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Dynamic Ocean Model - NEMO
The dynamic ocean model used for medium-range and seasonal forecasts of ocean structure is "NEMO" (the Nucleus for European Modelling of the Ocean) and is coupled within the ENS, extended-range and seasonal forecast models. It is a three-dimensional general circulation ocean model and can reproduce the general features of the circulation and the thermal structure of the ocean and their seasonal and inter-annual variations. The ocean model has approximately 0.25 degree (~28km) horizontal resolution (in ENS, extended-range and seasonal models). There are 75 vertical ocean levels with most of them concentrated towards the surface (top level is <1m depth).
The ENS (but not yet HRES) uses the atmosphere-wave-ocean coupling framework from the start of the forecast, since it is important to capture two-way feedback between the atmosphere and the sea-surface temperatures, sea ice extent and ocean waves (e.g. a slow-moving tropical cyclone can cool the sea surface).
The Numerical Structure
Whereas the atmospheric model covers the whole globe, the ocean model has the additional problem of lateral boundaries along coasts causing effects like boundary currents (example: the gulf stream). Also near the continents the sea depth becomes abruptly shallower towards coastal regions at the continental shelfs.
The vertical resolution varies with depth. There are 75 vertical ocean levels. The levels are closest together near the sea surface (18 levels in the first 50m, top level is approximately at 1m depth) to capture the temperature and salinity structure of the uppermost layers and the thermocline.
The ocean-atmosphere coupling is achieved by a two-way interaction: the atmosphere affects the ocean through its wind, heat and net exchange of moisture by precipitation and/or evaporation, whilst the ocean affects the atmosphere through its sea-surface temperature, ocean surface current and ice concentration. The ocean-atmosphere interaction is carried out every hour.
Handling of Sea Ice
Throughout the forecast period the changing extent of sea ice and the variation of the ice shelf with time have important effects upon the energy and moisture balance at the atmosphere/surface boundary. The Louvain-la-Neuve Sea Ice Model (LIM2) is a prognostic sea ice model that deals with the dynamic and thermodynamic evolution of the sea surface so that sea ice cover evolves dynamically. It is incorporated into the dynamic ocean model. The ice extent will change through the forecast period in response to sea and air temperatures, ocean currents and wind.
Three-Dimensional Data Assimilation - NEMOVAR
NEMOVAR is a three-dimensional variational assimilation (3D)-Var) system adapted to the NEMO model. Changes in the observed characteristics (temperature, salinity) are slow and a 3D-Var system is used since there is little need to fit observations to a precise time. It is designed to assimilate temperature and salinity profiles, sea-surface temperature (SST), and altimeter-derived sea level anomalies. Sea ice from ) is assimilated within NEMOVAR and combined with the ocean assimilation to provide a first guess for the next assimilation cycle and initial conditions for coupled model The ocean analysis system consists of a reanalysis stream (ORAS5) and a real-time stream (ORTS5).
Observed sea-surface temperatures are not assimilated directly but a strong relaxation towards the OSTIA (Operational Sea-Surface Temperature and Sea Ice Analysis) sea-surface temperature data is applied during the outer loops of the data assimilation cycle. Bathymetric observations are not used in regions where the total model depth is less than 500m in order to avoid assimilating data on the continental shelves where the model has poor representativeness.
The Ensemble of Data Assimilations for NEMO
It is important to represent uncertainty in the ocean initial conditions and in model structure. An oceanic EDA system achieves this. The perturbed analyses that result contribute through ocean-atmosphere coupling to the ensemble of forecasts used for probabilistic predictions at medium, monthly and seasonal ranges..
Additional Sources of Information
(Note: In older material there may be references to issues that have subsequently been addressed)
- For more information on the 75 ORCA ocean model levels (section2.3).
- Read an in-depth description of the NEMOVAR ocean data assimilation system.
- Read more on Coupling of NEMO and IFS.
- See Met Office OSTIA site