ALBERT

Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉This study addresses the evolution of global tidal dynamics since the Last Glacial Maximum focusing on the extraction of tidal levels that are vital for the interpretation of geologic sea‐level markers. For this purpose, we employ a truly‐global barotropic ocean tide model which considers the non‐local effect of Self‐Attraction and Loading. A comparison to a global tide gauge data set for modern conditions yields agreement levels of 65%–70%. As the chosen model is data‐unconstrained, and the considered dissipation mechanisms are well understood, it does not have to be re‐tuned for altered paleoceanographic conditions. In agreement with prior studies, we find that changes in bathymetry during glaciation and deglaciation do exert critical control over the modeling results with minor impact by ocean stratification and sea ice friction. Simulations of 4 major partial tides are repeated in time steps of 0.5–1 ka and augmented by 4 additional partial tides estimated via linear admittance. These are then used to derive time series from which the tidal levels are determined and provided as a global data set conforming to the HOLSEA format. The modeling results indicate a strengthened tidal resonance by M〈sub〉2〈/sub〉, but also by O〈sub〉1〈/sub〉, under glacial conditions, in accordance with prior studies. Especially, a number of prominent changes in local resonance conditions are identified, that impact the tidal levels up to several meters difference. Among other regions, resonant features are predicted for the North Atlantic, the South China Sea, and the Arctic Ocean.〈/p〉

Description: Plain Language Summary: We discuss changes in ocean tides during the last 21,000 years. This time marks the Last Glacial Maximum when large parts of the Earth's surface were covered by ice and the sea level was more than 100 m lower than today. Such a low sea level means that many regions of the Earth became land and the ocean's depth changed markedly. The distribution of land and water dominates changes in the tidal levels like the spring or neap tide. With a tidal computer model recently developed by our group, we determine these tidal levels for different times steps from 21,000 years to today. Tidal levels are important for geologists who want to understand former sea level changes with samples found at ancient shorelines. As many of such samples were deposited at a specific tidal level, our modeled information will help them to relate their height to the mean sea‐level. Of course, our model is not the only one that can estimate such changes, but we discuss the advantages of our recent development over previous tools available.〈/p〉

Description: Key Points: Evolution of four major partial tides from Last Glacial Maximum until present times.〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Validation of the employed ocean tide model with present‐day tide gauge data and dissipation rates.〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Diligent derivation of global tidal levels for the interpretation of sea level indexpoints.〈/p〉〈/list-item〉 〈/list〉 〈/p〉

Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Description: Sub-diurnal variations in Earth rotation parameters as obtained from time-series of space geodetic observations contain substantial variability even after correcting for the effects of oceanic tides. These residuals are in particular apparent at frequencies of 1, 2 and 3 cycles per solar day, where atmospheric tides, principally excited by water vapor absorption and ozone heating in the middle atmosphere, are known to occur. By means of hourly data of the chemistry-climate model WACCM, the potential of atmospheric tides on the excitation of UT1 variations is re-assessed. Tidal signals are separated into migrating and non-migrating zonal waves for individual height levels. Only standing waves of wavenumber zero are found to be effective in exciting UT1 variations, which are subsequently discussed in terms of their characteristic surface pressure and vertically varying wind amplitudes.

Description: Regional ocean models are extraordinarily useful tools as complements to global models, since they work at higher spatial and temporal resolutions and parameters can be adapted to the particular conditions in the region of interest. These advantages are bought with new potential issues at the boundaries of the modelled region. At open boundaries, a global model has to provide boundary conditions such as velocity, temperature, and salinity, necessarily obtained at coarser resolution and with less accuracy. The region we focus on is the surroundings of South Africa, comprising parts of the Southern Atlantic and Southern Indian Ocean as well as the Southern Ocean down to the ice shelves of Antarctica.We attempt to better understand the dynamics of the Agulhas Current, which has been shown to have far-reaching impacts also on the Meridional Overturning Circulation and, thereby, on the world’s climate. With our study region expanded southwards, including a fraction of the Antarctic Circumpolar Current (ACC), we investigate the local current-current interactions which are conveyed by small-scale turbulences. In our analysis, we focus on sea surface height and ocean bottom pressure and the different forcing terms that influence these two variables. We configure a version of the Regional Ocean Modelling System (ROMS) to simulate ocean dynamics around South Africa, forced with ERA-Interim atmospheric data, and explore the sensitivity to various choices of boundary conditions. The horizontal resolution of 0.25 degrees - 0.25 degrees at 32 vertical levels is supposed to resolve mesoscale eddies as well as the climatologically important shedding of Agulhas rings. To show the capabilities of our model, we compare the output in terms of sea-surface heights to altimetric measurements provided by AVISO. In-situ data of ocean bottom pressure measured in the ACC path adds to the observational database. The study area is especially promising as, additionally, we can show whether the simulations of an integrated ocean bottom pressure signal correspond to the residuals in measurements of the Superconducting Gravimeter in Sutherland, South Africa.

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: Time variable gravity fields, reflecting variations of mass distribution in the system Earth is one of the key parameters to understand the changing Earth. Mass variations are caused either by redistribution of mass in, on or above the Earth's surface or by geophysical processes in the Earth's interior. The first set of observations of monthly variations of the Earth gravity field was provided by the US/German GRACE satellite mission beginning in 2002. This mission is still providing valuable information to the science community. However, as GRACE has outlived its expected lifetime, the geoscience community is currently seeking successor missions in order to maintain the long time series of climate change that was begun by GRACE. Several studies on science requirements and technical feasibility have been conducted in the recent years. These studies required a realistic model of the time variable gravity field in order to perform simulation studies on sensitivity of satellites and their instrumentation. This was the primary reason for the European Space Agency (ESA) to initiate a study on ''Monitoring and Modelling individual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites''. The goal of this interdisciplinary study was to create as realistic as possible simulated time variable gravity fields based on coupled geophysical models, which could be used in the simulation processes in a controlled environment. For this purpose global atmosphere, ocean, continental hydrology and ice models were used. The coupling was performed by using consistent forcing throughout the models and by including water flow between the different domains of the Earth system. In addition gravity field changes due to solid Earth processes like continuous glacial isostatic adjustment (GIA) and a sudden earthquake with co-seismic and post-seismic signals were modelled. All individual model results were combined and converted to gravity field spherical harmonic series, which is the quantity commonly used to describe the Earth's global gravity field. The result of this study is a twelve-year time-series of 6-hourly time variable gravity field spherical harmonics up to degree and order 180 corresponding to a global spatial resolution of 1 degree in latitude and longitude. In this paper, we outline the input data sets and the process of combining these data sets into a coherent model of temporal gravity field changes. The resulting time series was used in some follow-on studies and is available to anybody interested.

Description: Angular momentum forecasts for up to 10 days into the future, modeled from predicted states of the atmosphere, ocean and continental hydrosphere, are combined with the operational IERS EOP prediction bulletin A to reduce the prediction error in the very first day and to improve the subsequent 90-day prediction by exploitation of the revised initial state and trend information. EAM functions derived from ECMWF short-range forecasts and corresponding LSDM and OMCT simulations can account for high-frequency mass variations within the geophysical fluids for up to 7 days into the future primarily limited by the accuracy of the forecasted atmospheric wind fields. Including these wide-band stochastic signals into the first days of the 90-day statistical IERS predictions reduces the mean absolute prediction error even for predictions beyond day 10, especially for polar motion, where the presently used prediction approach does not include geophysical fluids data directly. In a hindcast experiment using 1 year of daily predictions from May 2011 till July 2012, the mean prediction error in polar motion, compared to bulletin A, is reduced by 32, 12, and 3 % for prediction days 10, 30, and 90, respectively. In average, the prediction error for medium-range forecasts (30–90 days) is reduced by 1.3–1.7 mas. Even for UT1-UTC, where AAM forecasts are already included in IERS bulletin A, we obtain slight improvements of up to 5 % (up to 0.5 ms) after day 10 due to the additional consideration of oceanic angular momentum forecasts. The improved 90-day predictions can be generated operationally on a daily basis directly after the publication of the related IERS bulletin A product finals2000A.daily.

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: Ocean bottom pressure gradients deduced from the satellite gravity mission Gravity Recovery and Climate Experiment (GRACE) were previously shown to provide barotropic transport variations of the Antarctic Circumpolar Current (ACC) with up to monthly resolution. Here, bottom pressure distributions from GRACE with monthly (GFZ RL04) and higher temporal resolution (CNES/GRGS with 10 days, ITG-GRACE2010 with daily resolution) are evaluated over the ACC area. Even on time scales shorter than 10 days, correlations with in situ bottom pressure records frequently exceed 0.6 with positive explained variances, giving evidence that high-frequency nontidal ocean mass variability is captured by the daily ITG-GRACE2010 solutions not already included in the applied background models. Bottom pressure is subsequently taken to calculate the barotropic component of the ACC transport variability across Drake Passage. For periods longer than 30 days, transport shows high correlations between 0.4 and 0.5 with several tide gauge records along the coast of Antarctica. Still significant correlations around 0.25 are obtained even for variability with periods shorter than 10 days. Since transport variations are predominantly affected by time-variable surface winds, GRACE-based transports are contrasted against an atmospheric index of the Southern Annular Mode (SAM), which represents the Southern Hemispheric wind variability. Correlations between the SAM and GRACE-based transports are consistently higher than correlations between any of the available sea level records in all frequency bands considered, indicating that GRACE is indeed able to accurately observe a hemispherically consistent pattern of bottom pressure (and hence ACC transport) variability that is otherwise at least partially masked in tide gauge records due to local weather effects, sea ice presence and steric signals.

Description: Daily effective angular momentum functions from atmosphere, oceans, and continental hydrosphere that are consistent in terms of global mass conservation among the sub-systems are obtained from atmospheric data the most recent ECMWF re-analysis ERA Interim and corresponding simulations with the hydrological model LSDM and the ocean model OMCT covering 1989 - 2008. Correlations between simulations and geodetic excitation functions based on the EOP C04 polar motion series are generally improving when considering oceanic and even continental effects in addition to the atmosphere, with correlation coefficients that exceed values of 0.8 during the most recent years. While contributions to the annual wobble are found to be of similar amplitude and phase as in previous studies, both seasonal averaged and inter-annual variations are able to capture the main characteristics of individual peaks in the corresponding geodetic excitation functions. By decomposing the simulated global angular momentum functions into their regional contributions, atmospheric and oceanic pressure and current distributions in accordance with continental water storage variations are shown to be of similar importance for polar motion excitation on seasonal time-scales, whereas continental water flow contributions to the relative angular momentum of the Earth have been found to be three orders of magnitude lower than the corresponding effect of water storage changes. The data-sets discussed here are publicly available via the restructured Geophysical Fluids Center of the IERS.