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Based on the 1961-2010 NCEP/NCAR reanalysis, this work uses empirical orthogonal function (EOF) and composite analysis to study the distributions of zonal land-sea thermal contrast between Asia and the Pacific during transitions from the summer monsoon to the winter monsoon in East Asian subtropics, and investigates the interannual variations of the thermal contrast and their relationships with circulation systems over the East Asian subtropics. The findings are as follows. 1) In autumn, the interannual variations of the temperature deviation in the middle and upper troposphere show significant east-west out-of-phase teleconnection over Asia and the central and eastern Pacific, i.e. the Asian-Pacific Oscillation, or APO. 2) While not as significant as in summer with regard to coverage and intensity, the APO shows interannual variations in autumn that well depicts the change in the intensity of the subtropical monsoon. In the high (low) APO year, the current subtropical summer monsoon is strong (weak) and the winter monsoon is weak (strong) in East Asia as derived from the general circulation and wind field of the East Asian-Pacific region. [ABSTRACT FROM AUTHOR]