Boreal Summer Extratropical Intraseasonal Waves over the Eurasian Continent and Real-Time Monitoring Metrics

Abstract

Abstract Boreal summer extratropical intraseasonal oscillation (EISO) is crucial in modulating regional subseasonal variation and particularly causing extreme meteorological events, but it has yet to be well clarified and operationally monitored. This study first objectively sorts out three dominant EISOs trapped along two extratropical westerly jet streams over Eurasia, and then proposes the corresponding real-time metrics. The three dominant EISOs are (i) an 8–25-day eastward-propagating wave along the subtropical westerly jet (EISO-SJE) initiating at the exit of the North America–North Atlantic jet and strengthening over the Black Sea–Caspian Sea–arid central Asia region; (ii) a 10–30-day eastward-traveling wave along the polar front jet (EISO-PJE), starting near Scandinavia and enhancing from the East European Plain to the West Siberian Plain and then decaying over the Okhotsk region; (iii) a 10–40-day westward-migrating wave along the polar front jet (EISO-PJW), which enhances near the Ural Mountains and weakens over Scandinavia. The real-time metrics then, following the three EISOs, have been constructed, and they are able to capture the spatiotemporal features of three EISOs in application. Moreover, the close linkages between these EISOs and the regional extremes/the blocking occurrence have been clearly demonstrated, confirming the importance of real-time EISO metrics. Together with tropical intraseasonal oscillation, this study provides the subseasonal-to-seasonal (S2S) community with a well-portrayed unified picture of extratropical intraseasonal waves and the real-time metrics for monitoring boreal summer intraseasonal signals over Eurasia and facilitate subseasonal predictions. Significance Statement Boreal summer extratropical intraseasonal oscillation (EISO) has drawn increasing attention owing to its importance in triggering extreme weather events and affecting regional subseasonal prediction. However, despite the urgent need of the subseasonal-to-seasonal (S2S) community, a comprehensive delineation of EISO diversity and real-time EISO monitoring remain the gap of knowledge. This study objectively sorts out and comprehensively clarifies three dominant EISOs trapped along two extratropical westerly jet streams over Eurasia. More importantly, the well-portrayed real-time EISO metrics are constructed based on three EISOs, which are applicable for operational real-time monitoring, subseasonal prediction, and model evaluation. This study stimulates an extratropical focus in the S2S community as a complementary component in addition to monitoring the MJO’s teleconnection to the mid- to high latitudes.