Characteristics of East Asian Summer Weather Regimes and Their Predictability in GloSea6 Hindcasts

Abstract Weather regimes (WRs) provide a useful framework for organizing large-scale circulation variability and diagnosing subseasonal-to-seasonal predictability. Yet they remain poorly characterized during the East Asian peak summer. Using daily 500-hPa geopotential height anomalies, we identify five recurrent WRs over East Asia. Specifically, WR1 and WR2 are linked to extratropical wave-train activity over Eurasia and high latitudes, WR3 and WR4 resemble opposite phases of the Pacific-Japan-like variability and are closely tied to tropical sea surface temperature (SST) variability, and WR5 represents a distinct midlatitude circulation state. Each WR is associated with coherent but regime-specific surface anomalies, with WR1 and WR2 showing pronounced temperature responses across much of the domain and WR3 and WR4 exhibiting weaker temperature signals but stronger precipitation related to western North Pacific monsoon variability. Operational hindcasts show that forecast skill declines rapidly after about two weeks but differs markedly among regimes. SST-linked WR3 and WR4 retain detectable skill into weeks 3-4, whereas extratropical WR1 and WR2 lose skill more rapidly. At longer leads, the model increasingly overestimates WR3 and WR4, revealing systematic drift in regime statistics and linking regime origin to both extended-range predictability and model bias over East Asia.