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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.

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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:

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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.

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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.

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  • 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.  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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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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