The Origin of Systematic Forecast Errors of Extreme 2020 East Asian Summer Monsoon Rainfall in GloSea5

This study examined the origin of the systematic underestimation of rainfall anomalies over East Asia during July-August 2020 in operational forecasts. Through partial nudging experiments, we found that the East Asian rainfall anomalies were successfully predicted in GloSea5 with corrected tropical sea surface temperature (SST) forcing. Once the observed SST is applied over the Indian Ocean and tropical central-eastern Pacific, a low-level anticyclonic anomaly over the subtropical western Pacific, which transports warm-moist air from the tropics to increase the East Asian precipitation, is well reproduced as observed. By further separating the SST into climatological and anomalous components, we revealed that the cold and dry mean state bias over the Indian Ocean and central-eastern Pacific is responsible for the weak anomalous atmospheric teleconnection patterns from the tropics to East Asia. This implies that correcting the model mean climatological fields can directly impact the operational seasonal forecast skill