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Introduction, Data, and Methods Validation Steric Sea Level Trends 5th International Conference on Reanalysis Andrea Storto, Simona Masina (CMCC, Bologna, Italy), Karina von Schuckman, Clement Bricaud, Charles Desportes, Marie Drevillion, Yann Drillet, Clotilde Dubois, Gilles Garric, Coralie Perruche (Mercator Ocean, Toulouse, France), Hao Zuo, Magdalena Balmaseda, Patricia de Rosnay (ECMWF, Reading, UK), Drew Peterson, Laura Jackson, Matt Martin, Richard Wood (MetOffice, Exeter, UK), Sandrine Mulet, Isabelle Pujol (CLS, Toulouse, France) Contact: [email protected]! Introduction, data, and methods Steric sea level is the variation of the ocean volume due to density changes (expansion and contraction of water masses), through ocean salinity (halosteric) and ocean temperature (thermosteric) variations. Thermosteric variability is the dominant component of global steric sea level change. Salinity variations associated with freshwater tendencies at the sea surface that are redistributed in the ocean’s interior have a negligible effect on seawater density and thus on sea level changes on the global scale. On regional to basin scales, the role of halosteric effects through the addition and subtraction of freshwater or mixing processes can be large, and should not be neglected in sea level studies. Previous assessments of steric sea level from ocean reanalyses (e.g. Storto et al., 2017, Clim. Dyn.) showed that the ensemble mean of ocean reanalyses is able to capture the variability of steric sea level and its components, although the halosteric component and depths below 700 m exhibit non-significant results due to the large spread of reanalyses. Hence, it is crucial to i) monitor the steric sea level rise (see the CMEMS Ocean State Report), ii) continue assessing the performance of steric sea level from reanalyses to quantify the reliability of reanalysis products for multi-decadal investigations and iii) evaluate the advancements of reanalyses with new vintage of products. Here we use data from the GREP (Global ocean Reanalysis Ensemble Product) ensemble of ocean reanalyses produced within the Copernicus Marine Service (CMEMS). In particular the products are: 1) C-GLORSv7 (from CMCC, Italy); 2) GLORYS2v4 (from Mercator Ocean, France); 3) GLOSEA5v13 (from MetOffice, UK); 4) ORAS5 (from ECMWF) plus 5) GREP-EM (Ensemble Mean). Steric Sea Level (0-z) is calculated as the integrated density anomaly between the sea surface and depth z. For validation purposes (2003-2015), we estimate independent steric sea level (SLSTER) estimates from the relationship SLSTER = SLTOT – SLMASS where the total sea level SLTOT is derived from altimetry (T/P + Jason, Nerem et al., 2010, Mar. Geod.), and the mass component SLMASS from gravimetry data (GRACE, Chambers, 2013, J.G.R.). DATASET TREND CORR. [mm/yr] w.r.t. Validation ALT-GRV Steric Sea level trends ALT-GRV 1.20 +/- 0.05 - Significant trends are found for the thermosteric sea level at different depths, except in the deep ocean (deeper GREP-EM 1.96 +/- 0.08 0.83 than 1500m) where the small trend (~0.1 mm/yr) is not significant w.r.t. to the GREP spread. Large inconsistency GREP-EM 1.69 +/- 0.04 0.87 is found for the halosteric component, reflected in the Thermoster. total steric spread uncertainty (larger than the steric one). C-GLORS 1.61 +/- 0.04 0.91 Regional maps of trends show significant steric sea level rise in the Indian, western Pacific (PDO-driven) and FOAM 2.53 +/- 0.17 0.66 Atlantic Ocean. Halosteric trends are generally non- significant, except for the thermo-halo compensation in GLORYS 1.29 +/- 0.07 0.78 the Atlantic Ocean, the freshening around Australia and salinification within marginal seas. ORAS 2.28 +/- 0.09 0.84 Validation of global (above) and zonal means (below) steric sea level against independent estimates from altimetry minus gravimetry All reanalyses show significant correlations w.r.t. the verifying dataset. Correlation of the ensemble mean is greater when thermosteric is considered, implying low accuracy of the global halosteric signal. (C- GLORS exhibits the largest CORR). Zonally averaged correlations show that the GREP EM outperforms the individual Vertically integrated steric sea level trends (1993-2015) for the GREP EM and individual products. Map of regional trends for total, thermo and halo reanalyses, except in the ACC Red bars correspond to the standard deviations of ensemble member trends; steric sea level (1993-2015). Gray dots correspond to (GLORYS outperforming others) Black bars to the uncertainty of the ensemble mean trend (through bootstrapping). trends that are significant (w.r.t. the GREP spread). Comparing GREP ensemble consistency with the Impact of observing network previous vintage of reanalyses The Argo revolution TAO missing We compare the spread (consistency) of the GREP reanalyses with the subset of ORA-IP reanalyses (2012 vintage) from CMCC, ECMWF, MetOffice, Mercator Ocean, i.e. the previous vintage of reanalyses (called GREPv0). It is shown that for all latitudinal bands and steric components, the GREPv1 spread is smaller than that of GREPv0 (except the non- significant differences for the Southern ET thermosteric sea level). In particular (right panels) large reduction of uncertainty occurs in the Indo-Pacific Tropical areas for both Argo- poor and Argo-rich decades. Investigations of spread behavior with time are commonly adopted to assess the impact of a certain observing network on the resulting ensemble mean accuracy. Summary The top panels show the impact of the reduction of TAO/TRITON mooring GREP products, and notably the GREP Ensemble Mean, capture well the steric sea level variability, Difference of time-averaged spread data after 2012, which leads to a Between Argo-poor and Argo-rich period although large inconsistency is found below 1500m e for the halosteric component. The spread sudden increase of thermosteric, hence Large spread decrease in North Atlantic (larger total, steric sea level spread, in reproduces the evolution of the ocean observing network, and testifies the advancement of the for individual components) indicate better agreement with a similar work from Xue reanalyses, in terms of increased consistency, w.r.t. the previous vintage of reanalyses. thermo-halo separation in the Argo-rich period. et al. (2017, Clim. Dyn.). Low-resolution (1°x1°) GREP monthly mean data are released through CMEMS for the period 1993-2016 and are available at marine.copernicus.eu See the poster “The Copernicus Marine Service Global Reanalysis Ensemble Product GREPv1” by M. Drevillion et al. for more info on GREP data Acknowledgements: This study was supported by the Copernicus Marine Service (CMEMS) and by the COST Action “Evaluation of Ocean Synthesis” .
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