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Table of Contents

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 (HRES*, ENS, Extended-range and Seasonal forecast modelsmedium 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 decreases abruptly over the continental shelfsshelf.

The ocean-atmosphere coupling is carried out every hour and 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
  • the ocean affects the atmosphere through its sea-surface temperature, ocean surface current and ice concentration.

The HRES and ENS use medium range ENS uses the atmosphere-wave-ocean coupling framework from the start of the forecast.  This is because 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).  

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Additionally Airborne eXpendable BathyThermographs Bathy Thermographs (AXBTs) are small floats or buoys that are dropped to the sea surface on parachutes. Once afloat, these instruments deploy long wires with temperature sensors, transmitting ocean temperature versus depth as the sensors sink through the water column to about 500m.  AXBT data is frequently used to increase knowledge of the ocean structure along the forecast track of a tropical storm and is an important factor in determining the intensity a hurricane may reach.

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:

  • analyses received daily from the Met Office (OSTIA, 5 km resolution).
  • NEMO, and the LIM2 subprogram within it.  

NEMO and LIM2 forecast changes in the sea-surface temperature (SST) and sea ice evolution.  These are used interactively by all IFS atmospheric models.  HRES Medium range ENS and extended range ENS use the same initial ice extent.  See also remarks on water surface temperature and sea ice. 

Note: ECMWF uses LIM2 which is an earlier version of the Louvain-la-Neuve sea ice model that is currently available (Version 3.6, LIM3)    


Fig2.3-1.11: Sequence of sea-ice and sea-surface temperatures from the ENS CTRL run data time 00UTC 27 April 2017.  T+0hr (00UTC 27 April 17), T+120hr (00UTC 02 May 17),  T+240hr (00UTC 07 May 17), and  T+360hr (00UTC 12 May 17).  On such plots the climatological average sea ice cover is shown in pink (contour and stippling, for >50%), just discernible in the northern Gulf of Bothnia and in the White Sea.   Dark purple areas (SST between 0C and -2C) are prone to ice formation if not already in existence.   Areas of sea ice are shown as turquoise. 

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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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The Operational Sea Surface Temperature and Sea-Ice Analysis (OSTIA) provides sea-ice information.  This is combined with the NEMOVAR ocean assimilation to give initial conditions for coupled model and also to provide a first guess for the next NEMOVAR assimilation cycle.   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 sea-surface temperature data is applied during the outer loops of the data assimilation cycle.  

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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 SST distributions of sea surface temperature and ice cover distributions 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)