On the basis of surface data and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, the characteristics of long-term variability of summer southeast and southwest monsoon frequency over East Asia for the recent 30 years are examined by correlation and composite analysis by dividing the surface wind into southeast and southwest monsoons. The results show that the frequency of southeast and southwest monsoons and the snow depth over the Tibetan Plateau (TP) experienced interdecadal transitions after 2000 such that the frequencies of southwest and southeast monsoons have enhanced, while the snow depth over the TP has decreased. The interdecadal decrease of the snow depth over the TP indicates warming of the TP and strengthening of the South Asia high. The latter induces a strong anticyclonic anomaly at the lower levels of the troposphere (18°-28°N, 108°-118°E) and increased southwest monsoon frequency in East Asia. Moreover, in the presence of moisture feedback, the TP warming induced an increase in precipitation in the Huaihe River basin, which would favor the strengthening of the Western Pacific subtropical high couplet through the Sverdrup vorticity balance, and the northward shift of the subtropical high is beneficial to the interdecadal increase in southeast monsoon frequency over East Asia.