(a) The normalized time series of anomalous winter mean 2 m air temperature over East Asia (25–45°N, 105–145°E)
Figure 1. (a) The normalized time series of anomalous winter mean 2 m air temperature over East Asia (25–45°N, 105–145°E). Differences in air temperature (K) between AFT1985 and BEF1985 (b) at surface, (c) in latitude–height cross section averaged along 105–145°E, and (d) in longitude–height cross section averaged along 25–45°N. Contour intervals in (b) and ((c), (d)) are 0.5 K and 0.2 K, respectively. Shadings in (b), (c) and (d) represent the values significant at the 95% confidence level.
An analysis of the interannual variability of surface air temperature during the boreal winter in the East Asian (EA) region from 1960 to 2009 reveals that the East Asian winter monsoon (EAWM) significantly weakens after the mid-1980s. The robust warming over the EA region in the lower and middle troposphere as well as at the surface is caused mainly by changes in circulations over the North Pacific and Eurasian continent. The 300 hPa East Asian jet and 500 hPa trough over the EA region, which are closely linked to cold surges, significantly weaken after the mid-1980s. The weakened northerly wind in the Siberian high region and north of the EA region interfere with cold advection toward the EA region. The anomalous southeasterlies over the East China Sea due to an enhanced North Pacific oscillation (NPO)-like sea level pressure (SLP) pattern lead to anomalous warm advection over the EA region. It is also found that the advection of mean temperature by anomalous wind and the advection of anomalous temperature by mean wind mainly contribute to the anomalous warm advection in the EA region after the mid-1980s. Consequently, these anomalous circulations provide a more favorable environment for weakening of the EAWM.