Prediction of monsoon changes in the coming decades is important for infrastructure planning and sustainable economic development. The decadal prediction involves both natural decadal variability and anthropogenic forcing. Hitherto, the causes of the decadal variability of Northern Hemisphere summer monsoon (NHSM) are largely unknown because the monsoons over Asia, West Africa, and North America have been studied primarily on a regional basis, which is unable to identify coherent decadal changes and the overriding controls on planetary scales.

Tropical cyclones over the Arabian Sea in the pre-monsoon season (May–June) have intensified since 1997 owing to significant reductions in storm-ambient vertical wind shear (VWS) in the troposphere; these reductions have decreased on average by about 3 m s−1 from the pre-1997 epoch (1979–1997) to the recent epoch (1998–2010)1. The authors attribute the reduction of pre-monsoon VWS to the dimming effects of increased anthropogenic black carbon and sulphate emissions.

In the past decade there has been extensive research into tropical intraseasonal variability, one of the major components of the low frequency variability of the general atmospheric circulation. This paper briefly reviews the state-of-the-art in this research area: the nature of the Madden-Julian Oscillation, its relation to monsoonal and extratropical circulations, and the current theoretical understandings.

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