Prolonged high-skill windows for subseasonal forecasting of surface air temperature over East Asia in summer 1993 and 1994

Although there is an increasing demand for subseasonal prediction, the skill of subseasonal forecasting is currently limited. Under specific oceanic and atmospheric conditions, the subseasonal forecast skill can reach a high level intermittently during a long period. Based on the subseasonal-to-seasonal (S2S) database, the forecast skill windows for surface air temperature (SAT) over East Asia are identified using the Pattern Correlation Coefficient (PCC). Two long-lasting windows stand out over the past 30 years, namely the cold summer of 1993 and the hot summer of 1994 in central and north-east East Asia. The two windows lasted about 2 months with high forecast skill in week-3 SAT, even in week-4 and week-5 SAT. The persistent large-scale oceanic and atmospheric climate anomalies were generally reinforcing in these two summers, providing windows of opportunity for high forecast skill. The combination of the tropical western Pacific sea surface temperature anomalies (SSTA), the Japan Sea-Kuroshio-Kuroshio Extension (K-KE) SSTA, and the North Atlantic SSTA favored the atmospheric teleconnection, resulting in the cold event in 1993 and the hot event in 1994. The North Atlantic Oscillation (NAO) stayed in its negative or positive phase persistently, contributing to the climate anomalies over East Asia. Climate model experiments with prescribed SST variations have demonstrated that the SSTA in three regions influences East Asia SAT. An index based on preceding SSTA in the three regions can be used to help identify whether the current forecast case is within the real-time forecast window. This enhances the practical application of the study and has a positive impact on real-time operational forecasting.