Another look at interannual-to-interdecadal variations of the east Asian winter monsoon: The northern and southern temperature modes

This study investigates the causes of interannual-to-interdecadal variability of the East Asian (EA; 0°-60°N, 100°-140°E) winter monsoon (EAWM) over the past 50 yr (1957-2006). The winter mean surface air temperature variations are dominated by two distinct principal modes that together account for 74% of the total temperature variance. The two modes have notably different circulation structures and sources of variability. The northern mode, characterized by a westward shift of the EA major trough and enhanced surface pressure over central Siberia, represents a cold winter in the northern EA resulting from cold-air intrusion from central Siberia. The southern mode, on the other hand, features a deepening EA trough and increased surface pressure over Mongolia, representing a cold winter south of 408N resulting from cold-air intrusion from western Mongolia. The cold northern mode is preceded by excessive autumn snow covers over southern Siberia-Mongolia, whereas the cold southern mode is preceded by development of La Niña episodes and reduced snow covers over northeast Siberia. These remarkably different spatiotemporal structures and origins are primarily associated with interannual variations. On the decadal or longer time scale their structures are somewhat similar and are preceded by similar autumn sea surface temperature anomalies over the North Atlantic and tropical Indian Ocean. The two modes found for the EA region also represent the winter temperature variability over the entire Asian continent. Thus, study of the predictability of the two modes may shed light on understanding the predictable dynamics of the Asian winter monsoon.