September 23, 2025 Dataset Open Access

GECCO3 Ocean Synthesis (1˚x1˚ grid 41S6m)

Armin Köhl

The GECCO3 ocean synthesis is the German contribution of the Estimating the Circulation and Climate of the Ocean project (ECCO, see references) and is the new version of the GECCO2 synthesis already available at ICDC. The synthesis also uses the adjoint method to adjust the initial temperature and salinity in 1948 together with the air temperature, humidity, precipitation, shortwave radiation, and zonal and meridional wind every 10 days to bring the model into consistency with the data, which derive from the EN4.2.1 data base  AVISO along track SLA, DTU10 mean sea surface minus GOCO5s geoid to yield the mean dynamic topography, HadISST SST, and the WOA18 climatology.   The global model is based on the MITgcm model, has 40 levels, and uses the horizontal and vertical grid of the ocean component of MPI-ESM in the MR/HR configuration, which is with nominally 0.4° resolution eddy-permitting. The bathymetry is also adopted from this configuration. The background atmospheric state is taken from the 6 hourly NCEP/NCAR Reanalysis 1. 

GECCO3 and GECCO3S6m

The GECCO3 synthesis covers the period 1948-2018 and, unlike GECCO2, is optimized over only one assimilation window. The state after 41 iterations is available. There are two versions of the synthesis, which differ in the time constant of the relaxation of the surface salinity. For the present version 41S6m a time constant of 180 days was used. Globally and averaged over all data types, there is a slight advantage for the stronger relaxation. However, an evaluation with ESA-CCI surface salinity data for the years 2011-2018 showed a somewhat more realistic variability with less relaxation. In other variables that were examined, mainly differences between the two products on multi-decade time scales were found. The difference between the products gives an impression of the robustness of the estimate, but it must be taken into account that only one aspect was changed. Overall, the GECCO3S6m version can be recommended.

Model: MITgcm 
Resolution: 0.4°, 40 z-levels, partial cells 
Data assimilated: EN4.2.1 T/S (XBT corrected)+ AVISO along track SLA , GOCO5s MDT, HadISST SST, WOA18 climatology 
Assimilation method: 4D-VAR (adjoint) method 
Forcing and relaxation used: NCEP RA1 atm. Fields, Bulk formulae, Large and Yeager, 2009 

Coverage, spatial and temporal resolution

Period and temporal resolution:

• 1948-2018
• monthly
• SSH (daily upon request)

Coverage and spatial resolution:

• Global
• Spatial resolution: interpolated to 1˚x1˚ grid
• Dimension: 360x180x40
• Altitudes: 40 depth levels

Format:

• NetCDF4

Data quality

Variational methods fit the trajectory best during the middle section of the assimilation period. During the first few years spin-up effects are noticeable such that the variability during this period should be considered with care. Changes in the densities and type of assimilated data leave signatures in the product, in particular noticeable in year 1992 and at the beginning of the 2000s during the advent of altimeter and Argo data.

References

•  Köhl, A. (2020), Evaluating the GECCO3 1948-2018 Ocean Synthesis - a configuration for initializing the MPI-ESM climate model, Q. J. R. Met. Soc., doi: 10.1002/qj.3790. https://doi.org/10.1002/qj.3790