What? When? Where? Why?

To set the scene for this scientific blog I thought it would be appropriate to outline what the South Asian Monsoon is? When and where the monsoon occurs? And what the driving mechanisms of the monsoon are?

Fig1. Map showing average onset (Monsoon arrival) dates and
wind directions prevalent during the SAM.

The term ‘monsoon’ originated from the Arabic ‘mausim’ which means season. Ramage (1971) was the first to define and use the word ‘monsoon’ in terms of an annual reversal wind regime and a contrast between a rainy summer and a dry winter. Local inhabitants of monsoon-affected regions often refer to the monsoon as the rainy seasons as the it brings excessive rainfall over short periods from the beginning of June to end of September (see Fig. 1 and 2) with primarily dry conditions throughout the winter.  The focus of this blog is the South Asian Monsoon (SAM), which predominantly impacts the Indian subcontinent: Bangladesh, Bhutan, India, Nepal, Pakistan and Sri Lanka.

Fig 2. The average Indian rainfall. Over 80% occurs during the
monsoon season (June to September). 

The explanation of the SAM and many other monsoons are often over-simplified in many media articles due to the complexity of processes. The following explanation of the monsoon is from a June 2012 paper by A. Turner and H. Annamalai. As one of the most up to date journals in this field and as a review, it provides what I feel is a very detailed but an easily understandable description of the driving mechanisms.

At the most basic level, the seasonal cycle of solar heating through spring warms the land regions surrounding South and Southeast Asia faster than the adjoining oceans, owing to differences in heat capacity, and develops a large-scale Meridional surface temperature gradient. Subsequently, this forms a surface heat low over northern India in late spring; the north-south pressure gradient then induces a cross-equatorial surface flow and return flow aloft. The Himalayan and Tibetan Plateau ensure that sensible heating during spring occurs aloft, meaning that the large-scale Meridional temperature gradient exists not just at the surface but over significant depth in the troposphere, anchoring the monsoon onset, and intensity. The intense solar heating in late spring and summer gives thermodynamic conditions favouring the occurrence of convection poleward of the Equator, allowing the monsoon to be viewed as a seasonal migration of the Intertropical Convergence Zone. The north-northwest migration of winter convection from the equatorial region and its interaction with circulation leads to a positive feedback and deeper monsoon trough, enhancing the cross-equatorial flow in the lower troposphere that feeds moisture to the monsoon, as well as the Tibetan anticyclone and easterly jet with a return cross-equatorial flow at upper levels. The north-south-oriented East African Highlands anchor the low-level cross-equatorial flow and the Earth’s rotation aids in the formation of the low-level westerly jet as it approaches South Asia from across the Arabian Sea. The rapid intensification of rainfall and circulation during the onset can be attributed to wind-evaporation feedback as well as feedback between extratropical eddies and the tropical circulation.

The next blog shall look to provide some relevant statistics regarding the SAM, with a particular focus on recent changes to the monsoon. This shall provide a setting for a series of blogs about anthropogenic activities and whether or not these are the cause of changes to the monsoon.