Eurasian Summertime Intraseasonal Oscillation and its impact on compound hot-dry extremes

Mid- to high-latitude intraseasonal dynamics remain an underappreciated driver of summertime compound extremes over the Eurasian continent. Here, we identify the Eurasian Summertime Intraseasonal Oscillation (ESISO), a coherent propagating intraseasonal mode defined by the leading EOF pair of Eurasian summertime surface air temperature variability, using JRA-3Q reanalysis, and examine its future changes through large-ensemble climate simulations (d4PDF). The ESISO propagates southeastward from the Western Siberian Plain toward East Asia via Rossby wave energy dispersion, maintaining a predominantly equivalent barotropic structure. This propagation exerts striking phase-dependent control on compound hot-dry extremes across the Eurasian continent: the probability of simultaneously exceeding the 90th percentile thresholds of daily maximum temperature (TMX) and vapor pressure deficit (VPD) increases approximately 20-fold over East Asia during warm phases compared to cold phases (15.8% vs 0.8%). Under +4 K warming, the fundamental spatial structure of the ESISO remains robust, while the frequency of intense events (amplitude > 2σ) increases significantly in most phases and the associated anomalies strengthen modestly, amplifying compound hot-dry extreme risk along the propagation pathway. These results establish the ESISO as a distinct mid- to high-latitude contributor to Eurasian compound extremes that complements existing tropical and subtropical forcing frameworks, and as a key dynamical contributor with implications for subseasonal prediction of heat-drought co-occurrences and climate risk management across the Eurasian continent.