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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 (ensemble control forecast, medium range ensemble, sub-seasonal range ensemble, seasonal ensemble).  

The medium range ensemble 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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Throughout the forecast period, NEMO provides the oceanic temperature structure near and at the surface.   ECWAM provides wave data, and therefore an indication of surface roughness.   Full coupling, incorporating sea roughness effects, governs the fluxes of heat, moisture and momentum into the lowest layers of the atmosphere.  The formation, evolution and decay of ice over open waters is controlled by SI3 (part of NEMO).  In effect, NEMO and SI3 together move ice around (according to ocean drift etc.) and melt or form ice (according to sea-surface temperatures etc.). 

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The extent of sea-ice, and the variation of the ice shelf, have important effects upon the energy and moisture balance at the atmosphere/surface boundary.  This 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.  The varying areas of sea-ice diagnosed by sea-ice model SI³ are used to define albedo over sea-ice for each grid square.  Any snow on the ice surface is modelled as a single layer (unlike the four snow layers modelled over land).  Introduction of the sea-ice model SI³ reduces the warm bias seen in winter over the ice surface, especially in cloud free situations.

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NEMO forecasts changes in the sea-surface temperature (SST) and SI³ forecasts sea-ice evolution.  These are used interactively by all IFS atmospheric models.  Medium range ENS and sub-seasonal range ENS use the same initial ice extent.  See also remarks on water surface temperature and sea-ice. 

Fig2A.4-2: Sequence of sea-ice and sea-surface temperatures from the medium range ensemble control 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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(Note: In older material there may be references to issues that have subsequently been addressed)

See also Section on Atmosphere/Ocean coupling.

• For more information on the 75 ORCA ocean model levels.
• 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.
  
• An explanation of NEMO4 (para 1).
• An explanation of SI³ (para 6).

(FUG Associated associated with Cy50r1)