Assimilating Along-Track Altimetric Observations through Local Hydrostatic Adjustment in a Global Ocean Variational Assimilation System

Abstract

A global ocean three-dimensional variational data assimilation system was developed with the aim of assimilating along-track sea level anomaly observations, along with in situ observations from bathythermographs and conventional sea stations. All the available altimetric data within the period October 1992–January 2006 were used in this study. The sea level corrections were covariated with vertical profiles of temperature and salinity according to the bivariate definition of the background-error vertical covariances. Sea level anomaly observational error variance was carefully defined as a sum of instrumental, representativeness, observation operator, and mean dynamic topography error variances. The mean dynamic topography was computed from the model long-term mean sea surface height and adjusted through an optimal interpolation scheme to account for observation minus first-guess biases. Results show that the assimilation of sea level anomaly observations improves the model sea surface height skill scores as well as the subsurface temperature and salinity fields. Furthermore, the estimate of the tropical and subtropical surface circulation is clearly improved after assimilating altimetric data. Nonnegligible impacts of the mean dynamic topography used have also been found: compared to a gravimeter-based mean dynamic topography the use of the mean dynamic topography discussed in this paper improves both the consistency with sea level anomaly observations and the verification skill scores of temperature and salinity in the tropical regions. Furthermore, the use of a mean dynamic topography computed from the model long-term sea surface height mean without observation adjustments results in worsened verification skill scores and highlights the benefits of the current approach for deriving the mean dynamic topography.

Benchmarking the mesoscale variability in global ocean eddy-permitting numerical systems

“…They contribute to correct sub-surface temperature and salinity without directly changing the SSH. The overall effect of such a scheme on density profile is not limited to the mixed layer, but extents down to the thermocline or even below (Storto et al 2010) where the vertical redistribution is driven by the data assimilation background-error covariances. The reanalysis product is available on-line at C-GLORS website (www.cmcc.it/c-glors) for the 1982-2013 period.…”

Landfast ice thickness in the Canadian Arctic Archipelago from observations and models

“…See the appendix for a complete description of the method. We also investigate ice thickness values from a selection of the highest-resolution models (Storto et al, 2011;Forget et al, 2015;Haines et al, 2014;Masina et al, 2015), from the Ocean Reanalysis Intercomparison (ORA-IP) Chevallier et al, 2016) (Table 2) and from the Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS) (Zhang and Rothrock, 2003). Supporting 2 m temperature data were obtained from ERA-Interim (Dee et al, 2011).…”

Exploring the ability of current climate information to facilitate local climate services for the water sector

“…More specifically, the decadal predictions experiments have been performed following the WCRP/CMIP5 protocol (Taylor et al 2012) with simulations starting from 1960 and every five years up to 2005. Regarding initialization strategy, CMCC-CM was initialized using full-values from three different ocean analyses using the CMCC-INGV ocean syntheses (Bellucci et al 2007;Storto et al 2011) and different assimilation methods. For CNRM-CM5 initial states were produced from coupled experiments in which a surface flux correction was applied to constrain SST.…”