ENSO Signals in the Interannual Variability of East-Asian Winter Monsoon. Part Ⅰ: Observed Data Analyses
Mu Mingquan and Li Chongyin(State Key Laboratory of Numerical Modeling for Atmospheric Science and Geophysical Fluid Dynamics,Institute Of Atmospheric physics, Chinese Academy Of Science, Beijing 100029)
The interannual variability of winter monsoon in East Asia is investigated in detailwith the observed global month mean data from 1950 to 1989. Analysis results clearly show thatthe anomalous activities of winter monsoon in East Asia have the obvious relationship with theoccurrence of ENSO (EI Nino, La Nina). In other words, winter monsoon in East Asia becomesweaker after most EI Nino events occurred, and becomes stronger after most La Nina events occurred. During the past 40 years, there are 10 EI Nino cases and 7 La Nina cases. Through composite analyses for the two kinds of cases, respectively, it is shown that there are positive departure of500 hpa geopotential height and negative anomaly of sea level pressure in East Asia, which meanEast--Asian winter monsoon becomes weaker in the EI Nino wintertime. The anomalous circulation situation is well analogous to the observed PNA pattern in the Aleutian and North Americanarea. In the La Nina wintertime, the signs of the anomalous circulation in the Northern Hemisphere, especially in East Asia, are opposite to those during the same period with EI Nino. In themeantime, the analyses of the observed data also display that East--Asian winter monsoon is verystrong (weak) before EI Nino (La Nina) events burst out. Analyses of power spectrum clearly showthat the interannual variation of winter monsoon has 3- 5 years and quasi--biennial cycles. In other words, ENSO signals are included in the interannual anomaly of winter monsoon in East Asia.