Tropical–Extratropical Interactions Associated with East Asian Cold Air Outbreaks. Part I: Interannual Variability

Abstract

Interannual variability of winter-mean East Asian cold air outbreaks (CAOs) and its relationship with the tropical climate system during 56 boreal winters (DJF) are investigated. The magnitude of CAO is quantified as winter-mean equatorward cold airmass (CAM) flux below 280-K potential temperature across the 45°N latitude. EOF analysis shows that the interannual variation of East Asian CAOs is attributed mainly to the contributions from western and eastern CAOs. In particular, the western and eastern CAOs tend to be remotely forced by La Niña and El Niño events, respectively. The western and eastern CAOs have distinct climate variability. The western CAO, which is enhanced under the climatic anomalies of high pressure over northern Eurasia and low pressure over the western North Pacific, causes negative CAM anomalies over northern Eurasia and positive ones over midlatitude East Asia. In the tropical region, the western CAO negatively correlates with the eastern Pacific and Indian Ocean SST, both of which enhance precipitation over the Maritime Continent. On the other hand, the eastern CAO is enhanced by the strong Aleutian low and results in positive CAM anomalies in the western North Pacific and substantial negative anomalies in western North America. The eastern CAO positively correlates with the tropical SST anomalies and accordingly precipitation anomalies over the central Pacific. ENSO influences western and eastern CAOs through upper and poleward Rossby wave trains excited by convective anomalies over the Maritime Continent and central Pacific, respectively.