Global ocean forecast - eddy permitting

MERCATOR-GLOBAL-PSY3V2-FOR

Ocean forecast

Gridded (analysis)

General information : The target MERSEA 1/4° global forecasting system uses the NEMO (Nucleus for European Models of the Ocean) modelling system which presently includes the latest version of OPA, coupled to the thermodynamic-dynamic sea ice model LIM2 (Louvain sea Ice Model 2). The configuration was built within the framework of the DRAKKAR (www.ifremer.fr/lpo/drakkar) work group.

As shown by Barnier, the combination of recent implementation of an energy-enstrophy conserving scheme for momentum advection with a partial steps1 representation of the bottom topography yields significant improvements in the mean circulation and in the representation of western boundary currents such as the Gulf Stream and the downstream flow of the North Atlantic Current. Moreover, the model solution is often comparable to solutions obtained at 1/6° or 1/10° resolution on some aspects concerning mean flow patterns and distribution of eddy kinetic energy. In order to better resolve the upper layers, the vertical grid (now 50 levels) has been refined at the surface ending with a discretization of 1m until 20m depth and of 500m for the bottom layers. With this surface refined-mesh, this new vertical grid has been designed to improve the circulation in the coastal shelves and to represent more adequately the impact of the atmospheric diurnal cycle, which is planned to be explicitly modelled in the near future.

The atmospheric forcing fields which will drive the future system are calculated using the empirical bulk parameterization described by Goosse et al. A systematic bias in the precipitations is removed thanks to GPCP observations when available, and for the recent and real time analyses and forecast the bias is removed thanks to a predictor computed from these observations .

The sea ice is fully comprehensive with the implementation of the LIM2 model. With sea ice concentration, sea ice/snow thicknesses, sea ice drift and sea ice thermal content prognosed by this multi-layer model based on the Semtner 3-layers and the Hibler visco-plastic formulations, forecasts will handle most of the processes linked to the sea ice lifecycle.

These improvements prepare another MERSEA step: the development of the future global forecasting system at eddy resolving resolution (1/12°). A North Atlantic basin version is in progress and interannual experiments including all the updates listed in the previous section have been already performed.

The data assimilation technique that will be used in the next version of the 1/4° global forecasting system is a multi-data and multivariate assimilation algorithm consisting of a Singular Extended Evolutive Kalman (SEEK) filter analysis method. The SEEK filter is a reduced-order Kalman filter introduced by Pham14 in the context of mesoscale ocean models. This method assimilates jointly satellite SLA and SST, and in situ profiles of temperature and salinity. The error statistics are represented in a sub-space spanned by a small number of dominant 3D error directions.

• downwelling_radiative_flux_in_sea_water,W m-2,CF
• eastward_sea_water_velocity,m s-1,CF
• northward_sea_water_velocity,m s-1,CF
• ocean_barotropic_streamfunction,m3 s-1,CF
• ocean_mixed_layer_thickness,m,CF
• sea_surface_height_above_geoid,m,CF
• sea_water_salinity,1e-3,CF
• sea_water_temperature,K,CF
• surface_downward_eastward_stress,Pa,CF
• surface_downward_northward_stress,Pa,CF
• surface_downward_water_flux,kg m-2 s-1,CF

Global scale

1/4 degree

from 0 to -4000 meters.