Description: Sea-level variability as observed by satellite altimetry reflects the integral effect of various dynamic processes in the oceans including the response to atmospheric loading, density changes in the water column as well as re- distribution of ocean mass. Since the first two effects are reasonably estimated and removed from the data, altimetric observations of the remaining mass induced height changes can be used to validate direct measurements of ocean mass variations as obtained with the satellite gravimetry mission GRACE. In order to assess the accuracy of various GRACE products, recently released gravity field solutions from, e.g., GFZ Potsdam, CSR Austin, JPL Pasadena and the Universities of Bonn and Delft will be contrasted both against sterically corrected Jason 1 observations and mass anomalies simulated with the numerical ocean model OMCT. Focussing on the time period 2003 until 2007, regional distribution, intensity, and shape of mass variabilities as well as corresponding barotropic current anomalies are used to assess the accuracy and reliability of the latest GRACE results over the oceans separately for individual monthly solutions. The cross- comparisons will additionally allow for first order error estimates of the data sets involved, which are necessary to know for using these data in any type of inversion experiment.

Description: In the last years the application of high-quality terrestrial gravity observations to studies of mass transport has become more and more a focus of interest. This goes hand in hand with efforts to enhance the signal-to-noise ratios in the time series by more comprehensive and sophisticated reductions for geodynamic studies. In addition a precondition for a consistent combination of terrestrial and satellite-derived gravity field variations comprehends not only to apply reductions to the data sets as good as possible, but also to ensure that reductions for the same influences either are applied or evidence is provided that the respective effect is negligible. One of the effects which thus moved into focus encompasses non-tidal mass shifts in the oceans. A major benefit of including this effect in the reductions is the appropriate response of the oceans with respect to changes in atmospheric loading, when sea surface pressure anomalies are applied as additional forcing in the ocean simulations, redundantising assumptions with regard to inverted or non-inverted barometer response. For example from a study by Fratepietro et al. (2006) a significant influence of storm surge-related effects in the North Sea on gravity even for inland sites emerged. Based on the OMCT and the ECCO model the gravity effect of non-tidal oceanic mass shifts is computed for various sites equipped with a superconducting gravimeter (SG) esp. with a view on seasonal variations. A five year-long period covering the years 2002 through 2006 is considered. The results so far are ambiguous: The systematic seasonal change of about 10 nm/s2 peak-to-peak in gravity found for mid-European stations is presently not found in the observed gravity variations. Generally, the order of magnitude of the total effect of 22 to 27 nm/s2 peak-to-peak is quite large for stations at a distance of some 100 km from the coast. In some data sections an agreement between observed and modelled gravity variation can be found which then results in the removal of larger residuals. For the South-African station Sutherland a different result is obtained. Here the seasonal variation caused by the non-tidal oceanic mass shifts and gravity residuals correlate. In this instance the application of the additional reduction leads to an overall substantial improvement of the signal-to-noise ratio in the gravity observations. One explanation for the different results might be found in the principle accuracy of the global continental hydrological models. Such a model is needed in order to remove the effect of large-scale variations in continental water storage in the gravity observations in order to obtain residuals which contain mainly variations related to the non-tidal oceanic mass shifts. This reduction plays a greater role for European stations than for the South African site. A possible critical impact of the land-sea-mask of the oceanic models resp. the negligence of the shelf areas could not be confirmed. The OMCT and the ECCO model have with 1.875° and 1° a different spatial resolution. The incorporation of a regional high resolution model (5’ x 3’, resp. 50” x 30”) for the North and Baltic Sea from the BSH (Bundesamt für Seeschifffahrt und Hydrographie) does not change the principle order of magnitude of the seasonal effect. Thus, there are presently strong indications that the order of magnitude of the long-period contribution is real. This means the influence of non-tidal mass shifts in the oceans should not only be considered for studies in the short periods, but also in the long-period spectral range.

Description: An increase in accuracy of the Earth’s magnetic field observations by the future SWARM satellite mission strengths the interest of dealing with magnetic signals of small amplitudes but a global support. An example of such a signal is the magnetic field variation induced by global ocean dynamics. Since sea-water is a good electrical conductor, the ocean currents represent electrical currents moving in the main magnetic field. According to the Faraday’s law, they induce a secondary magnetic field that is, in principle, observable by ground and satellite observations. Although they are of small amplitudes, they have recently been identified for ocean currents forced by the tidal wave M2. The identification of small magnetic components in the total magnetic signal can be helpt by their numerical prediction. The 2-D theory for computing the magnetic field generated by ocean currents that has been proposed by Tyler et al. (1997) is combined with the ocean tidal flows simulated by the numerical ocean model for circulation and tides (OMCT; Thomas 2002) and used to calculate secondary magnetic field generated by lunar-solar tidal potential for individual tidal waves. Unlike to some recent studies where the ocean currents have been deduced from altimetry data by applying the geostrophic method, the ocean flows calculated by the OMCT approach are forced directly by the lunisolar tidal potential that is deduced from analytical ephemerides. As a result, the method provides the radial component of the ocean-induced magnetic signal at the sea surface and satellite altitude. A comparison with published results by Tyler et al. (2003) and Maus et al. (2004) shows a good agreement in terms of global spatial pattern and magnitude, though some minor differences occur. This new method simplifies the calculation of ocean-induced magnetic fields and allows the prediction of the secondary magnetic fields induced by the complete lunisolar tidal forcing. This numerical approach can be used to estimate the opportunities in detecting ocean-induced magnetic signals in satellite observations.

Description: Monthly mean gravity fields solely based on GRACE satellite tracking data are found to provide reliable ocean mass anomalies down to 500 km regional averages when comparing them to mass observations obtained from sterically corrected Jason 1 altimetry and simulated mass anomalies derived from the Ocean Model for Circulation and Tides (OMCT). Beside the assessment of systematic shortcomings of GRACE, Jason 1 and OMCT estimates, robust signals of mass anomalies in the North Pacific and in various regions of the Southern Ocean are identified in all three independent datasets. Correlations of up to 0.8 and rms values of differences of around 2 hPa indicate that uncertainties are well below the expected monthly mean mass signals of up to 6 hPa rms in these regions. By means of output of the numerical ocean model, mass anomalies are related to changes in barotropic ocean currents, providing in turn the opportunity to infer barotropic current anomalies from GRACE observations, and therefore principally allowing to monitor climate relevant changes of ocean currents from satellite observations.

Description: Die dynamischen Prozesse im Ozean haben aufgrund der hohen Wärmekapazität von Wasser einen herausragenden Einfluss auf die klimatischen Verhältnisse auf der Erde. Für die Untersuchungen der komplexen Wechselwirkungen mit der Atmosphäre und anderen klimatisch relevanten Teilsystemen wie der kontinentalen Hydrosphäre werden numerische Klimamodelle verwendet. Eine realistische Einschätzung der Resultate dieser Klimamodelle ist allerdings nur nach umfangreicher Validierung mit einer Vielzahl von unterschiedlichen Beobachtungsdaten möglich, zu der unter anderem auch geodätische Messverfahren wertvolle Beiträge liefern können. Unter Verwendung von Topex/Poseidon Altimetermessungen wurden beispielsweise von Kuragano und Kamachi (J. Geophys. Res., 2000) charakteristische Kennzahlen über die raum-zeitliche Variabilität der Meeresoberfläche veröffentlicht. Im Rahmen einer Studienarbeit im Fachgebiet Geodäsie an der Technischen Universität Dresden waren vergleichbare Werte aus simulierten Meereshöhen des Ozeanmodells für Zirkulation und Gezeiten (OMCT) zu bestimmen und mit den altimetrie-basierten Kennzahlen zu vergleichen. Dazu waren für jeden Datenpunkt des numerischen Modellgitters Autokorrelationskoeffizienten in zwei Raum- und einer Zeitkoordinate zu berechnen. Anschließend wurde für jede dieser Punktwolken ein im Sinne der kleinsten Quadrate bestanpassendes dreiachsiges Ellipsoid bestimmt, aus dessen sechs Parametern die entsprechenden Kennzahlen zu zweidimensionalen räumlichen Korrelationslängen, zeitlicher Korrelation und Advektion von Meereoberflächensignalen abgeleitet werden konnten. Dargestellt werden neben einer ersten Einschätzung der Realitätstreue der vorliegenden OMCTSimulationen vor allem die zu lösenden Probleme bei der iterativen Ausgleichung des nichtlinearen Gleichungssystems. Die Wahl des Iterationsverfahrens, der zu verwendenden Näherungswerte und das Festsetzen des Abbruchkriteriums hatten dabei entscheidenden Einfluss auf die Dauer und Anzahl der Iterationsschritte und letztlich den Erfolg der Untersuchungen.

Description: The advent and successive accuracy improvement of satellite altimetry as well as gravimetry missions have greatly enhanced our knowledge about the gravity field over the oceans. However, considerable efforts are still required in order to consistently combine both observation techniques as well as to properly account for the time-variations caused by the strong dynamics of the open oceans. In order to emphasize the importance of various physical processes, their impact on satellite-based observations is reviewed from an oceanographic perspective. In particular, gravimetry measurements allow the derivation of the geoid as an equipotential surface of the gravity field, while satellite altimetry basically measures the geometric surface of the oceans, which deviates from the geoid due to ocean currents. A consistent combination of both observation types requires the careful consideration of this so called mean dynamic topography based on independent in-situ observations or numerical ocean models. Moreover, both surfaces are affected by considerable variations in time. While barotropic motions associated with ocean tides and atmospheric loading have similar effects in both the surface and the geoid, additional surface deformations have to be accounted for due to steric compensations of density changes within the water column. Considering the limited resolution in time and space and the orbit configuration preferred for gravity field prospecting, these time-variable effects cannot be resolved from the measurements itself and have to be corrected for by independent methods as, e.g., numerical model approaches.