Seasonally Evolving Dominant Interannual Variability Modes of East Asian Climate*

A season-reliant empirical orthogonal function (S-EOF) analysis is applied to seasonal mean precipitation over East Asia for the period of 1979–2004. The first two dominant modes account for 44% of the total interannual variance, corresponding to post-ENSO and ENSO turnabout years, respectively. The first mode indicates that in El Niño decaying summer, an anomalous anticyclone appears over the western North Pacific (WNP). This anticyclone is associated with strong positive precipitation anomalies from central China to southern Japan. In the following fall, enhanced convection appears over the WNP as a result of the underlying warm SST anomalies caused by the increase of the shortwave radiative flux in the preceding summer. A dry condition appears over southeastern China. The anomalous precipitation pattern persists throughout the subsequent winter and spring. The second mode shows that during the El Niño developing summer the anomalous heating over the equatorial central Pacific forces a cyclonic vorticity over the WNP. This strengthens the WNP monsoon. Meanwhile, an anomalous anticyclone develops in the northern Indian Ocean and moves eastward to the South China Sea and the WNP in the subsequent fall and winter. This leads to the increase of precipitation over southeastern China. The anticyclone and precipitation anomalies are maintained in the following spring through local air–sea interactions. The diagnosis of upper-level velocity potential and midlevel vertical motion fields reveals a seasondependent Indian Ocean forcing scenario. The Indian Ocean basinwide warming during the El Niño mature winter and the subsequent spring does not have a significant impact on anomalous circulation in the WNP, because convection over the tropical Indian Ocean is suppressed by the remote forcing from the equatorial central-eastern Pacific. The basinwide warming plays an active role in impacting the WNP anomalous anticyclone during the ENSO decaying summer through atmospheric Kelvin waves or Hadley circulation.