Offline display program

The Atmosphere and Ocean De-Aliasing Level-1B (AOD1B) product provides a priori information about temporal variations in the Earth's gravity field caused by global mass variability in the atmosphere and ocean and is routinely used as background model in satellite gravimetry. The current version 06 provides Stokes coefficients expanded up to d/o 180 every 3 hours. It is based on ERA-Interim and the ECMWF operational model for the atmosphere, and simulations with the global ocean general circulation model MPIOM consistently forced with the fields from the same atmospheric data-set.

We here present preliminary numerical experiments in the development towards a new release 07 of AOD1B. The experiments are performed with the TP10 configuration of MPIOM and include (I) new hourly atmospheric forcing based on the new ERA-5 reanalysis from ECMWF; (II) an improved bathymetry around Antarctica including cavities under the ice shelves; and (III) an explicit implementation of the feedback effects of self-attraction and loading to ocean dynamics. The simulated ocean bottom pressure variability is discussed with respect to AOD1B version 6 as well as in situ ocean observations. A preliminary timeseries of hourly AOD1B-like coefficients for the year 2019 that incorporate the above mentioned improvements will be made available for testing purposes.

How to cite: Shihora, L. and Dobslaw, H.: Towards AOD1B RL07, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-29, https://doi.org/10.5194/gstm2020-29, 2020.

The accuracy of ocean tide models has increased drastically since the launch of Topex/Poseidon and its successors. However, in regions of imperfect altimetry coverage (e.g., polar seas and coastal areas) and for minor tides with small signal-to-noise ratios, unconstrained tidal models provide crucial estimates of tidal variability in both sea-level and ocean bottom pressure that are truly global in spatial coverage.

We will present improved results from the unconstrained hydrodynamic barotropic tidal model TiME (Weis et al., 2008) that was enhanced recently with various newly developed features including (i) a rotated grid avoiding the open sea coordinate singularity allowing for global simulations without the need of introducing open ocean boundaries or spherical caps; (ii) a revised scheme for feedbacks of self-attraction and loading on ocean dynamics; (iii) updated bathymetries that also include water column height information in cavities underneath the Antarctic ice-shelves; (iv) the ability to either simulate individual partial tides or transient tidal dynamics by means of full forcing by ephemerides including contributions of the third-order lunisolar potential (Hartmann and Wenzel, 1995). Most recently, (v) a topographic wave drag parametrization following the tensor scheme of Nycander (2005) was incorporated in order to properly represent also this energy dissipation channel in TiME.

We will concentrate on results for the principal lunar tide (M2) as well as on estimates of some minor tides that are not routinely included in modern tidal atlases. In a medium term perspective, tidal results from TiME will be considered as background information for the processing of GRACE and GRACE-FO gravity fields as currently explored within the research group NEROGRAV funded by the German Research Foundation.

How to cite: Sulzbach, R., Dobslaw, H., and Thomas, M.: Novel ocean tide solutions for application in satellite gravimetry including minor tides, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-39, https://doi.org/10.5194/gstm2020-39, 2020.

Ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment (GRACE) revealed Arctic Ocean circulation patterns and variability that were previously unknown (Morison et al., 2007; Morison et al., 2012; Peralta-Ferriz et al., 2014). OBP measurements from the GRACE Follow-On mission (GRACE-FO) are therefore increasingly important for monitoring Arctic Ocean variability, and critical for understanding and predicting the fate of the rapidly changing Arctic environment.

In this work we use GRACE data from 2002 to 2017 jointly with a 10-year record of in situ OBP at the North Pole (2005-2015) complemented with in situ OBP in the Canada Basin (2015-2018), and wind reanalysis products, to create a proxy representation of the OBP anomalies that explains the largest possible fraction of the observed OBP variability in the Arctic Ocean and the Nordic Seas. We do this by performing a linear regression analysis, combined with maximum covariance analysis (MCA) – a technique that was tested prior to the decommission of GRACE and the launch of GRACE-FO (Peralta-Ferriz et al., 2016). Here, the first predictor time series is the in situ OBP record at the North Pole and Canada Basin; the second predictor time series is the expansion coefficients time series of the leading mode of MCA between the GRACE OBP coupled with the winds. We use this proxy OBP to merge GRACE with the first 2 years of available GRACE-FO OBP. We compare our merged OBP field with OBP output from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS). Preliminary results suggest a good agreement between the proxy and predicted OBP fields and both GRACE and GRACE-FO data, especially in the central Arctic, but also in the Nordic Seas. The OBP variations from the merged GRACE and GRACE-FO and from PIOMAS will be also explored.

References:

• Morison, J. H., J. Wahr, R. Kwok and C. Peralta-Ferriz (2007), Recent trends in Arctic Ocean mass distribution revealed by GRACE, Res. Lett.,34, L07602, doi:10.1029/2006GL029016.
• Morison, J., R. Kwok, C. Peralta-Ferriz, M. Alkire, I. Rigor, R. Andersen and M. Steele (2012), Changing Arctic Ocean freshwater pathways. Nature, 481, 66-7
• Peralta-Ferriz, C., J. H. Morison, J. M. Wallace, J. Bonin and J. Zhang (2014), Arctic Ocean circulation patterns revealed by GRACE, of Climate, 27:1445–1468 doi:10.1175/JCLI-D-13-00013.1.
• Peralta-Ferriz, C., J. H. Morison and J. M. Wallace(2016), Proxy representation of Arctic ocean bottom pressure variability: Bridging gaps in GRACE observations,  Res. Lett., 43, 9183–9191, doi:10.1002/2016GL070137

How to cite: Peralta-Ferriz, C., Morison, J., and Bonin, J.: Arctic Ocean bottom pressure variations from 2002 to 2020: merging GRACE with GRACE-FO , GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-45, https://doi.org/10.5194/gstm2020-45, 2020.

Based on the latest GFZ release 06 of monthly gravity fields from GRACE satellite mission, area-averaged barystatic sea-level is found to rise by 2.02 mm/a during the period April 2002 until August 2016  in the open ocean with a 1000 km coastal buffer zone when low degree coefficients are properly augmented with information from satellite laser ranging. Alternative spherical harmonics solutions from CSR, JPL and TU Graz reveal  rates between 1.94 and 2.08 mm/a, thereby demonstrating that systematic differences among the centers are much reduced in the latest release. The results from the direct integration in the open ocean can be aligned to associated solutions of the sea-level equation when fractional leakage derived from two differently filtered global gravity fields is explicitly considered, leading to a global mean sea-level rise of 1.72 mm/a. This result implies that estimates obtained from a 1000 km coastal buffer zone are biased 0.3 mm/a high due the systematic omission of regions with below-average barystatic sea-level rise in regions close to substantial coastal mass losses induced by the reduced gravitational attraction of the remaining continental ice and water masses.

How to cite: Dobslaw, H., Dill, R., Bagge, M., Klemann, V., Boergens, E., Thomas, M., Dahle, C., and Flechtner, F.: Barystatic Sea-Level Rise from GRACE-based Solutions of the Sea-Level-Equation, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-48, https://doi.org/10.5194/gstm2020-48, 2020.

The currents of the Southern Ocean connect to every ocean basin and distribute water between them, making these currents an important quantity to monitor and understand as they change over time. In situ instrumentation is difficult to deploy within this region, however these currents create bottom pressure signals at major bathymetric obstacles which are observable by the GRACE and GRACE Follow-On satellites. 

    In this study, we examine the relationships between the Antarctic Circumpolar Current and the Antarctic Bottom Water within the Southern Ocean and the bottom pressure gradients at the Kerguelen Plateau seamount and the Drake Passage sill using the high resolution Southern Ocean State Estimate. Then we use these relationships along with bottom pressure gradients from the RL05 and RL06 mascon solutions from the Center for Space Research at the University of Texas at Austin to develop a real-life time series of the two currents.

How to cite: Meyer, J. and Chambers, D.: Constructing a Time Series for Southern Ocean Currents Using GRACE, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-67, https://doi.org/10.5194/gstm2020-67, 2020.