Bohai Sea and Yellow Sea as an example, and comparing MDT with the two international models CNES_2013_MDT and DTU15_MDT. The results show that the MDT has the potential to construct a vertical datum of the ocean and carry out related scientific research and application.

Improving the Coastal Mean Dynamic Topography by Geodetic Combination of Tide Gauge and Satellite Altimetry

The ocean mean dynamic topography (MDT) is the surface representation of the ocean circulation. The MDT may be determined by the ocean approach, which involves temporal averaging of numerical ocean circulation model information, or by the geodetic approach, wherein the MDT is derived using the ellipsoidal height of the mean sea surface (MSS), or mean sea level (MSL) minus the geoid as the geoid. The ellipsoidal height of the MSS might be estimated either by satellite or coastal tide gauges by connecting the tide gauge datum to the Earth-centred reference frame. In this article we present a novel approach to improve the coastal MDT, where the solution is based on both satellite altimetry and tide gauge data using new set of 302 tide gauges with ellipsoidal heights through the SONEL network. The approach was evaluated for the Northeast Atlantic coast where a dense network of GNSS-surveyed tide gauges is available. The typical misfit between tide gauge and satellite or oceanographic MDT was found to be around 9 cm. This misfit was found to be mainly due to small scale geoid errors. Similarly, we found, that a single tide gauge places only weak constraints on the coastal dynamic topography.

Description and release of Australian gravity field model testing data

ABSTRACTGravimetric geoid and/or quasigeoid models are routinely evaluated using co-located GPS-levelling and/or astrogeodetic vertical deflections, globally and regionally. This short note describes these ground-truth data for Australia as of August 2017, which are provided as Electronic Supplementary Material. We provide ∼7500 GPS-derived ellipsoidal heights, normal-orthometric heights from the 1971 adjustment of the Australian Height Datum, normal heights from a readjustment of levelling constrained to a model of the ocean's mean dynamic topography, and ∼1000 historical astrogeodetic vertical deflections. Updates to these data will be posted on the Intergovernmental Committee on Surveying and Mapping GitHub repository (https://github.com/icsm-au), together with a readme.txt file describing them.

Current state of art of satellite altimetry

Abstract One of the fundamental problems of modern geodesy is precise defi nition of the gravitational fi eld and its changes in time. This is essential in positioning and navigation, geophysics, geodynamics, oceanography and other sciences related to the climate and Earth’s environment. One of the major sources of gravity data is satellite altimetry that provides gravity data with almost 75% surface of the Earth. Satellite altimetry also provides data to study local, regional and global geophysical processes, the geoid model in the areas of oceans and seas. This technique can be successfully used to study the ocean mean dynamic topography. The results of the investigations and possible products of altimetry will provide a good material for the GGOS (Global Geodetic Observing System) and institutions of IAS (International Altimetry Service). This paper presents the achievements in satellite altimetry in all the above disciplines obtained in the last years. First very shorly basic concept of satellite altimetry is given. In order to obtain the highest accuracy on range measurements over the ocean improved of altimetry waveforms performed on the ground is described. Next, signifi cant improvements of sea and ocean gravity anomalies models developed presently is shown. Study of sea level and its extremes examined, around European and Australian coasts using tide gauges data and satellite altimetry measurements were described. Then investigations of the phenomenon of the ocean tides, calibration of altimeters, studies of rivers and ice-sheets in the last years are given.

Current State of Art of Satellite Altimetry

Abstract One of the fundamental problems of modern geodesy is precise definition of the gravitational field and its changes in time. This is essential in positioning and navigation, geo-physics, geodynamics, oceanography and other sciences related to the climate and Earth’s environment. One of the major sources of gravity data is satellite altimetry that provides gravity data with almost 75% surface of the Earth. Satellite altimetry also provides data to study local, regional and global geophysical processes, the geoid model in the areas of oceans and seas. This technique can be successfully used to study the ocean mean dynamic topography. The results of the investigations and possible products of altimetry will provide a good material for the GGOS (Global Geodetic Observing System) and institutions of IAS (International Altimetry Service). This paper presents the achievements in satellite altimetry in all the above disciplines obtained in the last years.

An Assessment of State‐of‐the‐Art Mean Sea Surface and Geoid Models of the Arctic Ocean: Implications for Sea Ice Freeboard Retrieval

AbstractState‐of‐the‐art Arctic Ocean mean sea surface (MSS) models and global geoid models (GGMs) are used to support sea ice freeboard estimation from satellite altimeters, as well as in oceanographic studies such as mapping sea level anomalies and mean dynamic ocean topography. However, errors in a given model in the high‐frequency domain, primarily due to unresolved gravity features, can result in errors in the estimated along‐track freeboard. These errors are exacerbated in areas with a sparse lead distribution in consolidated ice pack conditions. Additionally model errors can impact ocean geostrophic currents, derived from satellite altimeter data, while remaining biases in these models may impact longer‐term, multisensor oceanographic time series of sea level change in the Arctic. This study focuses on an assessment of five state‐of‐the‐art Arctic MSS models (UCL13/04 and DTU15/13/10) and a commonly used GGM (EGM2008). We describe errors due to unresolved gravity features, intersatellite biases, and remaining satellite orbit errors, and their impact on the derivation of sea ice freeboard. The latest MSS models, incorporating CryoSat‐2 sea surface height measurements, show improved definition of gravity features, such as the Gakkel Ridge. The standard deviation between models ranges 0.03–0.25 m. The impact of remaining MSS/GGM errors on freeboard retrieval can reach several decimeters in parts of the Arctic. While the maximum observed freeboard difference found in the central Arctic was 0.59 m (UCL13 MSS minus EGM2008 GGM), the standard deviation in freeboard differences is 0.03–0.06 m.

The coastal mean dynamic topography in Norway observed by CryoSat‐2 and GOCE

AbstractNew‐generation synthetic aperture radar altimetry, as implemented on CryoSat‐2, observes sea surface heights in coastal areas that were previously not monitored by conventional altimetry. Therefore, CryoSat‐2 is expected to improve the coastal mean dynamic topography (MDT). However, the MDT remains highly reliant on the geoid. Using new regional geoid models as well as CryoSat‐2 data, we determine three geodetic coastal MDT models in Norway and validate them against independent tide‐gauge observations and the operational coastal ocean model NorKyst800. The CryoSat‐2 MDTs agree on the ∼3–5 cm level with both tide‐gauge geodetic and ocean MDTs along the Norwegian coast. In addition, we compute geostrophic surface currents to help identifying errors in the geoid models. We find that even though the regional geoid models are all based on the latest satellite gravity data as provided by GOCE, the resulting circulation patterns differ. We demonstrate that some of these differences are due to erroneous or lack of marine gravity data. This suggests that there is significant MDT signal at spatial scales beyond GOCE, and that the geodetic approach to MDT determination benefits from the additional terrestrial gravity information provided by a regional geoid model. We also find that the border of the geographical mode mask of CryoSat‐2 coincides with the Norwegian Coastal Current, making it challenging to distinguish between artifacts in the CryoSat‐2 observations during mode switch and ocean signal.

A comparative assessment of coastal mean dynamic topography in Norway by geodetic and ocean approaches

AbstractThe ocean's mean dynamic topography (MDT) is the surface representation of ocean circulation. It may be determined by the ocean approach, using numerical ocean circulation models, or by the geodetic approach, where MDT is the height of the mean sea surface (MSS), or mean sea level (MSL), above the geoid. Using new geoid models, geodetic MDT profiles based on tide gauges, dedicated coastal altimetry products, and conventional altimetry are compared with six ocean MDT estimates independent of geodetic data. Emphasis is put on the determination of high‐resolution geoid models, combining ESA's fifth release (R5) of GOCE satellite‐only global gravity models (GGMs) with a regional geoid model for Norway by a filtering technique. Differences between MDT profiles along the Norwegian coast together with Taylor diagrams confirm that geodetic and ocean MDTs agree on the ∼3–7 cm level at the tide gauges, and on the ∼5–11 cm level at the altimetry sites. Some geodetic MDTs correlate more with the best‐performing ocean MDT than do other ocean MDTs, suggesting a convergence of the methods. While the GOCE R5 geoids are shown to be more accurate over land, they do not necessarily show the best agreement over the ocean. Pointwise monomission altimetry products give results comparable with the multimission DTU13MSS grid on the ∼5 cm level. However, dedicated coastal altimetry products generally do not offer an improvement over conventional altimetry along the Norwegian coast.

Seasonal Variation of Surface Circulation Along Peninsular Malaysia' East Coast

The sea surface circulation pattern over the coast of Peninsula Malaysia's East Coast during Northeast Monsoon (NE) and Southwest Monsoon (SW) are derived using the seasonally averaged sea level anomaly (SLA) data from altimetric data and 1992-2002 Mean Dynamic Ocean Topography. This altimetric data has been derived from multi-mission satellite altimeter TOPEX, ERS-1, ERS-2, JASON-1, and ENVISAT for the period of nineteen years (1993 to 2011) using the Radar Altimeter Database System (RADS). The estimated sea level anomaly (SLA) have shown similarity in the pattern of sea level variations observed by four tide gauges. Overall, the sea surface circulations during the NE and SW monsoons shows opposite patterns, northward and southward respectively. During the SW monsoon, an anti-cyclonic circulation has been detected around the Terengganu coastal area centred at (about 5.5° N 103.5° E) and nearly consistent with previous study using numerical modelling. The estimated geostrophic current field from the altimeter is consistent with the trajectories of Argos-tracked Drifting Buoys provided by the Marine Environmental Data Services (MEDS) in Canada.

Computation of a new mean dynamic topography for the Mediterranean Sea from model outputs, altimeter measurements and oceanographic in situ data

Abstract. The accurate knowledge of the ocean's mean dynamic topography (MDT) is a crucial issue for a number of oceanographic applications and, in some areas of the Mediterranean Sea, important limitations have been found pointing to the need of an upgrade. We present a new MDT that was computed for the Mediterranean Sea. It profits from improvements made possible by the use of extended data sets and refined processing. The updated data set spans the 1993–2012 period and consists of drifter velocities, altimetry data, hydrological profiles and model data. The methodology is similar to the previous MDT by Rio et al. (2007). However, in Rio et al. (2007) no hydrological profiles had been taken into account. This required the development of dedicated processing. A number of sensitivity studies have been carried out to obtain the most accurate MDT as possible. The main results from these sensitivity studies are the following: moderate impact to the choice of correlation scales but almost negligible sensitivity to the choice of the first guess (model solution). A systematic external validation to independent data has been made to evaluate the performance of the new MDT. Compared to previous versions, SMDT-MED-2014 (Synthetic Mean Dynamic Topography of the MEDiterranean sea) features shorter-scale structures, which results in an altimeter velocity variance closer to the observed velocity variance and, at the same time, gives better Taylor skills.

How well can we measure the ocean's mean dynamic topography from space?

AbstractRecent gravity missions have produced a dramatic improvement in our ability to measure the ocean's mean dynamic topography (MDT) from space. To fully exploit this oceanic observation, however, we must quantify its error. To establish a baseline, we first assess the error budget for an MDT calculated using a 3rd generation GOCE geoid and the CLS01 mean sea surface (MSS). With these products, we can resolve MDT spatial scales down to 250 km with an accuracy of 1.7 cm, with the MSS and geoid making similar contributions to the total error. For spatial scales within the range 133–250 km the error is 3.0 cm, with the geoid making the greatest contribution. For the smallest resolvable spatial scales (80–133 km) the total error is 16.4 cm, with geoid error accounting for almost all of this. Relative to this baseline, the most recent versions of the geoid and MSS fields reduce the long and short‐wavelength errors by 0.9 and 3.2 cm, respectively, but they have little impact in the medium‐wavelength band. The newer MSS is responsible for most of the long‐wavelength improvement, while for the short‐wavelength component it is the geoid. We find that while the formal geoid errors have reasonable global mean values they fail capture the regional variations in error magnitude, which depend on the steepness of the sea floor topography.

A comparison of GOCE and drifter-based estimates of the North Atlantic steady-state surface circulation

Over the last decade, due to the Gravity Recovery And Climate Experiment (GRACE) mission and, more recently, the Gravity and steady state Ocean Circulation Explorer (GOCE) mission, our ability to measure the ocean's mean dynamic topography (MDT) from space has improved dramatically. Here we use GOCE to measure surface current speeds in the North Atlantic and compare our results with a range of independent estimates that use drifter data to improve small scales. We find that, with filtering, GOCE can recover 70% of the Gulf Steam strength relative to the best drifter-based estimates. In the subpolar gyre the boundary currents obtained from GOCE are close to the drifter-based estimates. Crucial to this result is careful filtering which is required to remove small-scale errors, or noise, in the computed surface. We show that our heuristic noise metric, used to determine the degree of filtering, compares well with the quadratic sum of mean sea surface and formal geoid errors obtained from the error variance–covariance matrix associated with the GOCE gravity model. At a resolution of 100km the North Atlantic mean GOCE MDT error before filtering is 5cm with almost all of this coming from the GOCE gravity model.

Using Models of the Ocean's Mean Dynamic Topography to Identify Errors in Coastal Geodetic Levelling

Identifying errors (blunders and systematic errors) in coastal geodetic levelling networks has often been problematic, primarily for two reasons. First, mean sea level (MSL) at tide gauges cannot be directly compared to height differences from levelling because the geoid/quasigeoid and MSL are not parallel, being separated by the ocean's mean dynamic topography (MDT). Second, there is a the lack of redundancy at the edge of the levelling network. This article sets out a methodology to independently identify blunders and/or systematic errors (over long distances) in geodetic levelling using MDT models to account for the separation between the geoid/quasigeoid and MSL at tide gauges. This method is then tested in a case study using an oceanographic MDT model, MSL observations, GNSS data, and a quasigeoid model. The results are significant because the errors found could not be detected by standard levelling misclosure checks alone, with supplementary data from an MDT model, with cross-validation from GNSS-quasigeoid allowing their detection. In addition, it appears that an oceanographic-only MDT is as effective as GNSS and a quasigeoid model for detecting levelling errors, which could be particularly useful for countries with coastal levelling errors in their levelling networks that cannot be identified by conventional levelling closure checks.

The apparent British sea slope is caused by systematic errors in the levelling-based vertical datum

The spirit-levelling–based British vertical datum (Ordnance Datum Newlyn) implies a south–north apparent slope in mean sea level of up to 53 mm deg –1 latitude, due to the datum falling on heading northwards. Although this apparent slope has been investigated since the 1960s, explanations of its origin have remained inconclusive. It has also been suggested that, rather than a slope, the British vertical datum includes a step of about 240 mm affecting all sites north of about 53°N. In either case, the British vertical datum may be of limited use for any study requiring accurate heights or changes in heights, such as testing geoid models, groundwater and hydrocarbon extraction, the calibration and validation of satellite-based digital terrain models, and the unification of vertical datums internationally. Within the last decade, however, based on an apparent reduction in the slope to only −12 mm deg –1 latitude with respect to recent geoid models, it has been claimed that the British vertical datum does provide a physically meaningful surface for use in scientific applications. In this paper, we reinvestigate the presence of apparent south–north sea slopes around Britain and reported slopes in the vertical datum, using the EGM2008 global gravitational model, together with mean sea level and GPS data from British tide gauges, GPS ellipsoidal heights of 178 fundamental benchmarks across mainland Britain, and vertical deflection observations at 192 stations. We demonstrate a south–north slope in the British vertical datum of −(20–25) mm deg –1 latitude with respect to both mean sea level (corrected for the ocean's mean dynamic topography and the inverse barometer response to atmospheric pressure loading) and the EGM2008 quasigeoid model, while EGM2008 is shown to exhibit a negligible slope of (2 ± 4) mm deg –1 with respect to mean sea level. It is clear, therefore, that the slope can only arise from systematic errors in the levelling, although we are unable to isolate their exact origin. Using an offset detection method based on a penalized likelihood maximization using the Schwarz Information Criterion, we do not detect a step in the vertical datum affecting all sites north of 53°N, but do identify regional distortions that we attribute to the inhomogeneity in both the levelling data used and the least squares adjustment procedures used to realize the datum. We conclude that the British vertical datum remains unsuitable for scientific purposes.

Comparison of Geostrophic Current Estimation around the Korean Peninsula from Remote Sensing-based MSS Models

ABSTRACT Cho, J. M., Lee, D. H., Lee, M. R., Cho, J. H., Yun, H. S., Jeong, T. J., Jang, A. and Kim, T. Y., 2013. Comparison of Geostrophic Current Estimation around the Korean Peninsula from Remote Sensing-based MSS models Altimetry satellites like Geosat, ERS-1/2, Envisat and Topex-Poseidon, which have been developed and deployed since the 1970s, have collected immense arrays of data on the sea, and the data thus gathered have been used in oceanography as well as in a variety of scientific disciplines, such as geophysics and geodesy. The mean dynamic ocean topography or Sea Surface Topography (SST), obtained by calculating the difference between the Mean Sea Surface (MSS) and the geoid surface, plays a critical role in determining the earth's shape and geoid as well as in the precision modeling of the earth's gravity field, ocean currents, and tidal waves. In the study, the MSS_CNES_CLS10 (Centre National d'Etudes Spatiales Collectre Localisation Satellites 2010) and DTU10MSS (Danmarks Tekniske Universi...