Global Seasonal Forecasting System (GloSea) version 5 was implemented operationally in January 2014, and upgraded to the current version GloSea6-Global Couple (GC) 3.2 in February 2022, which is joint seasonal forecasting system between KMA and UK Met Office. Operational data are shared between both centres in real-time.

GloSea is an ensemble prediction system that is fully coupled with atmosphere, land, surface, ocean, and sea ice components. GloSea6-GC3.2 consists of the Unified Model (UM) Global Atmosphere (GA), version 7.2, for the atmosphere, Nucleus for European Modeling of the Ocean (NEMO) Global Ocean (GO), version 6.0, for the ocean, the Joint U.K. Land Environment Simulator (JULES Global Land (GL), version 8.0, for land, and the Los Alamos Sea Ice Model (CICE) Global Sea ice (GSI), version 8.1, for sea ice. These component models are coupled using the OASIS, version 3, coupler. The horizontal and vertical resolutions of GloSea6 for the atmosphere are N216 (0.556 latitude × 0.833 longitude degrees, approximately; 60 km) and L85 (85 vertical levels capped at 85 km above sea level), respectively. In the ocean model, resolution is 0.258 except for near the equator (20°S–20° N degrees; 1/ 3 degrees), and it has 75 vertical levels.

Two members run out to 240 days (for seasonal forecast) and two members run out to 72 days(for intra-seasonal forecast). The first 72 days of all forecast members (four members) are used for sub-seasonal products.

ECMWF ERA-Interim data are used to initialize the atmosphere and land surface in the hindcast members. Also, NEMOVAR for ocean and sea-ice is used for hindcast. The hindcast period is 24 years from 1993 to 2016 and the hindcasts are initialized 4 times a month (1, 9, 17 and 25 in each calendar month). Hindcast set consists of three members per start date per year.

Hindcasts of 24-yr (1993–2016) ensemble predictions starting on the 1st, 9th, 17th, and 25th of each month and integrated for 7 months. Each starting day of the hindcasts has three ensembles perturbed by the stochastic kinetic energy backscatter scheme, version 2 (SKEB2) and stochastic perturbation of tendencies scheme (SPT).

The initial atmospheric conditions for the forecast members are generated by the KMA’s operational numerical weather prediction (NWP) 4D‐Var data assimilation system (00Z). The ocean and sea‐ice initial conditions are taken from KMA’s Global Ocean forecasting and Data Assimilation System(GODAPS).

Reanalyses from the European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA‐Interim project are used to initialize the atmosphere in the hindcast members. The soil moisture in both the forecast and hindcast are initialised from ERA5 reanalysis using the JULES land surface model.