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The new operational ECWMF ocean analysis system (system 3 or S3) consists of two analysis streams:
- A historical reanalysis from 1959 which is 09.03.2007e (BRT). It is used to initialize seasonal forecasts.
- An early delivery ocean analysis, produced daily in near real time (NRT), used to initialize the monthly forecasts.
Although the historical reanalysis goes back to 1959, only the period 1981-2005 is used to initialize the calibrating hindcasts of the S3 seasonal forecasting system (Anderson et al., 2007). The earlier period of S3 ocean analysis will be used to initialize seasonal and decadal predictions within the ENSEMBLES project. As well as providing initial conditions for coupled model forecasts, the S3 ocean re-analysis, based on the synthesis of surface and subsurface ocean observations, surface fluxes from atmospheric analyses and reanalyses, and a general circulation ocean model, constitutes an important resource for climate variability studies.
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ECMWF has produced operationally daily global ocean analyses to provide initial conditions for the seasonal forecasting system since 1997. There have been two versions of the ocean analysis, linked to the operational seasonal forecasting system. System 1 (S1) started in 1997 (Alves et al., 2003) and provided the initial conditions for the first ECMWF operational seasonal forecasting system (Stockdale et al., 1998). System 2 (S2) was introduced in 2001 (Balmaseda 2004), and has provided initial conditions for the ECMWF operational seasonal forecasts since 2002 (Anderson et al. 2003, Oldenborgh et al. 2005a,b, Vialard et al. 2005). A comparison between S2 and S1 ocean analyses is given in Balmaseda 2004. In 2004 an extension of S2 was introduced in order to initialize the monthly forecasting system (Balmaseda 2005, Vitart 2005). In summer 2006 the S3 ocean analysis was implemented operationally (Balmaseda et al 2007).
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At the other end of the spectrum, moving towards the shorter time scales, we have the monthly and medium range forecasting activities, where the demand for ocean initial conditions is increasing. Monthly forecasting activities have stirred quite some interest in the last few years, and the benefits of having an active ocean in the forecasting system has been demonstrated (Vitart et al. 2006, Woolnough et al. 2007). The monthly forecasting system uses the same ocean model as the seasonal forecasting system though there are some differences in the way the ocean initial conditions are produced. In the near future it is planned that the medium range EPS forecasts will also be performed with a coupled model, with the consequent need for real-time ocean initial conditions. The EPS will also have a need for the historical ocean reanalysis, since the reliable prediction of extreme events also requires hindcasts or re-forecasting activities just as is done now for monthly and seasonal forecasting.
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- 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.
- 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.
- 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)
- 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.
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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).
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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).
3. The Ocean Observations and Quality Control
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Subsurface temperature observations that are available in near-real-time are currently provided by the TAO/TRITON and PIRATA arrays in the equatorial region (McPhaden 1998, Servain et al 1998) and the global Volunteer Observing Ship (VOS) programme which provides XBT (eXpandable BathyThermographs) measurements mainly along merchant shipping routes. More recently, observations are provided by the ARGO network of drifting profilers. Salinity data is available from ARGO and from the TRITON moorings.
Prior to 2004, the temperature and salinity profiles come from the quality controlled ENACT/ENSEMBLES data set (Ingleby and Huddleston 2006). From January 2005, the observations come from the GTS (Global Telecommunication System). An automatic quality control procedure (an extension of Smith et al., 1991) is then performed in several stages:
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The trend in global sea level is quite substantial. If this trend is produced by thermal expansion due to global warming, it can not be represented by the ocean model: in absence of fresh water fluxes, most ocean models used for climate are volume preserving, since they make use of the Boussinesq approximation. Therefore, if not treated correctly, the trend in sea level can be a problem when assimilating altimeter observations. In S3, the global sea level trend is removed from the altimeter sea level anomalies before they are assimilated via the CH96 scheme. In S3, the global sea level trend is later assimilated as described in Balmaseda et al 2007.
3.3 Sea Surface temperature
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When in 2004 the monthly forecasting system became operational at ECMWF, it was necessary to produce near-real-time ocean initial conditions, since the typical 11 days delay of the reanalysis product was not adequate for the monthly system. To this end, an early delivery ocean analysis system was introduced in May 2004. Details of the timing of the near real time S2 ocean analysis are given in Balmaseda 2005. In what follows, we denote the Near Real Time stream as NRT, and for BRT we refer to the reanalysis, which runs 11 days Behind Real Time. The BRT and NRT analysis systems are almost identical: they only differ in the details of the assimilation cycle and in the earlier cut-off of the assimilation windows. The BRT product is a continuation of the historical ocean reanalysis, while the NRT only exists for the recent period
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The operational ocean analysis is archived in the MARS mars MARS under stream=OCEA, expver=0001 and system=3.
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