The northern and southern modes of East Asian winter monsoon and their relationships with El Nino-Southern Oscillation

摘要

The characteristics of the northern and southern modes of the East Asian winter monsoon (EAWM) and their relationships with El Nino-Southern Oscillation (ENSO) are further investigated in this paper using reanalysis data. The results demonstrate that the northern mode shows strong inter-decadal variations, whereas the southern mode is marked by noticeable inter-annual variations. The west-east pressure gradient plays a leading role in influencing northern mode, and both the west-east and north-south pressure gradients have significant impacts on the southern mode. The two modes are closely related, but their relationship shows inter-decadal variations. The relationship between the southern mode and ENSO is significant and stable, but the relationship between the northern mode and ENSO is unstable and shows obvious inter-decadal variations. The relationship between the northern mode and ENSO is significant before the late 1960s and after late 1990s, but it is not significant during the period between the late 1960s and late 1990s. ENSO influences the activity of northern mode mainly by changing the west-east pressure gradient. During the periods of a weak relationship between northern mode and ENSO, the variations in west-east pressure gradient may be dominated by the climate systems at mid-high latitudes, and the effects of ENSO are confined or offset. The inter-decadal variations in the relationship between northern mode and ENSO may be modulated by the Pacific Decadal Oscillation (PDO) and Atlantic Multi-decadal Oscillation (AMO), but their roles and importance have varied over time. The relationship between EAWM and ENSO and its inter-decadal variations vary with the EAWM index, which is induced by the differences between the northern and southern modes. When the EAWM index is more closely correlated with the southern (northern) mode, its relationship with ENSO is (not) significant and stable (unstable).