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.

Strong Linkage of El Niño–Southern Oscillation to the Polar Cold Air Mass in the Northern Hemisphere

“…In the central North Pacific, equatorward flux anomalies appear (Figures 2b and 2d). This signature was observed in a previous study that showed that El Niño enhances the eastern part of the East Asian stream (Abdillah et al, 2017). The anomalous equatorward flow extends to midlatitudes and ultimately represents the increase in zonal-mean cold air outflows at the critical latitude of two-box model (Table 1).…”

“…In the east of the cyclonic anomaly center, poleward flux anomalies emerge that span from off the coast of western North America to inland North America. In the east of the Rocky Mountains, the poleward flux anomalies are situated over the climatological North American stream and indicates the decrease in equatorward advection of the PCAM which eventually causes a reduction in the PCAM amount (Abdillah et al, ; Kanno et al, ). In Alaska, westward flux anomalies prevail and also cause a significant PCAM reduction.…”

“…This reduction is consistent with the general agreement stating that El Niño causes warm winters or weak winter monsoons in East Asia (Chang et al, ). A detailed discussion on the impact of ENSO on the East Asian PCAM is documented in Abdillah et al (). The Rossby wave train excited by ENSO‐induced convection anomalies over the Maritime Continent/western Pacific plays a role in constructing the circulation anomalies over midlatitude East Asia (Abdillah et al, ; Zheng et al, ).…”

“…Previous studies reported that the distribution of the PCAM could be remotely controlled by variability in the tropics at both interannual (Abdillah et al, 2017;Kanno et al, 2017) and intraseasonal (Abdillah et al, 2018) time scales. The mechanisms appear to rely on the development of a poleward Rossby wave train excited by tropical convection anomalies (Hoskins & Karoly, 1981;Sardeshmukh & Hoskins, 1988;Trenberth et al, 1998).…”

“…The mechanisms appear to rely on the development of a poleward Rossby wave train excited by tropical convection anomalies (Hoskins & Karoly, 1981;Sardeshmukh & Hoskins, 1988;Trenberth et al, 1998). Circulation anomalies associated with the wave train affect PCAM streams and have implications on the changes in cold air outbreaks and PCAM distributions (Abdillah et al, 2017(Abdillah et al, , 2018. However, these previous studies are limited to regional PCAM patterns.…”