Ocean Tide Models Research Articles

Rotationally acceptable ocean tide models for determining the response of the oceans to atmospheric pressure fluctuations

Suitably generalized, ocean tide models can be used to determine the oceans' response to atmospheric pressure forcing; but the huge range of spatial and temporal scales of that forcing limits the relevance of state‐of‐the‐art tide modeling techniques, like data assimilation, for such determinations. With an interest in its effects on Earth's rotation, in 1998 I employed a generalized but non‐assimilating spherical harmonic tide model to determine the oceanic response to pressure forcing, however restricting its application to time scales exceeding a few days. This article revisits that spherical harmonic model in an attempt to improve its rotational predictions of short‐period tides. We find that increasing the resolution of the model ocean does not by itself affect the tidal solution much, but varying the model's frictional parameters can produce diurnal tides whose effects on Earth's polar motion are similar to those of a variety of other ocean tide models. Such an improved model will allow our calculations of the oceans' dynamic response to pressure forcing, and the effects of that response on Earth's rotation, to be extended down to diurnal time scales.

A novel high resolution model without open boundary conditions applied to the China Seas: first investigation on tides

We developed a Global Ocean Circulation and Tide Model (GOCTM) with coarse grids in the open deep ocean degrading ‘smoothly’ into the highly resolved China Seas (CS) of refined grids to study the tides and circulation there. GOCTM is based on the framework of the Finite Volume approach for better mass conservation through improved transports across the discrete individual control volume. It also takes a full advantage of the geometric flexibility of unstructured mesh using a realistic global topography including the Arctic Ocean. The CS are given a special focus by refining the unstructured grids, but they are embedded into global domain naturally. Furthermore, GOCTM not only successfully avoids the treatment of the open boundaries, but also optimizes the trade-off between computational cost and model accuracy. Meanwhile, GOCTM is driven by the astronomical tide-generating potential and the secondary tide-generating potential directly, together with the wind stress and heat flux. GOCTM succeeds in reproducing the global eight principal tidal harmonic constants. Particularly, the simulated tidal results in the CS are improved compared to some other regional models with the discrepancy of 3.9 cm for M2 tide. This idea of GOCTM can also be referred for other regional ocean study.

Coastal tides in West Greenland derived from tide gauge records

Based on pressure tide-gauge observations, sea-level records are derived for ten sites along the coast of West Greenland. The ocean tidal signal is extracted by a harmonic tidal analysis. The accuracy of the determined tidal constants is discussed in detail. The tides account for 85% of the observed sea-level standard deviation. The tide gauge records reveal significant shallow-water tidal effects, in particular compound and overtide amplitudes reaching 5 cm. The propagation of the tidal waves into the fjords depends strongly on local conditions and is in some cases accompanied by an amplification of the tidal amplitudes. The observed tidal signals are compared to the predictions of the global ocean tide model FES2004. At the outer coast, a good agreement is found. Inside the fjords, however, the model performs worse and tide gauge observations may still be indispensable when accurate tidal signals are required.

GOTIC2: A Program for Computation of Oceanic Tidal Loading Effect

Oceanic tidal loading signals are in general smaller than solid Earth tide signals, but they can be measured by geodetic technique and need to be accurately corrected for because they are in general a source of noise. In order to meet this requirement we have developed a program to compute the loading tide most accurately in Japanese region by using a combination of global and regional ocean tide models and fine-scale land-sea grids. The program computes six kinds of loading (radial and horizontal displacements, gravity, tilt, strain, and deflection of the vertical) for 21 constituents including long-period tide.

Spectral and geographical variability in the oceanic response to atmospheric pressure fluctuations, as inferred from “dynamic barometer” Green's functions

A decade ago, a novel theoretical approach was developed (Dickman, 1998) for determining the dynamic response of the oceans to atmospheric pressure variations, a response nicknamed the “dynamic barometer” (DB), and the effects of that response on Earth's rotation. This approach employed a generalized, spherical harmonic ocean tide model to compute oceanic Green's functions, the oceans' fluid dynamic response to unit‐amplitude pressure forcing on various spatial and temporal scales, and then construct rotational Green's functions, representing the rotational effects of that response. When combined with the observed atmospheric pressure field, the rotational Green's functions would yield the effects of the DB on Earth's rotation. The Green's functions reflect in some way the geographical and spectral sensitivity of the oceans to atmospheric pressure forcing. We have formulated a measure of that sensitivity using a simple combination of rotational Green's functions. We find that the DB response of the oceans to atmospheric pressure forcing depends significantly on geographic location and on frequency. Compared to the inverted barometer (IB) (the traditional static model), the DB effects differ slightly at long periods but become very different at shorter periods. Among all the responses, the prograde polar motion effects are the most dynamic, with large portions of the North Atlantic and some of the North Pacific no larger than one third of IB, but most of the Southern Hemisphere oceans at least 50% greater than IB.

Evaluation of Ocean Tide Models Used for Jason-2 Altimetry Corrections

It has been more than a decade since the last comprehensive accuracy assessment of global ocean tide models. Here, we conduct an evaluation of the barotropic ocean tide corrections, which were computed using FES2004 and GOT00.2, and other models on the Jason-2 altimetry Geophysical Data Record (GDR), with a focus on selected coastal regions with energetic ocean dynamics. We compared nine historical and contemporary ocean tide models with pelagic tidal constants and with multiple satellite altimetry mission (T/P, ERS-1/-2, Envisat, GFO, Jason-1/-2) sea level anomalies using variance reduction studies. All accuracy assessment methods show consistent results. We conclude that all the contemporary ocean tide models evaluated have similar performance in the selected coastal regions. However, their accuracies are region-dependent and overall are significantly worse than those in the deep-ocean, which are at the 2–3 cm RMS (root-mean-square) level. The Gulf of Mexico and Northwest Atlantic regions present the least reduction of altimetry sea surface height variability after ocean tides are removed, primarily because of large oceanic variability associated with loop currents in the Gulf of Mexico and the Gulf Stream in the Northwest Atlantic.

OCEAN TIDE LOADING AND RELATIVE GNSS IN THE BRITISH ISLES

Ocean tide loading (OTL) affects all parts of the British Isles to varying degree, causing peak-to-peak vertical displacements of up to 13 cm in South-West England over semi-diurnal and diurnal timescales. Lateral displacements are typically around one-third of the magnitude of vertical displacements at a point, so are also considerable for carrier phase GNSS surveying. Using a recent numerical ocean tide model, we predict that widespread residual displacements up to ∼1 cm remain in kinematic or short-occupation static relative GNSS positions computed with respect to the nearest continuously operating reference station. Even if OTL is not modelled explicitly, these errors will be mitigated by network GNSS to an extent dependent on the number and location of reference stations used, and the adjustment or error interpolation scheme adopted in the processing. For a selection of error interpolation algorithms, we predict that throughout most inland regions of Great Britain and Ireland, network processing reduces the residual OTL error to within the expected kinematic GNSS system noise. However, pockets of OTL error exceeding 1 cm may remain, especially in coastal locations and in South-West Ireland, South Wales, South-West England, and the islands off the west coast of Scotland. Residual OTL error at semi-diurnal periods dominates that at diurnal periods. We derive a simple heuristic for estimating the magnitude of this error from a sample of observations at a locality, valid within a short window of the spring/neap tidal cycle, and show how the residual error may, if necessary, be mitigated by a suitable averaging scheme.

Determination of ocean tide loading displacements in Hong Kong using GPS technique

Ocean tide loading (OTL) displacements of eight principal constituents at 12 sites in Hong Kong were determined using more than eight years of continuous GPS observations. Parameters of the OTL displacements were estimated using daily GPS solutions obtained with precise point positioning (PPP) technique. The results were compared with predictions from seven latest global ocean tide models. Gravity measurements of OTL in Hong Kong were also used to validate the GPS results. The study shows that the results from both the GPS and gravity measurements agree best with the GOT4.7 and NAO99b models, when the K1 and K2 constituents are excluded. The agreements between the GPS and the model estimates are generally at sub-millimeter level in both the horizontal and vertical directions, except for S2, K2 and K1 constituents that have relatively larger errors. After removing the systematic biases between the GPS and the model estimates, the misfits of M2, S2, N2, O1, P1 and Q1 at all sites are within 0.5 and 1.0 mm in the horizontal and the vertical directions, respectively, whereas K1 and K2 show relatively larger misfits of up to 2.3 mm. Both the GPS and the gravity S2 estimates have large biases with unknown reasons when compared with the modeled values, which needs to be further investigated. The study demonstrates that GPS is capable of estimating the OTL displacements with the same accuracy as the model predictions, especially for coastal areas.

Towards development of a consistent orbit series for TOPEX, Jason-1, and Jason-2

The TOPEX/Poseidon, Jason-1 and Jason-2 set of altimeter data now provide a time series of synoptic observations of the ocean that span nearly 17 years from the launch of TOPEX in 1992. The analysis of the altimeter data including the use of altimetry to monitor the global change in mean sea level requires a stable, accurate, and consistent orbit reference over the entire time span. In this paper, we describe the recomputation of a time series of orbits that rely on a consistent set of reference frames and geophysical models. The recomputed orbits adhere to the IERS 2003 standards for ocean and earth tides, use updates to the ITRF2005 reference frame for both the SLR and DORIS stations, apply GRACE-derived models for modeling of the static and time-variable gravity, implement the University College London (UCL) radiation pressure model for Jason-1, use improved troposphere modeling for the DORIS data, and apply the GOT4.7 ocean tide model for both dynamical ocean tide modeling and for ocean loading. The new TOPEX orbits have a mean SLR fit of 1.79 cm compared to 2.21 cm for the MGDR-B orbits. These new TOPEX orbits agree radially with independent SLR/crossover orbits at 0.70 cm RMS, and the orbit accuracy is estimated at 1.5–2.0 cm RMS over the entire TOPEX time series. The recomputed Jason-1 orbits agree radially with the Jason-1 GDR-C orbits at 1.08 cm RMS. The GSFC SLR/DORIS dynamic and reduced-dynamic orbits for Jason-2 agree radially with independent orbits from the CNES and JPL at 0.70–1.06 cm RMS. Applying these new orbits, and using the latest altimeter corrections for TOPEX, Jason-1, and Jason-2 from September 1992 to May 2009, we find a global rate in mean sea level of 3.0 ± 0.4 mm/yr.

Rotational evaluation of a long-period spherical harmonic ocean tide model

By exchanging angular momentum with the solid earth, tidal variations in ocean currents and sea level cause the rotation of the solid earth to change. Observations of earth rotation variations can therefore be used to evaluate ocean tide models. The rotational predictions of a spherical harmonic ocean tide model that is not constrained by any type of data are compared here to the predictions of numerical ocean tide models and to earth rotation observations from which atmospheric and non-tidal oceanic effects have been removed. The spherical harmonic ocean tide model is shown to account for the observed variations at the fortnightly tidal period in polar motion excitation but not in length-of-day. Overall, its long-period polar motion excitation predictions fit the observed tidal signals better than do the predictions of the numerical ocean tide models studied here. It may be possible to improve its agreement with length-of-day observations by tuning certain model parameters, as was done to obtain the close agreement reported here between the modeled and observed polar motion excitation; alternatively, the discrepancy in length-of-day may point to the need to revise current models of mantle anelasticity and/or models of the oceanic response to atmospheric pressure variations.

Accuracy Assessment of Global Ocean Tide Models Base on Satellite Altimetry

Tidal harmonics of 8 principal constituents (M2,S2,K1,O1,N2,K2,P1 and Q1)derived from ground observations at 152 tidal gauge stations are used to assess the accuracy of 7 global ocean tidal models.It is shown that for the deep ocean area these models have high accuracy.The root-mean-square values of tidal height differences (RMSd) are within the range from 1.0 to1.3 cm for M2,and the root-sum-square values of the RMSd of these 8 constituents lie in the range from 2.0 to 2.3 cm.Comparison of the global tidal models to 18 offshore and island tide gauge observations in the seas adjacent to China shows that the accuracy significantly decreases.The RMSd values of constituent M2 lie in the range of 4.4 to 10 cm,which is significantly greater than that in the deep ocean area.Among these models,the model NAO99 that assimilates T/P altimeter data and tide gauge observations into a hydrodynamic model,developed by the National Astronomical Observatory of Japan,has the best accuracy for the seas adjacent to China.

Space-borne gravimetric satellite constellations and ocean tides: aliasing effects

Ocean tides redistribute mass at high temporal frequencies. Satellite missions that aim to observe medium to low frequency mass variations need to take into account this rapidly varying mass signal. Correcting for the effects of ocean tides by means of imperfect models might hamper the observation of other temporal gravity field signals of interest. This paper explores different methods for mitigating aliasing errors for the specific example of observing mass variations due to land hydrology, including temporal filtering of time-series of gravity solutions, spatial smoothing and the use of satellite constellations. For this purpose, an Earth System Model (ESM) was constructed, which included state-of-the-art time varying components for ocean, atmosphere, solid Earth, hydrology, ice-sheets and ocean tides. Using the ESM, we simulated the retrieval of the hydrologically driven gravity field changes using a number of different satellite constellations. We find that (1) the ocean tide aliasing strongly depends on the satellite constellation, the choice of orbital parameters and the length of the data span; (2) the aliasing effect manifests itself differently for different geographical regions; (3) the aliasing causes a peculiar striping pattern along the ground track of the satellite orbits; (4) optimizing the choice of orbital parameters of a single GRACE-type tandem can be more effective at reducing the aliasing of ocean tide model errors than flying more tandems. Finally, we corroborate the experiences with GRACE data analysis that appropriate post-processing techniques can significantly improve the quality of the retrieved gravity changes.

Optimization of Boundary Condition and Physical Parameter in an Ocean Tide Model Using an Evolutionary Algorithm

Optimization of Boundary Condition and Physical Parameter in an Ocean Tide Model Using an Evolutionary Algorithm

GSFC DORIS contribution to ITRF2008

The NASA GSFC DORIS analysis center has provided weekly DORIS solutions from November 1992 to January 2009 (839 SINEX files) of station positions and Earth Orientation Parameters for inclusion in the DORIS contribution to ITRF2008. The NASA GSFC GEODYN orbit determination software was used to process the orbits and produce the normal equations. The weekly SINEX gscwd10 submissions included DORIS data from Envisat, TOPEX/Poseidon, SPOT-2, SPOT-3, SPOT-4, SPOT-5. The orbits were mostly seven days in length (except for weeks with data gaps or maneuvers). The processing used the GRACE-derived EIGEN-GL04S1 gravity model, updated modeling for time-variable gravity, the GOT4.7 ocean tide model and tuned satellite-specific macromodels for SPOT-2, SPOT-3, SPOT-4, SPOT-5 and TOPEX/Poseidon. The University College London (UCL) radiation pressure model for Envisat improves nonconservative force modeling for this satellite, reducing the median residual empirical daily along-track accelerations from 3.75 × 10 −9 m/s 2 with the a priori macromodel to 0.99 × 10 −9 m/s 2 with the UCL model. For the SPOT and Envisat DORIS satellite orbits from 2003 to 2008, we obtain average RMS overlaps of 0.8–0.9 cm in the radial direction, 2.1–3.4 cm cross-track, and 1.7–2.3 cm along-track. The RMS orbit differences between Envisat DORIS-only and SLR & DORIS orbits are 1.1 cm radially, 6.4 cm along-track and 3.7 cm cross-track and are characterized by systematic along-track mean offsets due to the Envisat DORIS system time bias of ±5–10 μs. We obtain a good agreement between the geometrically-determined geocenter parameters and geocenter parameters determined dynamically from analysis of the degree one terms of the geopotential. The intrinsic RMS weekly position repeatability with respect to the IDS-3 combination ranges from 2.5 to 3.0 cm in 1993–1994 to 1.5 cm in 2007–2008.

Reply to comment by M. S. Bos on “Anomalous ocean load tide signal observed in lake‐level variations in Tierra del Fuego”

[1] The comments of Bos [2010] are valuable and certainly contribute to an improved understanding of the tidal interactions in Tierra del Fuego. They are based on a thorough examination of the subject and emphasize the scientific relevance of this investigation. [2] Bos [2010, Figure 2] presents modelled lake-tide predictions that differ from those given in our article. The reason for this difference consists essentially in incorrect phase lags for the direct effect as given by Richter et al. [2009, Figure 2a]. Due to inconsistencies between the used ETERNA and TASK-2000 software tools an unnoticed time reference shift of one hour was introduced which resulted in the about 30° too large phase lags. [3] However, our modelled load tides (indirect effect) are not affected thereby. Still, the load tides predicted by Bos [2010] are about 10% larger in amplitude compared to our results. This is most likely explained by the application of different load models for the Magellan Strait (e.g., our Magellan Strait model produces a slight reduction in the total load tide amplitude in contrast to the statement by Bos). An impact of accounting for the complete global tidal load model rather than our limited domain in the order of 0.2 mm, 15° as stated by Bos seems to us questionable. Test computations had shown that using the tidal loading predictions of M. S. Bos and H.-G. Scherneck (Ocean tide loading provider, 2009, available at http://www.oso.chalmers.se/∼loading/) (implying convolution over the complete global model; FES2004 ocean tide model, Gutenberg-Bullen earth model) changes the load tide contributions at the three tide gauge sites by no more than 0.01 mm and 1.5°, respectively (crustal deformation effect only). [4] A corrected version of Richter et al.'s [2009] Figure 2a is displayed in Figure 1. It reveals now a smaller difference between observed and modelled lake tide signals. However, the remaining misclosure still exceeds the uncertainties of the observed signal and also the 3σ interval (which has been added in Figure 1). Here, σ represents the site-specific standard deviation of the M2 amplitude and M2 phase as determined from a 12-months lake-level observation record. For site B, the σ were derived directly as the standard deviation of the M2 tidal parameters of three independent 12-month records (σ12B). For sites A and C, for which only one year of observations is available, the respective σ were derived from the standard deviations of the M2 parameters of two independent 6-month records, scaled by the ratio σ12B/σ6B between the standard deviations of the 12-month solution and two 6-month solutions (σ6B) at site B (assuming that the gain in accuracy by increasing the record length from 6 to 12 months is equal at all three sites). The geometrical relation between the vectors (i.e., relative phase relation) continues to suggest that this significant difference between observation and model is to be attributed to an underestimation of the modelled load tide amplitudes. A satisfactory agreement would require a scaling of the modelled load tide amplitudes by about 120%. [5] The article was aimed to present our lake-tide observations and their deviation from the model predictions. Our considerations concerning the elastic earth model were only one general interpretation/speculation in an attempt to explain the obtained differences. We did, however, not pretend to infer a realistic elastic earth model from the tidal parameters. For such an undertaking our approach (constant scaling of the complete Green's function, no consideration of vertical crustal structure, etc.) would indeed be too simplistic. Therefore, the discussion of this topic by Bos [2010, Figure 1] represents a valuable extension of knowledge. [6] Thus, thanks to the comment by Bos [2010] the deviation of the modelled load tide from the observations has been reduced from 30% to 20%. Nevertheless, a 20% amplification of the theoretical load tides is still important compared to other published observational results. And, in our opinion, it is still difficult to explain it by uncertainties of the observations and the load (ocean tide) model only. On the other hand, a substantial amplification of elastic loading effects has been observed also in other regions in South America [Bevis et al., 2004]. However, we agree with Bos that the differences between the observations in Lago Fagnano and the predictions of the internet ocean tide loading provider should be interpreted with care. At present, the authors are continuing the investigations of tidal interactions in Tierra del Fuego and the results will be made public to the scientific community in due time.

Improving gravity anomalies over China marginal sea from retracked Geosat and ERS-1 data

The quality of altimeter data and ocean tide model is critical to the recovery of coastal gravity anomalies. In this contribution, three retracking methods (threshold, improved threshold and Beta-5) are investigated with the aim of improving the altimeter data over a shallow water area. Comparison indicates that the improved threshold is the best retracking method over China Sea. Two ocean tide models, NAO99b and CSR4.0, are analyzed. Results show that different tide models used in the processing of altimeter data may result in differences more than 10 mGal in recovered coastal gravity anomalies. Also, NAO99b is more suitable than CSR4.0 over the shallow water area of China Sea. Finally, gravity anomalies over China Sea are calculated from retracked Geosat/GM and ERS-1/GM data by least squares collocation. Comparison with shipborne gravimetry data demonstrates that gravity anomalies from retracked data are significantly superior to those from non-retracked data. Our results have the same order as the other two altimeter-derived gravity models: Sandwell&Smith(V16) and DNSC08.

Tidal Simulation around Japanese Coastal Region with a Global Ocean Tide Model

This study examines the sensitivity of tidal simulation to the model-domain size, and validates the accuracy of tidal simulation with broad model domain as the storm surge simulation. The tidal simulation models which adopt the driving force given by the global ocean tide model are developed. The models cover the Seto Inland Sea and coastal region of Kyushu-Shikoku with three different model-domain sizes. The size of the broadest domain is just as the storm surge simulation. Validation with the tide-gauge data shows that the accuracy is high in the narrower domain cases, however, in the broadest domain case the calculational error of tide becomes large in Bungo Channel and eastern coastal region of Kyushu.

GPS 정밀단독측위기법을 이용한 해양조석하중 분조성분 검출

이 연구에서는 GPS(Global Positioning System) 정밀단독측위 기법을 이용하여 해양조석하 중(Ocean Tide Loading, OTL) 분조성분을 검출하고, 그 결과를 조석모델에 의한 예측값과 비교하였다. 고정밀 GPS 자료처리 프로그램인 GIPSY를 사용하여 인천, 서산, 제주 GPS 상시관측소를 대상으로 10분 단위의 수직좌표 시계열을 산출하였으며, 시계열의 조석조화분해를 통해 OTL 지각변위를 일으키는 주요 4대 분조(<TEX>$M_2$</TEX>, <TEX>$S_2$</TEX>, <TEX>$K_1$</TEX>, <TEX>$O_1$</TEX>)의 진폭과 위상을 검출하였다. 조석모델의 예측값을 기준으로 GPS로 산출한 OTL 분조의 정확도를 평가한 결과 진폭은 제주관측소, 위상은 인천관측소에서 가장 높은 정확도를 나타내었다. 주요 11개 분조 중 최대비율을 차지하는 <TEX>$M_2$</TEX>분조의 경우 진폭성분에서 평균 23.8%의 상대오차를 보였다. In this study, the Ocean Tide Loading (OTL) constituents were detected by the Precise Point Positioning (PPP) technique using GPS. Then, the GPS estimates of OTL constituents were compared with the predictions of the ocean tide models. We picked three permanent GPS stations as test sites and they are ICNW, SEOS, and CJUN. To detect the OTL constituents using GPS, we created vertical coordinate time series at 10-minute intervals using the PPP approach implemented in the GIPSY software. Through the tidal harmonic analysis of this height time series, the four major constituents (<TEX>$M_2$</TEX>, <TEX>$S_2$</TEX>, <TEX>$K_1$</TEX>, <TEX>$O_1$</TEX>) were determined. The amplitude obtained from the GPS height time series of the OTL constituents showed best match with the model predictions at CJUN, while the phase showed closest match at ICNW. The amplitude accuracy of the <TEX>$M_2$</TEX>, which is the dominant factor out of the 11 major constituents, was 24.8% on average.

Least squares self-coherence for sub-nGal signal detection in the superconducting gravimeter records

Abstract In this paper, we use superconducting gravimeter (SG) data recorded at three stations of the global geodynamics project (GGP) network, with good geographical distribution, to search for possible significant peaks in the gravity spectrum that are in the assumed period range of the Slichter triplet. Seven-year long series from Cantley (Canada), and four-year long series from both Canberra (Australia) and Moxa (Germany) stations are used. First, a solid Earth and ocean tide model is subtracted from the data, followed by a local atmospheric pressure correction based on a frequency-, and location-dependent admittance estimated by the least squares response method. Subsequently, the residual series are filtered with a Parzen-based bandpass filter with a passband (12 h–78 s). A sub-nGal detection level is confirmed by injecting an artificial sine wave of different amplitudes. The Least Squares Self-Coherency spectrum shows the existence of many apparently statistically significant peaks at the 95% confidence level in the band (3–8 h). Although a few peaks are close to the claimed Slichter periods in previous research, the large number of candidate peaks may be related to other mechanisms such as global pressure variations, or hydrology.

Ocean tidal effects on Earth rotation

Abstract Tidal forces due to the tide-raising potential deform the solid and fluid regions of the Earth, causing the Earth's inertia tensor to change, and hence causing the Earth's rate of rotation and length-of-day to change. Because both the tide-raising potential and the solid Earth's elastic response to the tidal forces caused by this potential are well-known, accurate models for the effects of the elastic solid body tides on the Earth's rotation are available. However, models for the effect of the ocean tides on the Earth's rotation are more problematic because of the need to model the dynamic response of the oceans to the tidal forces. Hydrodynamic ocean tide models that have recently become available are evaluated here for their ability to account for long-period ocean tidal signals in length-of-day observations. Of the models tested here, the older altimetric data-constrained model of Kantha et al. (1998) is shown to still do the best job of accounting for ocean tidal effects in length-of-day, particularly at the fortnightly tidal frequency. The model currently recommended by the IERS is shown to do the worst job.