FASST: A Compact Stateless Atmosphere Model for Long-Term Ocean Coupling

Coupled climate simulations are computationally expensive, largely because of the atmosphere. We introduce FASST, a coupled climate model that retains the prognostic POP2 ocean while replacing the atmosphere and sea ice with lightweight U-Net emulators. The atmospheric emulator is diagnostic, predicting surface fluxes from the evolving ocean--ice state every 6 simulated hours, while the sea-ice emulator advances the ice state monthly. For 140 years of unforced integration, FASST maintains a stable global-mean climate without SST restoring, with post-transient global-mean SST of 291.30 +/- 0.22K compared with 291.25 +/- 0.10K in CESM2 piControl. FASST also captures interannual Niño3.4 variability, although with weaker amplitude than CESM (0.62 versus 1.02). These results demonstrate that a stateless learned atmosphere can sustain stable, free-running coupled climate simulations over century-scale integrations.