Description: One decade of time-variable gravity field observations from the GRACE satellite mission reveals low-frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year-long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea-level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA-Interim sea-level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favorably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE-based OBP observations are indeed sensitive to changes in the prevailing sea-level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.

Description: This paper deals with the reconstruction of sea level anomalies using altimetry and GPS-corrected tide gauge data. In the first part of this paper, the reconstruction algorithm used ("Optimal interpolation" (OI (C)) is investigated in more detail. It is found that it shows a strong element of stochasticity. The reconstruction is strongly dependent on the relative spatial position of the tide gauges to the empirical orthogonal Functions (EOF) used in the reconstruction. In the second part of the paper, a reconstruction of global sea level anomalies for the period of 1970-2001 is performed. The tide gauges used for this purpose are corrected for land movement using vertical GPS trends. Also, a a smoothing algorithm is introduced which helps to fill the errors resulting from gaps in the tide gauge records.

Description: Precise weekly positions of 403 Global Positioning System (GPS) stations located worldwide are obtained by reprocessing GPS data of these stations for the time span from 4 January 1998 until 29 December 2007. The processing algorithms and models used as well as the solution and results obtained are presented. Vertical velocities of 266 GPS stations having a tracking history longer than 2.5 yr are computed; 107 of them are GPS stations located at tide gauges (TIGA observing stations). The vertical velocities calculated in this study are compared with the estimates from the co-located tide gauges and other GPS solutions. The formal errors of the estimated vertical velocities are 0.01–0.80 mm yr−1. The vertical velocities of our solution agree within 1 mm yr−1 with those of the recent solutions (ULR5 and ULR3) of the Université de La Rochelle for about 67–75 per cent of the common stations. Examples of typical behaviour of station height changes are given and interpreted. The derived height time series and vertical motions of continuous GPS at tide gauges stations can be used for correcting the vertical land motion in tide gauge records of sea level changes.

Description: Variations in the earth rotation parameters are strongly influenced by atmospheric and oceanic variability patterns. In order to develop a climate index from Earth rotation parameters, the influence of known large-scale climate variability features on Earth rotation must be assessed. This can be done using Atmosphere-Ocean General Circulation Models (CGCMs), simulating the climate system in a physically consistent way. The analysis performed is based on the computation of effective angular momentum functions derived from: a) an ocean model (OMCT) driven with ECMWF (ERA Interim/ERA40) atmospheric reanalysis data, and with a 500 year run of the ECHAM5/OM1 model, developing its climate without an observational forcing. Results obtained from re-analysis and the simulated ocean can be directly a compared with the observational IERS geodetic earth orientation data (C04 excitation functions). Data from the free model run shall demonstrate in how far the fully coupled model is able to reproduce the same features for the geodetic variations. One of the variability features investigated is the North Atlantic Oscillation (NAO), the dominant atmospheric winter teleconnection pattern for the Northern Hemisphere. Its influence on polar motion(e.g. Chao and Zhou, 1998) was thought to be caused largely by mass redistribution. This assumption is, however, inconsistent with the inverted barometer assumption, telling that atmospheric pressure anomalies over the ocean (where the larger part of the NAO anomalies lies) should be outweighed by an elastic response of the ocean surface. Our results suggest that, instead of atmospheric mass redistribution, the influence of the NAO on polar motion is exerted through changes in wind speed and resulting oceanic transport, mainly via the x1 motion components of the atmospheric (AAM) and oceanic (OAM) effective angular momentum (EAM) functions. As a second variability feature, the possible influence of the Quasi-Biannual Oscillation (QBO) on polar motion is examined. Because of the link between NAO and QBO (significantly correlated with r=0.22 in the ERA Interim dataset), an indirect influence of the QBO on earth orientation would be expected. However, no significant correlation between the EAM functions and the QBO index is found. To investigate this connection further, Granger causality is used, a statistical tool to determine whether the knowledge of past values of one timeseries (y) is useful in predicting future values of a second timeseries (x) over and above the knowledge of past values of x alone. It is shown that, for the ERA Interim period, the QBO “grangercauses” the winter AAM x-1 mass component as well as the OAM x-1 motion component at 99% significance level, meaning that previous QBO index values may influence the earth orientation. The comparison with data from the ECHAM5/OM1 model - which does not include a well resolved stratosphere and fails to reproduce correctly the QBO - is used to determine whether the influence of the NAO on earth rotation is modified by the existence of the QBO.

Description: One decade of time-variable gravity field observations from the GRACE satellite mission reveals low-frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year-long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea-level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA-Interim sea-level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favourably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE-based OBP observations are indeed sensitive to changes in the prevailing sea-level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.

Description: Earth orientation parameters (EOPs) are strongly influenced by atmospheric and oceanic mass and motion variations, and therefore may help provide an independent measure of climate variability. Coupled Atmosphere-Ocean General Circulation Models (GCMs) simulate the variations in the atmosphere and the ocean in a physically consistent way. Thus, the GCMs can be inter-compared with respect to the derived EOP variations. Global warming has been shown to exert a major effect on Length-of-Day, caused by an enhancement in atmospheric motion. However, a comprehensive assessment of the impact of climate change on polar motion excitation has not yet been presented. In this paper, an inter-model comparison of a Climate Change signal (A1B – 20C) in Polar Motion is provided for a set of model runs from the WCRP CMIP-3 campaign. The models used in the comparison are the ECHAM5/OM1, GFDL CM2, NCAR CCSM3, and UK MetOffice HadCM3. As an additional fifth model, we use tidal and non-tidal runs from the ECOCTH model, which consists of the ECHAM5/OM1 with a tidal coupler. First, a basic consistency check was performed for multi-century control runs of the models. The twodimensional excitation fields for atmospheric mass and motion, as well as oceanic mass and motion are compared. Also, the globally integrated EOPs are analysed both in time and spectral domain. The comparison yields, e.g., for the atmospheric mass component of polar motion excitation, very good agreement between the models with respect to the annual cycle. In the Taylor diagrams comparing the main EOFs from the two-dimensional excitation fields calculated from the atmospheric mass distribution, we also obtain good agreement. All five main EOFs show correlations in the range of 0.75 to 0.98 in the inter-model comparison. In a second step, the impact of climate change signal, i.e. the difference between two 30-year periods from the beginning and the end of the A1B run, is analysed.