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Abstract

The 2010 summer monsoon was marked by a series of flood events over Pakistan and Northwest India and an intriguing aspect during this season was the strong westward shift of West Pacific Subtropical High (WPSH) by nearly 15° longitude relative to the climatology. This period also witnessed the evolution of an intense La Niña in the Pacific Ocean. La Niña episodes are generally characterized by wetter-than-normal monsoons over South Asia, although the situation was rather different in 2010. Large areas covering east-central India, the Bay of Bengal and extending eastward into the South China Sea and the Philippines received deficit monsoon rains during 2010, while rainfall activity was enhanced mostly over northern India and adjoining northwest Pakistan. By conducting a detailed diagnostic analysis of observed and reanalysis datasets, it is found that the anomalous westward shift of the WPSH and suppression of convection around the Bay of Bengal region were crucial in sustaining the rainfall activity over northwest India and Pakistan during 2010. The results suggest that the westward shift of the WPSH favoured the setting up of an anomalous trough extending over the sub-tropical areas near India and Pakistan, so that the anomalous transport of moisture from the Arabian Sea sustained the local rainfall activity. An analysis of SST records during the last 60+ years, shows a long-term background La Niña-like warming trend in the tropical Pacific. Based on the present results, it is suggested that generation of La Niña events in the backdrop of the SST warming trend have significant potential in influencing the rainfall activity over the South Asian monsoon-trough region. © 2012 Royal Meteorological Society.