What's the story?

We've seen the IPCC Headlines for the global picture but what's going on with the South Asian monsoon, and more importantly why? But first a bit of science...

Figure 1 - (IPCC AR5: Chapter 12)

Much of the monsoon response is explained by basic climate dynamics - global precipitation and global temperature exhibit a fairly linear relationship (IPCC AR5; Figure 1). This is very much governed by the Clausius Clapeyron relation, whereby the water-holding capacity of the air is increased by ~8% per °C increase in temperature, but is somewhat limited by the changes in the net radiative cooling rate with the troposphere (AR5). Furthermore, the gradual increase of CO₂ over time will cause an increase in both temperature and water vapour, and thus increase precipitation (Held and Soden, 2006 in AR5).

With regards to the South Asian monsoon, the tropical Indian Ocean SSTs (sea surface temperature) are projected to rise and the temperature of the land to rise further still (aided by the shear amount of land in the northern hemisphere), creating a larger temperature gradient. This leads to increased evaporation and enhanced moisture flux from the ocean to the land, increasing precipitation (Liepert and Previdi, 2012) despite the circulation of the South Asian monsoon weakening. Chung and Ramanathan (2006) have linked this weakening to a trend whereby the SSTs of the equatorial Indian Ocean have warmed but those of the northern Indian Ocean have not. This is evidenced with reduction in the meridional SST gradient, causing the monsoon circulation to weaken, and leading to a re-distribution of rainfall within the South Asian monsoon. Figure 2 from the IPCC report summarises what is going on rather nicely and makes linkages between factors more visible.

Figure 2 - (a) and (b) are fairly self explanatory but (c) is the water vapour flux convergence in the lower troposphere and (d) is the convergence of winds in the lower troposphere. RCP 2.6 = dark blue line, RCP 4.5 = light blue line, RCP 6.0 = orange line and RCP 8.5 = red line (IPCC AR5: Chapter 14). 

Another major change to the South Asian monsoon is the increase of extreme rainfall events and the decrease of weaker events (Goswami et al., 2006), and CMIP5 models also suggest an earlier onset and later retreat leading to a lengthening on the monsoon (AR5). Hopefully this has given you an insight into the basic science behind the anthropogenic influence upon the South Asian monsoon. Over the next few posts I'll be exploring the drivers in more detail and their manifestations...