New aspects on the interdecadal variability of East Asian winter monsoon

Analysis of interannual variability of surface air temperature in the East Asian region from 1960 to 2009 indicates that the East Asian winter monsoon (EAWM) significantly weakens after the mid-1980s. Both the Siberian high (SH) and the Aleutian low (AL) have dominant effects on the EAWM before the mid-1980s. Since the mid-1980s, influences of circulations associated with arctic oscillation (AO) and North Pacific oscillation (NPO)-like sea level pressure (SLP) pattern on the EAWM become important. Weakening of the SH and northeastward shifting of the deepened AL are distinct after the mid-1980s. The cold advection is interfered by the anomalous southwesterly, which is related to the positive phase of AO, in the SH region and north of the East Asian region. The change in the AL is linked to the enhanced NPO-like SLP pattern. The anomalous southeasterlies toward EA region related to the easterlies from the anomalous anticyclone due to the NPO-like SLP pattern give rise to the anomalous warm advection, inducing the weakening of the EAWM after the mid-1980s. Consequently, the effects of anomalous circulations associated with the positive AO phase, weakened SH, and enhanced NPO-like SLP pattern are responsible for the weakening of EAWM after the mid-1980s. Moreover, it is found that the most existing EAWM indices show the inadequacy in describing this weakening of EAWM in recent decades. Present study suggests that the suitable modification of EAWM indices is required to describe more accurately the interannual and interdecadal variations of the EAWM for the whole period.