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Dynamic Ocean Model - NEMO

Purpose

The dynamic ocean model used for medium-range and seasonal forecasts of ocean structure is the Nucleus for European Modelling of the Ocean (NEMO).  It is coupled with all IFS forecast models (medium range ENS, extended range ENS, Seasonal).  NEMO 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 Numerical Structure

Whereas the atmospheric model covers the whole globe, the ocean model has the additional problem of lateral boundaries along coasts causing effects such as boundary currents (e.g. the gulf stream).  Also near the continents the sea depth becomes abruptly more shallow at the continental shelfs.

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These data are assimilated by NEMOVAR.

 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 temperatures and air temperatures, ocean currents and wind.

Sea-surface temperature (SST) and ice concentration

Sea-surface temperature are initialised using:

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• Movement of ice (turquoise) in the northern Gulf of Bothnia  due to the winds.
• Steady rise of sea-surface temperatures in the Black Sea, and especially in the shallow waters of both the Sea of Azov and the northern Caspian Sea.  In the White Sea (east of Finland, top of plot) sea-ice cover is less than the climatological average for this time of year.  Using these plots, the user can assess where sea-ice cover is above/below average.

Three-Dimensional Data Assimilation  - NEMOVAR

NEMOVAR is a three-dimensional variational assimilation (3D-Var) system adapted to the NEMO model.  The observed characteristics (temperature, salinity) vary only slowly and a 3D-Var system is used since there is little need to fit observations to a precise time.  NEMOVAR assimilates:

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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.

Considerations

The impacts of differently-evolving distributions of sea surface temperature and ice cover should be considered when comparing different forecasts, even when they are from the same data time.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

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