Recent Trends in the South Asian Monsoon

Changes to the SAM have gained a lot of attention in recent years, particularly as climate change science has developed. The ability to measure the monsoon is rather complex, but in terms of impacts on populations it is generally determined by precipitation patterns. Anthropogenic forcing’s, such as greenhouse gases, have been associated with recent changes. The changes to the amount of precipitation derived from the monsoon is illustrated in figure 1. In general there has been a decreasing trend in precipitation since the early to middle 1900s. The figure outlines the results from a series of models and recorded data, thus helping provide an evaluation of changes to the amount of precipitation.

Figure 1: Time series of mean summer (June-September) precipitation averaged over land points. AIR, CRU and IMD illustrate the observed data. The remainder of data is that of climate models. Of particular importance is the inset, which compares the recorded data. The faint thin black line illustrates the observations without smoothing. For those interested the modelled changes following the present day are the projection for an A1B scenario (2000-2100). The graph is from Turner and Annalamalai (2012).  

However, in truth, the amount of precipitation is not a fair reflection of changes to the monsoon. Much of society throughout Southern Asian not only rely on the amount of precipitation but the stability and consistency of rainfall. Many journals highlight how the total precipitation has only decreased slightly but the intensity of rainfall has become far more sporadic throughout the affected regions. Subsequently, leading to droughts in some areas and catastrophic flooding in others. As would be expected there is a degree of natural variability, especially at the decadal level, however, there is large concern over the stability of monsoon rainfall, particularly throughout India.  Turner and Slingo (2009) have produced a review journal of the extremes of precipitation and active-break cycles of the SAM. The journal provides a lengthy overview of multiple recorded and modelled studies that have highlighted the change to rainfall in terms of intensity. The authors focus particularly on active-break cycles; the period of rainfall, followed by a break in rainfall. Unlike previous records, different parts of the impacted region are experiencing greater active-break cycles, meaning some areas witness less active periods than before and others witnessing more, thus leading to extremes in climatic conditions. 

For a far more in depth analysis of changes to the monsoon see:

Turner, A. and Slingo, J. (2009) Subseasonal extremes in precipitation and active-break cycles of the Indian summer monsoon in a climate-change scenario. Quarterly Journal of the Royal Meteorological Society. 135: 549-567.

And

Turner, A. and Annamalai, H. (2012) Climate change and the South Asian summer monsoon. Nature. 2:587-595.