Clustering of Tibetan Plateau vortices by 10-30-day intraseasonal oscillation

Tibetan Plateau (TP) vortices and the related 10-30-day intraseasonal oscillation in May-September 1998 are analyzed using the twice-daily 500-hPa synoptic weather maps, multiple reanalysis datasets, and satelliteretrieved brightness temperature. During the analysis period, distinctively active and suppressed periods of TP vortices genesis are noticed. In 1998, nine active periods of TP vortices occurred, which were largely clustered by the cyclonic circulations associated with the intraseasonal oscillation of 500-hPa relative vorticity. In addition to the well-recognized 30-60-day oscillation, the clustering of TP vorticity in the 1998 summer are more likely modulated by the 10-30-day oscillation, because all active periods of TP vortices fall into the positive phase of the 10-30-day oscillation in 1998. Even in the negative (i.e., anticyclonic) phases of the 30-60-day oscillation, the positive (i.e., cyclonic) 500-hPa 10-30-day oscillation can excite the clustering of TP vortices. This result indicates that the 10-30-day oscillation more directly modulates the activities of TP vortices by providing a favorable (unfavorable) cyclonic (anticyclonic) environment. The analysis of the 10-30-day atmospheric oscillation suggests that the westerly trough disturbances, in conjunction with convective instability due to low-level warm advection from the Indianmonsoon region, are important in the clustering of TP vortex activities. In particular, the moisture flux from the southwest boundary of TP is essential to the accumulation of convective energy. Thus, a better understanding and prediction of the 10-30-day intraseasonal oscillation is needed to advance the extended-range forecasting of TP vortices and their downstream impacts on the weather and climate over East Asia.