Systematic variation in wintertime precipitation in East Asia by MJO-induced extratropical vertical motion

The variations in the wintertime precipitation over East Asia and the related large-scale circulation associated with the Madden-Julian oscillation (MJO) are examined. By analyzing the observed daily precipitation for the period 1974-2000, it is found that the MJO significantly modulates the distribution of precipitation over four East Asian countries; the precipitation rate difference between wet and dry periods over East Asia, when the centers of MJO convective activities are located over the Indian Ocean and western Pacific, respectively, reaches 3-4 mm day(-1), which corresponds to the climatological winter-mean value. Composite analysis with respect to the MJO suggests that the MJO-precipitation relation is mostly explained by the strong vertical motion anomalies near an entrance region of the East Asia upper-tropospheric jet and moisture supply in the lower troposphere. To elucidate different dynamic origins of the vertical motion generated by the MJO, diagnostic analysis of a generalized omega equation is adopted. It is revealed that about half of the vertical motion anomalies in East Asia are induced by the quasigeostrophic forcings by the MJO, while diabatic heating forcings explain a very small fraction, less than 10% of total anomalies.