Understanding and predicting the summer monsoon in South Asia is important for more than a billion people since their lives depend on the four summer months (June to September) to bring 80% of annual rainfall. Knowledge of the timing, intensity and duration of the monsoon are important for agriculture and industry, while extreme rainfall can cause devastation to society.
Most future projections of monsoon rainfall from global models of the ocean and atmosphere suggest generally small increases (albeit not of the same size over the whole region). However a recent paper has suggested that the monsoon circulation has weakened during the last 50 years.
The low-level monsoon circulation in South Asia consists of a south-westerly flow bringing air laden with moisture from the southern Indian Ocean, across the equator and Arabian Sea to India and nearby countries. A weakening of this circulation can be understood in terms of an overall stabilization (slowing down) of the tropical circulation, which has been shown both generally and in the tropical east-west Walker Circulation. This is a simple consequence of mass conservation since increases in moisture held by the lower atmosphere accelerate beyond changes in global mean precipitation as the planet warms.
Since the monsoon circulation affecting South Asia is spread over such a large portion of the tropics, it is also expected to slow down under warming. But what does this mean for the finer details of regional precipitation? When the monsoon flow first hits India, it rises up over the Western Ghat mountains, dumping considerable precipitation there. Further east, there is a lull or rain shadow, over south-eastern India. Further downstream, the monsoon flow again hits mountains: this time the Arakan Range of Burma. It is not then unreasonable to think that if the horizontal flow incident upon these mountain ranges is reduced, then the resulting uplift and orographic precipitation will also reduce. A range of trends in observations of circulation and precipitation support this. In particular, negative rainfall trends are measured over the Western Ghats along the west coast of India (see Figure 1).
Figure 1: Spatial map of linear trend of rainfall rate for JJAS season based on the (left) APHRODITE (0.5° × 0.5° resolution), (right) IMD (1° × 1° resolution) daily gridded rainfall datasets. The units are mm/day/57years for the period 1951–2007. Copyright © Springer-Verlag, 2012.
It is only in this new study, where an ultra-high resolution global atmospheric model is used to make estimates of future monsoon rainfall, that future monsoon rainfall over the Western Ghat mountains also declines. The model used runs on a grid spacing of 20km, much finer than the 100-200km grid scale typical of other state-of-the art climate models. This allows it to better resolve the steep slopes of the narrow Western Ghats. But are its results believable and is this where the future will take the South Asian monsoon? Results from the coarser resolution models referred to in the IPCC reports show considerable uncertainty among the sign and pattern of rainfall changes over South Asia. While the model used here is at much higher resolution allowing the flow to better interact with orographic features, it lacks an ocean component and prevents important feedbacks between the monsoon circulation and sea surface temperatures. Ultimately the same result needs to be verified in a wider number of high-resolution models, with coupled ocean components.