Summer rainfall variability in low-latitude highlands of China and subtropical Indian Ocean dipole.

摘要

Based on reanalysis and observational datasets, this study proposes a reasonable mechanism for summer rainfall variations over the low-latitude highlands (LLH) of China, in which a subtropical Indian Ocean dipole (SIOD)-like pattern is the key external thermal forcing. In summers with a positive SIOD-like pattern, sea surface temperature (SST) anomalies may lead to lower-tropospheric divergence over the tropical Indian Ocean and convergence over the subtropical southwestern Indian Ocean and Arabian Sea. The convergence over the Arabian Sea can induce easterly anomalies of the divergent wind component off the eastern coast of the Bay of Bengal (BOB), while the divergence over the tropical Indian Ocean can change the interhemispheric vertical circulation and produce a descending motion over the same area and cyclonic anomalies in the rotational wind component over the Indian peninsula. The combined effect of the divergent and rotational wind anomalies and enhanced interhemispheric vertical circulation facilitates easterly anomalies and weakens climatological water vapor flux to the northern BOB. Therefore, anomalous water vapor divergence and less precipitation are observed over the LLH. In summers with a negative SIOD-like pattern, the situation is approximately the same but with opposite polarity and a weaker role of the divergent wind component. Further analyses indicate that the summertime SIOD-like pattern can be traced to preceding seasons, especially in positive SIOD-like years. The SST-wind-evaporation feedback mechanism could account for maintenance of the SIOD-like pattern. These results provide efficient prediction potential for summer rainfall variations over the LLH.