1. Forecast system version

System name: GloSea6-GC5.1

First operational forecast run: 3 March 2026

2. Configuration of the forecast model

Is it a coupled model? Yes: Atmosphere, land, ocean and sea-ice.

Coupling frequency: Hourly coupling between atmosphere-land and ocean-sea-ice.

The coupled model Global Coupled 5.1 (GC5.1) is described in Xavier et al., 2023.

2.1 Atmosphere and land surface

Model	Met Office Unified Model (UM) - Global Atmosphere and Land 9.1
Horizontal resolution and grid	N216: 0.83 degrees x 0.56 degrees (approx 60km in mid-latitudes)
Atmosphere vertical resolution	85 levels
Top of atmosphere	85km
Soil levels (layers)	4 Layer 1 : 0 - 0.1 m Layer 2 : 0.1 - 0.35 m Layer 3 : 0.35 - 1.0 m Layer 4 : 1.0 - 3.0 m
Time step	15 minutes

Detailed documentation:

JULES documentation

2.2 Ocean and cryosphere

Ocean model	NEMO v4.0.4 - Global Ocean Sea Ice 9.0
Horizontal resolution	ORCA 0.25
Vertical resolution	L75
Time step	30 minutes
Sea ice model	SI³ - Global Ocean Sea Ice 9.0
Sea ice model resolution	ORCA 0.25
Sea ice model levels	5 categories + open water
Wave model	N/A
Wave model resolution	N/A

Detailed documentation: NEMO documentation

GOSI9: Guiavarc'h et al, 2025.

3. Boundary conditions - climate forcings

Greenhouse gases	CO₂, CH₄, N₂O, CFC12, HCFC134a - SSP2-4.5
Ozone	CMIP6. Zonal mean climatology 1993-2014.
Tropospheric aerosols	Derived from climatology of Model with full aerosol scheme
Volcanic aerosols	Backgroud aerosol
Solar forcing	CMIP6

Detailed documentation:

Eyring et al., 2016

4. Initialization and initial condition (IC) perturbations

4.1 Atmosphere and land

	Hindcast	Forecast
Atmosphere initialization	ERA-5	Met Office Global Hybrid 4D-VAR
Atmosphere IC perturbations	None	None
Soil moisture & temperature initialization	Met Office JULES-ERA5 analysis	Met Office JULES-ERA5 analysis
Snow initialization	Met Office JULES-ERA5 analysis	Met Office JULES-ERA5 analysis
Unperturbed control forecast?	No	No

Detailed documentation:

Met Office Global Hybrid 4D-VAR: Clayton et al, 2013

ERA5: Hersbach et al., 2020

4.2 Ocean and cryosphere

	Hindcast	Forecast
Ocean initialization	GloSea Ocean Sea-Ice Analysis (GS-OSIA)	Forecast Ocean Assimilation Model (FOAM)
Ocean IC perturbations	No	No
Unperturbed control forecast?	No	No

Detailed documentation:

The GS-OSIA and the FOAM system both use the Nucleus for European Modelling of the Ocean data assimilation system (NEMOVAR). This is a 3d-VAR data assimilation scheme. The GS-OSIA uses different surface forcing (ERA-5) and observation sets as it is a historical analysis. FOAM uses surface forcing from the Met Office Global NWP model and real-time observations.

The common NEMOVAR system is described in Blockley et al, 2014.

5. Model uncertainties perturbations:

Model dynamics perturbations	None
Model physics perturbations	Atmosphere stochastic physics scheme, SKEB2
If there is a control forecast, is it perturbed?	No control

Detailed documentation:

SKEB2: Bowler et al, 2009

6. Forecast system and hindcasts

Forecast frequency	daily
Forecast ensemble size	2 per day
Hindcast years	24 (1993-2016)
Hindcast ensemble size	7 per start date
Hindcast start dates	1, 9, 17, 25 of each month
On-the-fly or static hindcast set?	on-the-fly

7. Other relevant information

Interpolation details

Horizontal Interpolation

Atmosphere: area weighted regrid from N216 to 1x1 degree.

Ocean: uses a two-stage regrid.

Area weighted regrid fom ORCA025 grid to N216 grid (using same weights as used in model coupling)
Area weighted regrid from N216 to 1x1 degree

Vertical interpolation

Atmosphere: linear interpolation from the model hybrid height grid onto pressure levels.

8. Where to find more information

GloSea system:

Model description references:

Initialisation references:

Forcing references:

Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization. Geoscientific Model Development, 9, 1937–1958, https://doi.org/10.5194/gmd-9-1937-2016.