An Integrated East Asian Winter Monsoon Index and Its Interannual Variability

The collaborative relationship among members of the East Asian winter monsoon (EAWM) system is first examined through the multivariate EOF (MV-EOF). Univariate EOF is then used to define the intensity index of each single system. Based on the above analyses, an integrated index of the East Asian winter monsoon (EAWMII) is defined. The new index exhibits distinct interannual variability and takes into account variations of Siberian high, East Asian trough, and meridional shear of upper-tropospheric zonal wind. Results show that the EAWMII can capture well the continuous weakening of the EAWM since the mid-1980s. It is also statistically significantly correlated with variations of both atmospheric circulation fields and surface temperature during winter. In addition, the EAWMII is closely related to the Arctic Oscillation (AO) index, the Nio3.4 sea surface temperature index, and the North Pacific Oscillation (NPO) index. The impact of AO on the East Asian surface temperature primarily occurs in the middle and high latitudes of Eurasia, Northeast China, and North Japan, etc. The influence of NPO is mainly registered in South China, East China, North Korea, South Korea, and South Japan. It is very likely that the AO affects the EAWM via the NPO.