T. F. Baker

Sea level drop in the Mediterranean Sea: An indicator of deep water salinity and temperature changes?

Coastal sea level data from seven tide gauges in the Western Mediterranean and the Adriatic show decreasing sea levels after 1960. Control stations in the Black Sea and in the northeastern Atlantic indicate sea levels still rising after 1960. The sea level trend in the Mediterranean before 1960 was between 1.2 and 1.5 mm yr−1, while in the Atlantic and the Black Sea stations it was between 1.8 and 2.2 mm yr−1. After 1960 the sea level in the Mediterranean is decreasing with rates up to −1.3 mm yr−1, while in the Black Sea the sea level trend remains unaltered and at the Atlantic stations sea level keeps rising with reduced rates of 1.0–1.2 mm yr−1. The change of the Mediterranean sea level trends, which is in excess of the sea level trend reduction at the Atlantic sites, is consistent with increases in temperature and salinity of the Mediterranean Deep Water. The reduction of sea level trends at the Atlantic sites is probably related to the North Atlantic Oscillation.

Using continuous GPS and absolute gravity to separate vertical land movements and changes in sea-level at tide-gauges in the UK

Researchers investigating climate change have used historical tide-gauge measurements from all over the world to investigate the changes in sea-level that have occurred over the last century or so. However, such estimates are a combination of any true sea-level variations and any vertical movements of the land at the specific tide-gauge. For a tide- gauge record to be used to determine the climate related component of changes in sea-level, it is therefore necessary to correct for the vertical land movement component of the observed change in sea-level. In 1990, the Institute of Engineering Surveying and Space Geodesy and Proudman Oceanographic Laboratory started developing techniques based on the Global Positioning System (GPS) for measuring vertical land movements (VLM) at tide-gauges in the UK. This paper provides brief details of these early developments and shows how they led to the establishment of continuous GPS (CGPS) stations at a number of tide-gauges. The paper then goes on to discuss the use of absolute gravity (AG), as an independent technique for measuring VLM at tide-gauges. The most recent results, from CGPS time-series dating back to 1997 and AG time-series dating back to 1995/1996, are then used to demonstrate the complementarity of these two techniques and their potential for providing site-specific estimates of VLM at tide-gauges in the UK.

Absolute gravity measurements at UK tide gauges

Absolute gravity near 3 of the United Kingdoms (UK) core tide gauges was repeatedly measured using an FG5 gravimeter over a period of 3–4 years to determine vertical land movements at those gauges. The absolute gravity sites were established at Newlyn and Aberdeen in 1995 and in Lerwick in 1996. Assuming a height change of 1 mm causes a change in gravity of 0.2 µgal we see vertical land movements of 1.0±1.4 mm/yr at Newlyn, −3.8±1.6 mm/yr at Lerwick and 0.9±3.1 mm/yr at Aberdeen. These land movements are, within the error estimates, in agreement with land movement predicted by a model of post-glacial rebound/subsidence. To ensure that our absolute gravimeter is giving accurate results it has been regularly intercompared with other absolute gravimeters and was verified to be in agreement at the 2 µgal level.

Ocean tide loading displacements in western Europe: 2. GPS‐observed anelastic dispersion in the asthenosphere

GPS-observed vertical ocean tide loading displacements show in Cornwall, southwest England, and in Brittany, northwest France, discrepancies of 2–3 mm with predicted values based on the isotropic Preliminary Reference Earth Model for the main tidal harmonic M2, yet in central Europe the agreement is better than 0.5 mm. By comparison of ocean tide models and validation with tide gauge observations, we demonstrate that the uncertainties in the former are too small to cause this disagreement. Furthermore, we find that different local models of the crust and different global elastic reference models derived from seismological observations can only reduce the observed discrepancies to 1–2 mm, which still exceeds the GPS observational uncertainty of 0.2–0.4 mm. It is customary to use the elastic properties of the Earth as given by seismic models. Previously, there has been insufficient evidence to determine how to modify these properties during the transformation from seismic to tidal frequencies to account for possible anelastic dispersion in the asthenosphere, and so this effect has been ignored. If we include this effect, then our discrepancies reduce further to 0.2–0.4 mm. This value is of the same order as the sum of the remaining errors due to uncertainties in the ocean tide models and in the GPS observations themselves. This research provides evidence in western Europe of a reduction of around 8–10% of the seismic shear modulus in the asthenosphere at tidal frequencies. In addition, we find that the asthenosphere absorption band frequencies can be represented by a constant quality factor Q.

Monitoring changes in mean-sea-level to millimeters using GPS

The fiducial GPS technique is now well established in geodesy and geophysics, for determining horizontal and vertical land movements. In particular, the technique is used in crustal dynamics and in monitoring changes in mean-sea-level. The main limiting factor to achieving accuracies of a few millimeters is the reference frame (or datum), as defined by the adopted coordinates of the fiducial stations. This paper summarises a series of tests which have been carried out to assess the quality of alternative global reference frameworks, in order to achieve the highest accuracies for tide gauge heights in the UK.

Using GPS to Separate Crustal Movements and Sea Level Changes at Tide Gauges in the UK

Changes in mean sea level recorded by tide gauges are corrupted by crustal movements, which can be of a similar order of magnitude. A network of permanent, continuous GPS (CGPS) stations has been established in the UK with five stations being sited at tide gauges. Data from these and four other CGPS stations have been analysed. A common mode filtering technique was successfully applied in order to reduce the effect of annually repeating signals on station velocity estimates. The effect of time-dependent correlations in the coordinate time series were accounted for when computing station velocity uncertainties.

Ocean tide loading displacements in western Europe: 1. Validation of kinematic GPS estimates

GPS has been extensively used to estimate tidal ground displacements, but the accuracy of this has not been systematically verified. Using more than 20 sites distributed across western Europe, we show that postprocessed kinematic precise point positioning GPS with appropriately tuned process noise constraints is capable of recovering synthetic tidal displacements inserted into real data, with a typical accuracy of 0.2 mm depending on the time series noise. The kinematic method does not result in erroneous propagation of signals from one coordinate component to another or to the simultaneously estimated tropospheric delay parameters. It is robust to the likely effects of day-to-day equipment and reference frame changes, and to outages in the data. A minimum data span of 4 years with at least 70% availability is recommended. Finally, we show that the method of reducing apparent coordinate time series noise by constraining the tropospheric delay to values previously estimated in static batch GPS analysis, in fact, results in the suppression of true tidal signals. Using our kinematic GPS analysis approach, periodic displacements can be reliably observed at the 0.2 mm level, which is suitable for the testing and refinement of ocean tide and solid Earth response models.

Application of the dual-CGPS concept to monitoring vertical land movements at tide gauges ☆

Abstract Recently the concept of using dual-continuous GPS (dual-CGPS) stations for monitoring vertical land movements at tide gauge sites was proposed by a working group of the European Sea Level Observing System (EOSS). In this concept, one CGPS station is established at the tide gauge, in order to monitor the local vertical land movements, and a second CGPS station is established on stable rock, within a few kilometers of the tide gauge further inland. Since 1997, a network of CGPS stations has been established to monitor vertical land movements in the UK. For the dual-CGPS station concept, pairs formed by the CGPS stations at Newlyn tide gauge and Camborne and at Lowestoft tide gauge and Hemsby, have been analyzed over a time span of approximately 2.7 and 2.0 years respectively. It has been shown by numerous authors that temporal and spatial correlations in CGPS coordinate time series can introduce biases in the estimated station velocities and their uncertainties. By analyzing the coordinate time series of two CGPS stations close to each other, spatial correlations can be removed successfully by differencing and a cleaner, difference time series for each coordinate component can be obtained. In this paper, the observed coordinate time series of four CGPS stations have been used to produce synthetic coordinate time series of 6 years in length. Station velocities and their uncertainties have been derived for both observed and synthetic coordinate time series and compared in order to assess the capabilities of the dual-CGPS concept to better describe local and geophysical vertical land movements.

Combined Use of Altimetry and In Situ Gravity Data for Coastal Dynamics Studies

Accurate local geoids derived from in situ gravity data will be valuable in the validation of GOCE results. In addition it will be a challenge to use GOCE data in an optimal way, in combination with in situ gravity, to produce better local geoid solutions. This paper discusses the derivation of a new geoid over the NW European shelf, and its comparison with both tide gauge and altimetric sea level data, and with data from ocean models. It is hoped that over the next few years local geoid methods such as these can be extended to cover larger areas and to incorporate both in situ and satellite measured gravity data.

Using a ‘GPS/MSL Geoid’ to Test Geoid Models in the UK

GPS has been used to compute the height of mean sea level (MSL) above a reference ellipsoid at a network of 14 tide gauge sites, several hundred kilometres apart in the UK. The resultant ‘GPS/MSL geoid’ heights have then been compared to values taken from a global, an European and a national geoid model. Through these comparisons an assessment of the vertical datum offsets, and of the magnitude of any long wavelength errors in the geoid models, has been carried out.