Weakening of East Asian Winter Monsoon in the Middle of 1980s

Among the monsoons of the world, the Asian monsoon including the South Asian and East Asian monsoons is the most conspicuous due to the significant land-sea thermal contrast between the Asian continent and the adjacent Oceans. The East Asian region is influenced by the large low-level cyclonic and anticyclonic monsoon circulations in summer and winter. The edge of the East Asian coast is located between high pressure (i.e., Siberian high) and low pressure (i.e., Aleutian low). Therefore, the East Asian winter monsoon is highly related to the pressure gradient between these two major system. In the present study, based on the calculations of the sea level pressure provided by ERA-40 in 1958-2002, 4 major areas along the coast of the East Asia with large pressure gradient are identified. They are areas A (Sakhalin Island to Hokkaido), B1 (Korean peninsula), B2 (East China Sea to the north of Taiwan) and B3 (south of Yangtze River to South China Sea).

By calculating the meridional wind in these 4 regions, this study found that the intensities of the northerly wind in areas A and B2 are significantly weakened in 1985 based on the change-point analysis of the regime shift index. In area A, the winter monsoon weakened (according to the observations of low-level northerly wind and pressure gradient) due to weakening of the Siberian high and the eastward moving of the Aleutian low. Noted that the Aleutian low is strengthening during the same period. Therefore, the weakening of the Siberian high and the strengthening of the Aleutian low alone can not explain the cause of the weakening of the winter monsoon along area A. In the lower latitudes of the East Asian coast (B1, B2 and B3), their decadal variations are not directly influenced by the Siberian high and Aleutian low. The weakening of the winter monsoon systems in B2 area is possibly due to the strengthening of the Hadley cell which produces a downward branch at the latitude of B2 region which receives the influence from the lower latitudes in the tropics.

In summary, this study found that the weakening of the East Asian winter monsoon in areas A (Sakhalin Island to Hokkaido) and B2 (East China Sea to the north of Taiwan) in 1985. The cause of the weakening in area A is due to the weakening of the Siberian high and the eastward moving of the Aleutian low. On the other hand, the reason for the weakening of monsoon system in area B2 is mainly related to the strengthening of the Hadley cell which produces a downward branch at the area.