Contrasting seasonal roles of external forcing and internal variability in East Asian temperature

How to reliably separate external forcing (EXF) and internal variability (IV) components of regional temperature variation is of critical importance in East Asia. We implement here an EOF-based semi-empirical framework to avoid spurious teleconnections often present with conventional methods. By combining observations (1901–2020) with CESM2 large ensemble simulations, we obtain seasonally-contrasted results in summer versus winter, and EXF versus IV. It is shown that EXF drives uniform warming in summer but a north–south contrast in winter, while IV produces the opposite. EXF dominates summer variations (68%), but shares a comparable role with IV (57% against 43%) in winter. The widely reported winter Atlantic Multidecadal Oscillation influence is statistically unstable and sensitive to incomplete EXF extraction, indicating a fragile long-term relationship rather than a robust physical teleconnection. The true winter IV is robustly linked to the Interdecadal Pacific Oscillation and the Warm Arctic–Cold Continents pattern. These results raise new challenges for regional climate reconstruction and attribution.