Seasonal Prediction Skill and Biases in GloSea5 Relating to the East Asia Winter Monsoon

The simulation and prediction of the climatology and interannual variability of the East Asia winter monsoon (EAWM), as well as the associated atmospheric circulation, was investigated using the hindcast data from Global Seasonal Forecast System version 5 (GloSea5), with a focus on the evolution of model bias among different forecast lead times. While GloSea5 reproduces the climatological means of large-scale circulation systems related to the EAWM well, systematic biases exist, including a cold bias for most of China’s mainland, especially for North and Northeast China. GloSea5 shows robust skill in predicting the EAWM intensity index two months ahead, which can be attributed to the performance in representing the leading modes of surface air temperature and associated background circulation. GloSea5 realistically reproduces the synergistic effect of El Niño–Southern Oscillation (ENSO) and the Arctic Oscillation (AO) on the EAWM, especially for the western North Pacific anticyclone (WNPAC). Compared with the North Pacific and North America, the representation of circulation anomalies over Eurasia is poor, especially for sea level pressure (SLP), which limits the prediction skill for surface air temperature over East Asia. The representation of SLP anomalies might be associated with the model performance in simulating the interaction between atmospheric circulations and underlying surface conditions.