Effect of global and regional SST biases on the East Asian Summer Monsoon in the MetUM GA7 and GC3 configurations

Abstract Climate-length experiments of the Met Office Unified Model Global Atmosphere 7.0 (GA7) and Global Coupled 3.0 (GC3) configurations are evaluated against observations and reanalyses for the simulation of the East Asian summer monsoon (EASM). The results show systematic model biases, such as overestimated rainfall over southern China and underestimated rainfall over northern China, suggesting a monsoon that does not penetrate northward enough. We evaluate the effects on the EASM of regional errors in sea-surface temperature (SST) conditions in three regions: the Pacific, the Indian, and the Atlantic Oceans. The global SST biases in GC3 configuration substantially shift the EASM seasonal cycle: a late northward progression of the EASM in the early/mid-monsoon season, and an early retreat of the monsoon that also reduces rainfall over most of northern China. The EASM seasonal rainfall bias in the EASM region is linked to changes in the locations and strength of the western North Pacific subtropical high, which is associated with biases in local evaporation and moisture transport towards South China. GC3 biases in the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) teleconnection pathways also influence the EASM biases. GC3 biases weaken the ENSO teleconnection to the EASM and cause a strong dry bias in southeast China during developing El Niño.