Mass evolution of Mediterranean, Black, Red, and Caspian Seas from GRACE and altimetry: accuracy assessment and solution calibration
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We present new measurements of mass evolution for the Mediterranean, Black, Red, and Caspian Seas as determined by the NASA Goddard Space Flight Center (GSFC) GRACE time-variable global gravity mascon solutions. These new solutions are compared to sea surface altimetry measurements of sea level anomalies with steric corrections applied. To assess their accuracy, the GRACE- and altimetry-derived solutions are applied to the set of forward models used by GSFC for processing the GRACE Level-1B datasets, with the resulting inter-satellite range-acceleration residuals providing a useful metric for analyzing solution quality. We also present a differential correction strategy to calibrate the time series of mass change for each of the seas by establishing the strong linear relationship between differences in the forward modeled mass and the corresponding range-acceleration residuals between the two solutions. These calibrated time series of mass change are directly determined from the range-acceleration residuals, effectively providing regionally-tuned GRACE solutions without the need to form and invert normal equations. Finally, the calibrated GRACE time series are discussed and combined with the steric-corrected sea level anomalies to provide new measurements of the unmodeled steric variability for each of the seas over the span of the GRACE observation record. We apply ensemble empirical mode decomposition (EEMD) to adaptively sort the mass and steric components of sea level anomalies into seasonal, non-seasonal, and long-term temporal scales.
KeywordsGRACE Sea surface altimetry Steric sea level Inter-satellite range-acceleration EEMD
Support for this work was provided by the NASA GRACE and GRACE Follow-On Science Team Grants NNH10ZDA001N and NNH15ZDA001N. We gratefully acknowledge the quality of the GRACE Level-1B products produced by our colleagues at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena. We thank J.P. Boy and R.D. Ray for contributions to the forward models applied in our GRACE data reduction and analysis. We especially acknowledge the numerous contributions of D.D. Rowlands and T.J. Sabaka in developing the foundation of algorithms and software necessary to carry out this research. We also thank the reviewers who provided valuable feedback towards improving this manuscript. The GRACE Level-1B products are available at the Physical Oceanography Distributed Active Archive Center (PO.DAAC) at https://podaac.jpl.nasa.gov/. The sea level anomaly grids are produced by the Sea Level Thematic Assembly Centre (SL-TAC) and hosted by Copernicus Marine Environment Monitoring Service at http://marine.copernicus.eu/. The Gibbs Seawater Oceanographic Toolbox for implementing TEOS-10 is available at http://www.teos-10.org/. MERRA-2 data is available at http://disc.sci.gsfc.nasa.gov/mdisc/.
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