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Effect of unmodelled tidal displacements in GPS and GLONASS coordinate time series Abraha, Kibrom Ebuy ; Teferle, Felix Norman ; Hunegnaw, Addisu et al in Geophysical Journal International (2018), 214(3), 2195-2206 This study demonstrates the different effects of unmodelled (sub-)daily tidal displacement in Global Positioning System (GPS) and GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) coordinate time ... [more ▼] This study demonstrates the different effects of unmodelled (sub-)daily tidal displacement in Global Positioning System (GPS) and GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) coordinate time-series. The results show that more than two propagated periodic signals appear in GPS and GLONASS Precise Point Positioning (PPP) coordinate time-series in the presence of an unmodelled M2 /O1 tidal displacements as a result of a non-overlapping 24-hr data sampling. To summarize the propagated periodic signals at the fortnightly period, an unmodelled M2 tidal displacement propagates predominately into two long-period signals at 13.6x (x is a positive integer) and 14.76 d for GPS, while only one significant propagated periodic signal at 14.76 d is discernible for GLONASS. Similarly, significant propagated periodic signals at 13.6x and 14.19 d for GPS and only at 14.19 d for GLONASS are evident as a result of an unmodelled O1 tidal displacement. However, an unmodelled M f (long- period) signal results in a strong power of similar magnitude at 13.6x d (∼13.66 d) for both GPS and GLONASS solutions. The appearance of different periodic signals as a result of the same unmodelled tidal displacement is attributed to the different ground repeat periods of the constellations. The latter is likely to explain the reason why the 13.6x-d fortnightly signal is present only in GPS solutions. Comparing the powers of the M2 propagated periodic signals at 13.6x and 14.76 d on average from 32 globally distributed stations, the amplitude of the former is larger than for the latter by an order of magnitude. The results of this study demonstrate that the 13.6x-d periodic signal in GPS/GNSS (Global Navigation Satellite System) derived products is a joint contribution of the propagation of unmodelled (sub-)daily tidal displacements and errors at longer periods with the former appearing to contribute more. Significant reduction of the propagated periodic signals is achieved from combined-system solutions where including Galileo (the European GNSS) to the combined solution already shows benefits by reducing the effect even before the system has reached its full constellation. Combined GNSS solutions will benefit the applications of GNSS time-series for retrieving tidal harmonic signals such as Mf as they reduce constellation specific propagation effects. [less ▲] Detailed reference viewed: 270 (27 UL)GRACE era variability in the Earth’s oblateness: A comparison of estimates from six different sources Meyrath, Thierry ; ; van Dam, Tonie in Geophysical Journal International (2017), 208(2), 1126-1138 We study fluctuations in the degree-2 zonal spherical harmonic coefficient of the Earth's gravity potential, $C_{20}$, over the period 2003-2015. This coefficient is related to the Earth's oblateness and ... [more ▼] We study fluctuations in the degree-2 zonal spherical harmonic coefficient of the Earth's gravity potential, $C_{20}$, over the period 2003-2015. This coefficient is related to the Earth's oblateness and studying its temporal variations, $\Delta C_{20}$, can be used to monitor large-scale mass movements between high and low latitude regions. We examine $\Delta C_{20}$ inferred from six different sources, including satellite laser ranging (SLR), GRACE and global geophysical fluids models. We further include estimates that we derive from measured variations in the length-of-day (LOD), from the inversion of global crustal displacements as measured by GPS, as well as from the combination of GRACE and the output of an ocean model as described by \cite{sunetal2016}. We apply a sequence of trend- and seasonal moving average filters to the different time series in order to decompose them into an interannual, a seasonal and an intraseasonal component. We then perform a comparison analysis for each component, and we further estimate the noise level contained in the different series using an extended version of the three-cornered-hat method. For the seasonal component, we generally obtain a very good agreement between the different sources, and except for the LOD-derived series, we find that over 90\% of the variance in the seasonal components can be explained by the sum of an annual and semiannual oscillation of constant amplitudes and phases, indicating that the seasonal pattern is stable over the considered time period. High consistency between the different estimates is also observed for the intraseasonal component, except for the solution from GRACE, which is known to be affected by a strong tide-like alias with a period of about 161 days. Estimated interannual components from the different sources are generally in agreement with each other, although estimates from GRACE and LOD present some discrepancies. Slight deviations are further observed for the estimate from the geophysical models, likely to be related to the omission of polar ice and groundwater changes in the model combination we use. On the other hand, these processes do not seem to play an important role at seasonal and shorter time scales, as the sum of modelled atmospheric, oceanic and hydrological effects effectively explains the observed $C_{20}$ variations at those scales. We generally obtain very good results for the solution from SLR, and we confirm that this well-established technique accurately tracks changes in $C_{20}$. Good agreement is further observed for the estimate from the GPS inversion, showing that this indirect method is successful in capturing fluctuations in $C_{20}$ on scales ranging from intra- to interannual. Obtaining accurate estimates from LOD, however, remains a challenging task and more reliable models of atmospheric wind fields are needed in order to obtain high-quality $\Delta C_{20}$, in particular at the seasonal scale. The combination of GRACE data and the output of an ocean model appears to be a promising approach, particularly since corresponding $\Delta C_{20}$ is not affected by tide-like aliases, and generally gives better results than the solution from GRACE, which still seems to be of rather poor quality. [less ▲] Detailed reference viewed: 298 (23 UL)GNSS related periodic signals in coordinate time-series from Precise Point Positioning Abraha, Kibrom Ebuy ; Teferle, Felix Norman ; Hunegnaw, Addisu et al in Geophysical Journal International (2016) In Global Navigation Satellite System (GNSS) coordinate time series unrecognised errors and un-modelled (periodic) effects may bias non-linear motions induced by geophysical signals. Hence, understanding ... [more ▼] In Global Navigation Satellite System (GNSS) coordinate time series unrecognised errors and un-modelled (periodic) effects may bias non-linear motions induced by geophysical signals. Hence, understanding and mitigating these errors is vital to reducing biases and on revealing subtle geophysical signals. To assess the nature of periodic signals in coordinate time series Precise Point Positioning (PPP) solutions for the period 2008 to 2015 are generated. The solu- tions consider Global Positioning System (GPS), GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) or combined GPS+GLONASS (GNSS) observations. We assess the pe- riodic signals of station coordinates computed using the combined International GNSS Service (IGS) and four of its Analysis Centers (ACs) products. Furthermore, we make use of different filtering methods to investigate the sources of the periodic signals. A faint fortnightly signal in our PPP solution based on Jet Propulsion Laboratory (JPL) products and the existence of an 8-day period for those ACs generating combined GPS+GLONASS products are the main features in the GPS-only solutions. The existence of the 8-day period in the GPS-only solution indicates that GPS orbits computed in a combined GNSS solution contain GLONASS-specific signals. The GLONASS-only solution shows highly elevated powers at the 3rd draconitic harmonic ( ~ 120-day period), at the 8-day period and its harmonics (4 days, 2.67 days) besides the well-known annual, semi-annual and other draconitic harmonics. We show that the GLONASS constellation gaps before December 2011 contribute to the power at some of the frequencies. However, the well known fortnightly signal in GPS-only solutions is not discernible in the GLONASS-only solution. The combined GNSS solution contains periodic signals from both systems, with most of the powers being reduced when compared to the single-GNSS solutions. A 52% reduction for the horizontal components and a 36% reduction for the vertical compo- nent are achieved for the fortnightly signal from the GNSS solution compared to the GPS-only solution. Comparing the results of the employed filtering methods reveals that the source of most of the powers of draconitic and fortnightly signals are satellite-induced with a non-zero contribution of site-specific errors. [less ▲] Detailed reference viewed: 308 (30 UL)Temporal variation of tidal parameters in superconducting gravimeter time-series ; ; Francis, Olivier et al in Geophysical Journal International (2016), 205(1), 284-300 Analysing independent 1-yr data sets of 10 European superconducting gravimeters (SG) reveals statistically significant temporal variations of M2 tidal parameters. Both common short-term (<2 yr) and long ... [more ▼] Analysing independent 1-yr data sets of 10 European superconducting gravimeters (SG) reveals statistically significant temporal variations of M2 tidal parameters. Both common short-term (<2 yr) and long-term (>2 yr) features are identified in all SG time-series but one. The averaged variations of the amplitude factor are about 0.2‰. The path of load vector variations equivalent to the temporal changes of tidal parameters suggests the presence of an 8.85 yr modulation (lunar perigee). The tidal waves having the potential to modulate M2 with this period belong to the 3rd degree constituents. Their amplitude factors turn out to be much closer to body tide model predictions than that of the main 2nd degree M2, which indicates ocean loading for 3rd degree waves to be less prominent than for 2nd degree waves within the M2 group. These two different responses to the loading suggest that the observed modulation is more due to insufficient frequency resolution of limited time-series rather than to time variable loading. Presently, SG gravity time-series are still too short to prove if time variable loading processes are involved too as in case of the annual M2 modulation known to appear for analysis intervals of less than 1 yr. Whatever the variations are caused by, they provide the upper accuracy limit for earth model validation and permit estimating the temporal stability of SG scale factors and assessing the quality of gravity time-series. [less ▲] Detailed reference viewed: 161 (12 UL)Reply to Comment on: ‘The quest for a consistent signal in ground and GRACE gravity time series’ ; ; et al in Geophysical Journal International (2014), 199(3), 1818-1822 Detailed reference viewed: 238 (4 UL)The quest for a consistent signal in ground and GRACE gravity time series ; ; et al in Geophysical Journal International (2014), 197 Detailed reference viewed: 151 (15 UL)An assessment of degree-2 Stokes coefficients from Earth rotation data Meyrath, Thierry ; van Dam, Tonie ; Weigelt, Matthias et al in Geophysical Journal International (2013), 195((1)), 249-259 Variations in the degree-2 Stokes coefficients C20, C21 and S21 can be used to understand long and short-term climate forcing. Here, we derive changes in these coefficients for the period 2003 ... [more ▼] Variations in the degree-2 Stokes coefficients C20, C21 and S21 can be used to understand long and short-term climate forcing. Here, we derive changes in these coefficients for the period 2003 January–2012 April using Earth rotation data. Earth rotation data contain contributions from motion terms (the effects of winds and currents) and contributions from the effects of mass redistribution. We remove the effects of tides, atmospheric winds and oceanic currents from our data. We compare two different models of atmospheric and oceanic angular momentum for removing the effects of winds and currents: (1) using products from the National Centers for Environmental Prediction and (2) using data from the European Centre for Medium-range Weather Forecasts (ECMWF). We assess the quality of these motion models by comparing the two resulting sets of degree-2 Stokes coefficients to independent degree-2 estimates from satellite laser ranging (SLR), GRACE and a geophysical loading model. We find a good agreement between the coefficients from Earth rotation and the coefficients from other sources. In general, the agreement is better for the coefficients we obtain by removing winds and currents effects using the ECMWF model. In this case, we find higher correlations with the independent models and smaller scatters in differences. This fact holds in particular for C20 and C21, whereas we cannot observe a significant difference for S21. At the annual and semiannual periods, our Earth rotation derived coefficients agree well with the estimates from the other sources, particularly for C21 and S21. The slight discrepancies we obtain for C20 can probably be explained by errors in the atmospheric models and are most likely the result of an over-/underestimation of the annual and semiannual contributions of atmospheric winds to the length-of-day excitation. [less ▲] Detailed reference viewed: 188 (13 UL)Detecting storm surge loading deformations around the southern North Sea using subdaily GPS ; ; Teferle, Felix Norman et al in Geophysical Journal International (2012), 191(2), 569-578 A large storm surge event occurred on 2007 November 2009 in the southern North Sea where strong winds caused the sea level to rise drastically by up to 3 m within several hours. Based on the Proudman ... [more ▼] A large storm surge event occurred on 2007 November 2009 in the southern North Sea where strong winds caused the sea level to rise drastically by up to 3 m within several hours. Based on the Proudman Oceanographic Laboratory storm surge model, the predicted loading displacements at coastal stations can reach a few centimetres in the vertical and several millimetres in the horizontal directions. In this study, we used two-hourly global positioning system (GPS) positions at 26 stations around the southern North Sea to identify the loading displacements caused by this storm surge event. We find that the mean rms of the differences between the estimated and predicted displacements are 4.9, 1.3 and 1.4 mm, which are insignificant compared to the one-sigma GPS positioning errors of 5.1, 2.0 and 2.4 mm for the Up, East and North components, respectively. More interestingly, in both vertical and horizontal directions, the estimated displacements successfully tracked the temporal evolution of the storm surge loading effects. In addition, within the whole of 2007 November, we used the predicted displacements to correct the two-hourly GPS positions, and consequently reduced the rms of the estimated displacements on average from 9.3, 3.0 and 2.9 mm to 7.8, 2.8 and 2.8 mm for Up, East and North components, respectively. Therefore, subdaily loading effects due to storm surges should be paid attention to in the GPS positioning that contributes to crustal-motion studies around shallow seas such as the North Sea, the Baltic Sea and the Gulf of Mexico. [less ▲] Detailed reference viewed: 157 (1 UL)Spaceborne Gravimetric Satellite Constellations and Ocean Tides: aliasing effects ; ; et al in Geophysical Journal International (2010), (181), 789-805 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 ... [more ▼] 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. [less ▲] Detailed reference viewed: 112 (2 UL)Glacial Isostatic Adjustment of the British Isles: New constraints form GPS measurements of crustal motion ; ; Teferle, Felix Norman et al in Geophysical Journal International (2009), 178(178), 14-22 We compared estimates of crustal velocities within Great Britain based on continuous global positioning system (CGPS) measurements to predictions from a model of glacial isostatic adjustment (GIA). The ... [more ▼] We compared estimates of crustal velocities within Great Britain based on continuous global positioning system (CGPS) measurements to predictions from a model of glacial isostatic adjustment (GIA). The observed and predicted values for vertical motion are highly correlated indicating that GIA is the dominant geodynamic process contributing to this field. In contrast, motion of the Eurasian plate dominates the horizontal motion component. A model of plate motion was adopted to remove this signal in order to estimate intraplate horizontal motion associated with GIA. However, a coherent pattern of horizontal motion was not evident in the resulting velocity field. We adopted a recently published model of the British–Irish ice sheet to predict vertical crustal motion for a large number of spherically symmetric Earth viscosity models. Our results show that the adopted ice model is capable of producing a high-quality fit to the observations. The CGPS-derived estimates of vertical motion provide a useful constraint on the average value of viscosity within the upper mantle. Values of model lithospheric thickness and lower mantle viscosity are less well resolved, however. A suite of predictions based on an alternative ice model indicates that the vertical motion data are relatively insensitive to uncertainties in the ice loading history and so the constraints on upper mantle viscosity are robust. [less ▲] Detailed reference viewed: 95 (4 UL)Trends in UK Mean Sea Level revisited ; Teferle, Felix Norman ; et al in Geophysical Journal International (2009), 179(179), 19-30 This paper presents estimates of rates of mean sea level (MSL) change around the UK based on a larger tide gauge data set and more accurate analysis methods than have been employed so far. The spatial ... [more ▼] This paper presents estimates of rates of mean sea level (MSL) change around the UK based on a larger tide gauge data set and more accurate analysis methods than have been employed so far. The spatial variation of the trend in MSL is found to be similar to that inferred from geological information and from advanced geodetic techniques, which is a similar conclusion to that arrived at in previous, less precise and complete studies. The tide gauge MSL trends for 1901 onwards are estimated to be 1.4 +/- 0.2 mm/year larger than those inferred from geology or geodetic methods, suggesting a regional sea level rise of climate change origin several 1/10s mm/year lower than global estimates for the 20th century. However, UK MSL change cannot be described in terms of a simple linear increase alone but includes variations on interannual and decadal timescales. The possible sources of variation in a ‘UK sea level index’ are explored. Air pressure is clearly one such possible source but its direct local forcing through the ‘inverse barometer’ accounts for only one third of the observed variability. A number of larger scale atmospheric and ocean processes must also play important roles, but modelling them satisfactorily and separating the individual contributions presents a major challenge. As regards future regional UK sea level changes, one concludes that there is no basis for major modification to existing projections for the 2080s included in the 2002 UK Climate Impacts Programme studies. [less ▲] Detailed reference viewed: 83 (3 UL)Crustal Motions in Great-Britain: Evidence from continuous GPS, Absolute Gravity and Holocene Sea-Level Data Teferle, Felix Norman ; ; et al in Geophysical Journal International (2009), 178(1), 23-46 Two independent continuous global positioning system (CGPS) processing strategies, based on a double-difference regional network and a globally transformed precise point positioning solution, provide ... [more ▼] Two independent continuous global positioning system (CGPS) processing strategies, based on a double-difference regional network and a globally transformed precise point positioning solution, provide horizontal and vertical crustal motion estimates for Great Britain. Absolute gravity and geological information from late Holocene sea level data further constrain the vertical motion estimates. For 40 CGPS stations we estimate station velocities and associated uncertainties using maximum likelihood estimation, assuming the presence of white and coloured noise. Horizontal station velocity estimates agree to <1 mm yr−1 between the two CGPS processing strategies and closely follow predicted plate motions. Residual velocities, generally <1 mm yr−1, follow no regular pattern, that is, there is no discernible internal deformation, nor any dependence on station monumentation or time-series length. Vertical station velocity estimates for the two CGPS processing strategies agree to ∼1 mm yr−1, but show an offset of ∼1 mm yr−1 with respect to the absolute gravity (AG) estimates. We attribute this offset to a bias related to known issues in current CGPS results and correct for it by AG-alignment of our CGPS estimates of vertical station velocity. Both CGPS estimates and AG-aligned CGPS estimates of present-day vertical crustal motions confirm the pattern of subsidence and uplift in Great Britain derived from Holocene sea level data for the last few thousand years: ongoing subsidence on Shetland, uplift in most areas of Scotland, and subsidence in large areas of England and Wales. [less ▲] Detailed reference viewed: 189 (11 UL)Basis functions for the consistent and accurate representation of surface mass loading ; ; et al in Geophysical Journal International (2007), 171(1), 1-10 Inversion of geodetic site displacement data to infer surface mass loads has previously been demonstrated using a spherical harmonic representation of the load. This method suffers from the continent-rich ... [more ▼] Inversion of geodetic site displacement data to infer surface mass loads has previously been demonstrated using a spherical harmonic representation of the load. This method suffers from the continent-rich, ocean-poor distribution of geodetic data, coupled with the predominance of the continental load (water storage and atmospheric pressure) compared with the ocean bottom pressure (including the inverse barometer response). Finer-scale inversion becomes unstable due to the rapidly increasing number of parameters which are poorly constrained by the data geometry. Several approaches have previously been tried to mitigate this, including the adoption of constraints over the oceanic domain derived from ocean circulation models, the use of smoothness constraints for the oceanic load, and the incorporation ofGRACEgravity field data. However, these methods do not provide appropriate treatment of mass conservation and of the ocean’s equilibrium-tide response to the total gravitational field. Instead,we propose a modified set of basis functions as an alternative to standard spherical harmonics. Our basis functions allow variability of the load over continental regions, but impose global mass conservation and equilibrium tidal behaviour of the oceans. We test our basis functions first for the efficiency of fitting to realistic modelled surface loads, and then for accuracy of the estimates of the inferred load compared with the known model load, using synthetic geodetic displacements with real GPS network geometry. Compared to standard spherical harmonics, our basis functions yield a better fit to the model loads over the period 1997–2005, for an equivalent number of parameters, and provide a more accurate and stable fit using the synthetic geodetic displacements. In particular, recovery of the low-degree coefficients is greatly improved. Using a nine-parameter fit we are able to model 58 per cent of the variance in the synthetic degree-1 zonal coefficient time-series, 38–41 per cent of the degree-1 non-zonal coefficients, and 80 per cent of the degree-2 zonal coefficient. An equivalent spherical harmonic estimate truncated at degree 2 is able to model the degree-1 zonal coefficient similarly (56 per cent of variance), but only models 59 per cent of the degree-2 zonal coefficient variance and is unable to model the degree-1 non-zonal coefficients. [less ▲] Detailed reference viewed: 93 (3 UL)Seasonal effect on vertical positioning by Satellite Laser Ranging and GPS on Absolute Gravity at the OCA geodetic station, Grasse, France ; ; et al in Geophysical Journal International (2006), 167(3), 1127-1137 We present a comparison of the vertical displacement monitored by independent techniques at the geodetic observatory of Grasse (France). Both Satellite Laser Ranging and Global Positioning System (GPS ... [more ▼] We present a comparison of the vertical displacement monitored by independent techniques at the geodetic observatory of Grasse (France). Both Satellite Laser Ranging and Global Positioning System (GPS) vertical position time-series over the period 1998–2003 show a prominent annual signal with a magnitude of 5–6 mm and reaching a maximum every year in July. Results from 14 absolute gravity measurements are also discussed. We investigate the possible origin of the observed signal by comparing it with predictions from various local and regional contributions. GPS results from a local network indicate that the periodic annual elastic deformation of the ∼1270 m high karstic plateau due to local water storage loading does not exceed 1–2 mm. In contrast, a combination of global model prediction for atmospheric and hydrological loading explains more than 70 per cent of the annual and semi-annual observed signals. [less ▲] Detailed reference viewed: 65 (0 UL)A geophysical interpretation of the secular displacement and gravity rates observed at Ny-Alesund, Svalbard in the Arctic - effects of the post-glacial rebound and present-day ice melting ; ; et al in Geophysical Journal International (2006), 165(3), 729-743 Detailed reference viewed: 94 (0 UL)Indication of the uplift of the Ardenne in long-term gravity variations in Membach (Belgium) Francis, Olivier ; ; van Dam, Tonie et al in Geophysical Journal International (2004), 158(1), 346-352 We report on the results of 7 yr of collocated gravity observations made with an FG5 abso- lute (AG) gravimeter and a GWR C-Series superconducting gravimeter (SG) located at the Membach Geophysical ... [more ▼] We report on the results of 7 yr of collocated gravity observations made with an FG5 abso- lute (AG) gravimeter and a GWR C-Series superconducting gravimeter (SG) located at the Membach Geophysical Station in eastern Belgium. The SG gravity residuals track changes in gravity periodically observed by the AG, at the microgal level. Further, in the SG resid- ual signal we distinguish a quasi-seasonal term that can be mostly explained by variations in local water storage effects. In the AG time-series we observe a small trend in the gravity of −0.6 ± 0.1 μGal yr−1 perhaps indicating that the Membach Station is being displaced up- wards by about 3.0 mm yr−1. An uplift of the region is confirmed by Global Positioning System (GPS) measurements performed 3 km away. We are able to explain the features in the gravity time-series in terms of water storage variability, post-glacial rebound and tectonic activity. [less ▲] Detailed reference viewed: 114 (3 UL)Predictions of crustal deformation and of geoid and sea level variability caused by oceanic at atmospheric loading van Dam, Tonie ; ; et al in Geophysical Journal International (1997), 129(3), 507-517 Detailed reference viewed: 75 (2 UL)Atmospheric load response of the oceans determined using Geosat data van Dam, Tonie ; in Geophysical Journal International (1993), 113(1), 1-16 Approximately one year's worth of altimeter-derived sea-surface heights are compared with global sea-level pressure fields to verify the open ocean inverted barometer response (-1 cm mb-1). When pressure ... [more ▼] Approximately one year's worth of altimeter-derived sea-surface heights are compared with global sea-level pressure fields to verify the open ocean inverted barometer response (-1 cm mb-1). When pressure is fit to the sea-surface height along individual altimeter tracks, the response is found to be only 60–70 per cent of the theoretical response or approximately -0.6 to -0.7 cm mb-1. Fits at fixed geographic locations show a clear dependence on latitude. There is a steady decrease in the absolute value o the regression coefficient between 70° and 20°, and then an abrupt increase again closer to the equator. A simple error analysis demonstrates that errors in the pressure data would reduce the along-track regression values, as is observed, and could produce a similar latitude dependence. But, the errors are unlikely to be large enough to explain the entire departure from inverted barometer. We estimate that pressure errors are apt to perturb the along-track track results by no more than about 0.1-0.2 cm mb-1. The possibility that the remaining disagreement is due to a global coherence between wind- and pressure-driven sea-surface height variability is considered. Winds driven by the pressure gradients of synoptic storms induce a sea-surface height response that is opposite in direction to that caused by the pressure cell. the wind-driven response is estimated for a stationary storm over a homogeneous barotropic ocean and for a moving storm over a two-layer baroclinic ocean by modeling the pressure cell as an idealized Gaussian distribution. the model results indicate that the wind-induced sea-surface height depends on both the radius and the translational velocity of the pressure cell. But, the winds associated with storms moving at average speeds of 10 ms-1 are apt to lower the theoretical pressure response in the model by only approximately 0.1 cm mb-1. the surface stress associated with those winds has the same latitudinal trend between 70° and 20° as the regression coefficients. But, the response of the ocean to that stress does not appear to exhibit the same trend. Nevertheless, the abrupt change in the regression coefficients near the equator suggests the apparent non-inverted barometer response may reflect a real change in sea-surface height related to atmospheric forcing (though the results near the equator are not as well defined as those at higher latitudes). [less ▲] Detailed reference viewed: 67 (3 UL) |
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