The spatial and temporal structures of atmospheric circulation and SST patterns associated with the South Asian monsoon rainfall variability on quasi-biennial (2-3 year) and lower-frequency (3-7 year) time scales are investigated, using the Indian rainfall, the NCAR/NCEP reanalysis, and Reynolds SST data. The former is referred to as the monsoon-TBO (Tropospheric Biennial Oscillation) mode and the latter is regarded as the monsoon-LFV (Low-Frequency Variability) mode.

We took both time-filtering and composite analysis approaches. The observational analyses indicate that physical processes that determine the monsoon rainfall variation on the 2-3 year and 3-7 year time scales differ significantly. The biennial variability of the monsoon is primarily determined by local processes over the monsoon region. On the TBO time scale the Indian Ocean SST anomaly (SSTA) and local moisture flux anomaly are highly correlated with the monsoon at a leading time of 3-6 months. We argue that a positive SSTA in the Indian Ocean increases local moisture due to enhanced surface evaporation, and the accumulation of the moisture may lead to a strong summer monsoon through mean flow advection. Due to stable stratification prior to the monsoon onset, the convergent anomalous moisture can only be used to moisten local air; once the monsoon starts the excess moisture can enhance large-scale convection and cause a strong monsoon.

The low-frequency variability of the monsoon, on the other hand, is primarily attributed to remote SST forcing in the Pacific. Three mechanisms may contribute to the monsoon LFV. The first is the change of large-scale east-west circulation and vertical overturning induced by the Pacific SSTA, which can be readily seen from the velocity potential difference field. The second is attributed to the effect of the SSTA in the western Pacific, which induces anomalous cyclonic (or anticyclonic) circulation and enhances (or suppresses) convective activity along the northwest Pacific monsoon trough. The change of convective activity along the monsoon trough can further impact the strength of the Indian monsoon through northwestward propagating synoptic-scale waves. The third is the remote tropical forcing on midlatitude atmospheric circulation. A strong north-south land-ocean thermal contrast occurs 3-6 months prior to a wet monsoon, which persists from spring to summer and is responsible for summer mean circulation changes.