Biases and teleconnections in the Met Office Global Coupled Model version 5.0 (GC5) – insights for seasonal prediction and Australia

Abstract

The Australian Bureau of Meteorology (The Bureau) has been involved in the package testing and assessment process of the UK Met Office Global Coupled Model Version 5.0 (GC5) configuration. GC5 will underpin the Met Office’s next seasonal prediction system, global coupled numerical weather prediction (NWP) system and Earth System Model. It will also likely be the next version of The Bureau’s seasonal prediction system, and the version to replace the global atmosphere-only NWP system to be the first global coupled NWP system at The Bureau. The GC5 configuration includes a new sea-ice model and substantial updates to almost all areas of model physics. We have evaluated the present-day climate simulation, and compared it to observations and with previous versions GC4 and GC2. Our assessment focuses on the climate mean state and variabilities relevant to Australian seasonal prediction, including the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Southern Annular Mode and the Madden–Julian Oscillation. Notably, in comparison to its predecessor (GC4), GC5 shows significant improvements in the eastern Pacific mean state but a slight degradation in the Indian Ocean in terms of the mean state and variability. These and other results provide us with early insights of the potential performance of the next sub-seasonal or seasonal forecast system. Longstanding issues in the seasonal prediction system associated with the equatorial eastern Indian Ocean biases and an overactive ENSO and IOD will likely remain; however, improvements over the eastern equatorial Pacific in GC5 hold promise of improved prediction skill of ENSO and its teleconnections.