Decadal-Scale Variation of South Asian Summer Monsoon Onset and Its Relationship with the Pacific Decadal Oscillation

The variation of the summer monsoon onset over South Asia was investigated by using long-term data of the onset over Kerala, India, during the 64-yr period from 1948 to 2011. It was found that the onset over Kerala shows variation on a multidecadal scale. In early-onset years, the sea surface temperature (SST) anomaly over the northern Pacific Ocean was very similar to the negative Pacific decadal oscillation (PDO). The stationary wave train related to the negative PDO reaches into central Asia and generates a warm anomaly, thereby intensifying the land-sea thermal contrast, which promotes summer monsoon onset over South and Southeast Asia. The correlation between the onset date over Kerala and the PDO has strengthened since 1976. Analysis of zonal wind in the upper-level troposphere for the period 1958-2002 indicates that the change in the correlation is related to the change in the wave train path. The wave train propagating from the northern Pacific Ocean to western Russia could propagate eastward more easily in 1976-2002 than in 1958-75.