2. The Ocean Data Assimilation System

As for the previous systems, the ocean data assimilation system for S3 is based on the HOPE-OI scheme: The first guess is given by forcing the HOPE (Hamburg Ocean Primitive Equations) ocean model with daily fluxes of momentum, heat, and fresh water, while the observations are assimilated using an Optimal Interpolation (OI) scheme.

2.1 The analysis cycle

2.2 The ocean model

2.3 Observations and surface fluxes data sets

2.1 The analysis cycle

The ocean analysis is performed every 10 days. All the observations within a centered 10-days window are gathered and quality controlled. In S3 major upgrades have been introduced in the HOPE-OI system. In addition to subsurface temperature, the OI scheme now assimilates altimeter derived sea-level anomalies and salinity data. All the observations in the upper 2000m are assimilated (in S2 only the observations in the upper 400m were used). More information about the origin of the observation streams and processing can be found here. Before the observations are combined with the model background via the OI, the model background is bias corrected according to the procedure described in Balmaseda et al 2007.Then, the different data streams are assimilated sequentially as follows:

1. Assimilation of sea level anomaly maps (from altimeter) :The detrended altimeter-derived sea level anomalies are combined with the bias-corrected model first-guess using the Cooper and Haines 1996 scheme to produce a first analysis.
2. Assimilation of subsurface temperature (from ARGO,XBTs,Moorings): The result of the previoues step is then used as a first guess for a second assimilation step, where only subsurface temperature data are assimilated, and salinity is updated by imposing conservation of the model temperature/salinity (T/S) relationship (Troccoli et al 2002), while the sea level and velocity field remain unchanged.
3. Assimilation of Salinity (from ARGO, Moorings): In a third assimilation step, the information provided by the salinity observations is used to modify the model T/S relationship. In this step, the T/S information is spread along isotherms following the scheme of Haines et al., 2006. Only salinity is modified in this step which results in the analysis.After this 3rd assimilation step, velocity updates are derived from the temperature and salinity increments imposing geostrophic balance (Burgers et al., 2002)
4. Assimilation of global sea level trend (from from altimeter): Finally, the trend in global (area averaged) sea level is assimilated. By combining the altimeter-derived trend in global sea level with the model trend in global dynamic height, it is possible to make the partition between changes in the global volume and changes in the total mass. By doing so, the global fresh water budget is closed and the global surface salinity and sea level adjusted accordingly.

Analysis increments in temperature, salinity and velocity are calculated using the OI scheme and balance relationships. To avoid exciting gravity waves, and to allow the model dynamics to adjust gradually to the changes in the density field, an Incremental Analysis Update (IAU) method (Bloom et al., 1996) is used: the increment is added slowly over the subsequent 10 days (IAU-10), after which a new background field is available, and the cycle repeated.

2.2 The ocean model

The HOPE ocean model (Wolff et al., 1997) uses an Arakawa E grid horizontal discretization. Several modifications took place over the years at ECMWF (Balmaseda 2004, Anderson and Balmaseda 2006). The horizontal resolution was increased to 1 x 1 degrees with equatorial refinement, i.e., the meridional resolution increases gradually towards the equator, where it is 0.3 degrees in the meridional direction. There are 29 levels in the vertical, with a typical vertical thickness of 10 meters in the upper ocean compared to 20 levels. The vertical mixing is based on Peters et al., 1998. The barotropic solver, originally implicit, was made explicit as described in Anderson and Balmaseda (2006).

2.3 Observations and fluxes data sets

n S3, the observations come from the quality controlled dataset prepared for the ENACT and ENSEMBLES projects until 2004 (Ingleby and Huddleston 2006), and from the GTS thereafter (ENACT/GTS). The OI scheme is now 3-dimensional, the analysis being performed at all levels simultaneously down to 2000m, where09.03.2007evel independently and only to 400m. In addition, the decorrelation scales depend on the density gradient, which favours the propagation of information along isopycnals. A pictorial view of the various data sets used in S3 is given in fig 1. The analysis of SST is not produced using the OI-Scheme. Instead, the model SSTs are strongly relaxed to analyzed SST maps. The maps are daily interpolated values derived from the OIv2 SST product (Reynolds et al 2002) from 1982 onwards. Prior to that date, the same SST product as in the ERA40 reanalysis was used.

In S3, altimeter data are assimilated for the first time in the ECMWF operational ocean analysis. The altimeter information is given by maps of merged satellite product, provided by Ssalto/DUACS and distributed by AVISO, with support from CNES. Twice a week (on Wednesday and on Saturday mornings) (1/3x1/3 degrees) Maps of Sea Level Anomaly (MSLA) for a merged product combining all satellites (Envisat, Jason, Topex/Poseidon, ERS2, GFO) using optimal interpolation and accounting for long wavelength errors are produced (Le Traon et al., 1998, Ducet et al., 2000)

Figure 1: Upper panel shows the surface forcing used in the ocean analysis and the initial conditions for the calibration hindcasts for S3. Lower panel shows the origin of the subsurface data surface temperature fields used.

The first-guess is obtained from integrating the HOPE ocean model from one analysis time to the next, forced by ERA40/OPS fluxes (ERA40 fluxes from the period January 1959 to June 2002 and NWP operational analysis thereafter). In S2 the fluxes were from ERA15/OPS, but the wind stresses were not directly used: instead, the wind stress was derived from the analyzed winds using an off-line bulk formula. The representation of the upper ocean interannual variability is improved when using the ERA40 wind stress (Uppala et al., 2006), although the stresses are biased weak in the equatorial Pacific. The fresh water flux from ERA-40 (Precipitation - Evaporation, denoted P-E) is known to be inaccurate. S3 uses a better but by no means perfect estimate, obtained by 'correcting' the ERA-40 precipitation values (Troccoli and Kallberg 2004)..