System Overview

The NASA GMAO seasonal forecasts are produced with the Goddard Earth Observing System (GEOS) Atmosphere-Ocean General Circulation Model and Data Assimilation System Version S2S_2.1. The new system replaces version S2S-1_0 described in Borovikov et al (2017), and includes upgrades to many components of the system.

The atmospheric model includes an upgrade from a pre-MERRA-2 version running on the latitude-longitude grid at ~1 degree resolution to a current version running on the cubed sphere grid at approximately 1/2 degree resolution. The important developments are related to the dynamical core (Putman et al., 2011), the moist physics (“two-moment microphysics” of Barahona et al., 2014) and the cryosphere (Cullather et al., 2014). As in the previous GMAO S2S system, the land model is that of Koster et al (2000).

GMAO S2S-2_1 now includes the Goddard Chemistry Aerosol Radiation and Transport model (GOCART, Colarco et al., 2010) single moment interactive aerosol model that includes predictive aerosols including dust, sea salt and several species of carbon and sulfate. The ocean model includes an upgrade from MOM4 to MOM5 (Griffies 2012), and continues to be run on the tripolar grid at approximately 1/2 degree resolution in the tropics with 40 vertical levels. As in S2S Version 1, the sea ice model is from the Los Alamos Sea Ice model (CICE4, Hunke and Lipscomb 2010). The Ocean Data Assimilation System (ODAS) has been upgraded from the one described in Borovikov et al., 2017 to one that uses the LETKF (Local Ensemble Transform Kalman Filter, Penny, 2014), and so now assimilates along-track altimetry, and does a nudging to MERRA-2 SST and sea ice boundary conditions. The atmospheric data assimilation fields used to drive the ODAS have been upgraded from MERRA to a MERRA-2 like system. The system is initialized using MERRA-2 atmospheric reanalysis (Gelaro et al. 2017) and the GMAO S2S Ocean Analysis. Ensemble members are produced with initial states at 5-day intervals, with additional members at the end of the month based on perturbations of the atmospheric and ocean states.

Both subseasonal and seasonal forecasts are submitted to the National MultiModel Ensemble (NMME) project, and are part of the US/Canada multi-model seasonal forecasts. As part of our participation in NMME, a large suite of retrospective forecasts (“hindcasts”) have been completed, and contribute to the calculation of the model’s baseline climatology and drift, anomalies from which are the basis of the seasonal forecasts. All data collections from GMAO S2S-2_1 are provided on the same horizontal grid. This grid has 720 points in the longitudinal direction and 361 points in the latitudinal and are on the regular 0.5°×0.5° longitude-by-latitude grid.