@article{PAR00000124,
  title = {{I}ncorporating state-dependent temperature-salinity constraints in the background error covariance of variational ocean data assimilation},
  author = {{R}icci, {S}. and {W}eaver, {A}. {T}. and {V}ialard, {J}. and {R}ogel, {P}.},
  editor = {},
  language = {{ENG}},
  abstract = {{S}everal studies have illustrated how the univariate assimilation of temperature data can have a detrimental effect on the ocean-state variables (salinity, currents, etc.) not directly constrained by the data. {I}n this paper, the authors describe how the salinity adjustment method proposed by {T}roccoli and {H}aines can be included as a multivariate temperature-salinity ({T}-{S}) constraint within a background-error covariance model for variational data assimilation. {T}he method is applied to a three-dimensional variational assimilation (3{DVAR}) system for a tropical {P}acific version of the {O}cean {P}arallelise ({OPA}) ocean general circulation model. {A}n identical twin experiment is presented first to illustrate how the method is effective in reconstructing a density profile using only temperature observations from that profile. {T}he 3{DVAR} system is then cycled over the period 1993-98 using in situ temperature data from the {G}lobal {T}emperature and {S}alinity {P}ilot {P}rogramme. {R}elative to a univariate ({T}) 3{DVAR}, the multivariate ({T}, {S}) 3{DVAR} significantly improves the salinity mean state. {A} comparison with salinity data that are not assimilated is also presented. {T}he fit to these observations is improved when the {T}-{S} constraint is applied. {T}he salinity correction leads to a better preservation of the salinity structure and avoids the development of spurious geostrophic currents that were evident in the univariate analysis. {T}he currents at the surface and below the core of the undercurrent are also improved. {E}xamination of the heat budget highlights how the temperature increment must compensate for a perpetual degradation of the temperature field by abnormally strong advection in the univariate experiment. {W}hen the {T}-{S} constraint is applied, this spurious advection is reduced and the mean temperature increment is decreased. {E}xamination of the salt budget shows that spurious advection is also the main cause of the upper-ocean freshening. {W}hen the {T}-{S} constraint is applied, the salinity structure is improved allowing for a better representation of the advection term and better preservation of the salt content in the upper ocean. {T}he {T}-{S} constraint does not correct for all problems linked to data assimilation: vertical mixing is still too strong, and the surface salinity state and currents still have substantial errors. {I}mprovements can be expected by including additional constraints in the background error covariances and by assimilating salinity data.},
  keywords = {},
  booktitle = {},
  journal = {{M}onthly {W}eather {R}eview},
  volume = {133},
  numero = {1},
  pages = {317--338},
  ISSN = {0027-0644},
  year = {2005},
  DOI = {10.1175/{MWR}2872.1},
  URL = {https://www.documentation.ird.fr/hor/{PAR}00000124},
}