Processes that Determine the Quasi-Biennial and Lower-Frequency Variability of the South Asian Monsoon

[摘要] The spatial and temporal structures of atmospheric circulation and SST associated with the Indian monsoon rainfall variability on quasi-biennial (2-3 year) and lower-frequency (3-7 year) time scales were investigated, using the domain-averaged Indian rainfall, the NCAR/NCEP reanalysis and Reynolds SST data. We took both time-filtering and composite analysis approaches. The results indicate that physical processes that determine the monsoon rainfall variation on the 2-3 year and 3-7 year time scales are different. The quasi-biennial variability of the monsoon is primarily determined by local processes in the Indian Ocean. Both local SST and moisture flux convergence anomalies are highly correlated with the monsoon at a lagged time of 3-6 months. It is argued that a positive SST anomaly in the Indian Ocean increases local moisture due to enhanced surface evaporation. The accumulation of these moistures leads to a strong monsoon through anomalous moisture advection by summer mean flows. The lower-frequency variability of the monsoon is primarily attributed to remote forcing mechanisms. Three possible processes may contribute to the monsoon variability on the 3-7 year time scale. The first is through the change of large-scale, east-west circulation induced by the eastern Pacific SST anomaly. The second is attributed to the effect of the SST anomaly in the Northwestern Pacific through enhanced (or suppressed) convective activity along the monsoon trough. The third is attributed to the tropicalmidlatitude teleconnection—a strong north-south, land-ocean thermal contrast occurs six months prior to a wet monsoon, and it persists from the preceding winter to summer and is responsible for the monsoon intensity change.

[学科分类] 大气科学