Variation of the East Asian Monsoon and the tropospheric biennial oscillation

Study of the tropospheric biennial oscillation (TBO) has attracted significant interest since the 1980s. However, the mechanism that drives this process is still unclear. In the present study, ECMWF daily data were applied to evaluate variation of the East Asian monsoon and its relationship to the TBO. First, the general East Asian monsoon index (EAMI) was delineated on the basis of a selected area using the 850 hPa u and v components. This new index may describe not only the characteristics of summer monsoons, but also the features of winter monsoons, which is crucial to understand the transition process between summer and winter monsoons. The following analysis of EAMI shows that there is a close relationship between summer and winter monsoons. In general, strong East Asian winter monsoons are followed by strong East Asian summer monsoons, and weak winter monsoons lead to weak summer monsoons. While strong (weak) summer monsoons followed by weak (strong) winter monsoons form a kind of 2-year cycle, which may be the possible mechanism leading to the TBO over the East Asian region.