The Development of the South Asian Summer Monsoon and the Intraseasonal Oscillation

Abstract

Fourteen years (1980–93) of National Aeronautics and Space Administration reanalysis data are used to document and study the variability in the development of the South Asian summer monsoon associated with the Intraseasonal Oscillation (ISO). The focus is on the coupling of the large-scale upper-level divergent circulation with the low-level southwesterlies and the associated developing regions of moisture convergence and precipitation, which serve to define the onset times of the various regions of the South Asian monsoon. The impact of the ISO on the development of the low-level southwesterlies is both local and remote, and depends on the strength and phasing of the ISO with the seasonal cycle. Of the 14 yr examined here, 6 showed a strong contribution to the northeastward progression and onset of the monsoon rains over India. In these cases, the ISO is initially (about 2 weeks prior to onset of rains over India) out of phase with, and therefore suppresses, the seasonal development of the regions of large-scale rising and sinking motion. As the ISO moves to the northeast, the rising branch enters the Indian Ocean and acts to enhance the latent heating in the region of the emerging Somali jet. At low levels the response takes the form of an anticyclonic circulation anomaly over the Arabian Sea, and a cyclonic circulation anomaly to the south, which acts to inhibit the eastward progression of the Somali jet. As the ISO moves in phase with and enhances the seasonal mean upper-level divergent circulation, there is an abrupt and intense development of the southwesterly winds leading to an unusually rapid northeast shift and intensification of the monsoon rains over India and the Bay of Bengal. The general northeast progression of the anomalies may be viewed as an initial suppression and then acceleration of the "normal" seasonal cycle of the monsoon.