Land-use & the monsoon

The monsoon circulation is primarily driven by large scale pressure gradients, created by the thermal disparity between the ocean and the land (Clift and Plumb, 2008:17). The South Asian monsoon should therefore be classified as a coupled atmosphere-ocean-land phenomena, with the land as influential as the ocean (Yasunari, 2007). Douglas et al. (2006) note that most studies have focussed on the larger scale dynamics of the monsoon, and as such relatively few studies have investigated the potential effects of regional land-use. With 18% of the world's population on just 2.4% of the global land surface, and the projection that India will become the world's most populous country by 2050 followed closely by China, the pressures on the environment are intensifying and any effect upon the South Asian monsoon could be increasing (Ray, 2011).

Although much of India had experienced deforestation and been transformed into agricultural land by 1700, significant agricultural expansion into the north-west around the Himalayan foothills began in 1940. Further intensification coincided with the significant change in farming practices, including the increased use of groundwater irrigation, with the Green Revolution of the 1960s (Roy et al., 2007). Douglas et al. (2006) looked at the influence of agricultural irrigation on evapotranspiration (vapour flux) and surface radiative balance and concluded that these can affect the local convection and rainfall. Furthermore, it was suggested that vapour flux and rainfall changes can influence monsoon and global circulations through teleconnections, if the components cover a large enough area. 

Figure 1 - GA analysis showing strong and negative
trends in rainfall (Niyogi et al., 2010)

Niyogi et al. (2010) employed an Empirical Orthogonal Function (EOF) approach to explore climatic trends within the South Asian monsoon region. They found that there been significant increases in summer rainfall over east India and reductions over north and north-west India over the last five decades. Interestingly two north-western states most affected by an intensification of irrigated agriculture since the 1960s, Punjab and Haryana, have experienced significant reductions in surface sensible heat flux leading to a reduction in rainfall (Douglas et al., 2006). Niyogi et al. (2010) performed additional Genetic Algorithm (GA) analysis, the use of which ties spatial patterns to strong temporal trends, which confirmed the same result (Figure 1). This trend has been linked to an increase in soil moisture and vegetation changes, both of which can effect the radiation balance, causing regional surface cooling (Niyogi et al., 2010) along with changing regional convection (Douglas et al., 2006). 

The culprit for the increase in soil moisture is the increased use of groundwater irrigation; the area utilised for paddy cultivation tripled during 1960-1990, with much of it occurring in the premonsoon season (up to 4 weeks earlier than in the past) due to the diminishing reliance on monsoon rains to kick-start the growing season. Bringing the growing season forward increases the normalized differential vegetation index at a crucial stage of monsoon circulation evolution. Through increased albedo, the intensity of the negative pressure anomaly can reduce and thus weaken the pressure gradient, influencing rainfall amount and distribution (Niyogi et al., 2010). Other factors such as urbanization could also influence the precipitation, however Kaufmann et al. (2007) demonstrate this effect reduces rainfall, but not during the summer monsoon as the mechanism overwhelms local urban effects. Arguably it is the intensification of agriculture and the associated surface feedbacks where the evidence is most compelling. Indeed, Douglas et al. (2006) suggest that monsoonal rainfall variability is regulated by antecedent land surface conditions. 

Due to the complexities of the South Asian monsoon system and the relatively youthful nature of this area of study, it is difficult to definitively attribute the changes in the monsoon to land-use change, although there is compelling evidence of its influence. Arguably, it is the role of land-use changes in exacerbating monsoonal effects, as opposed to their effect of upon the monsoon itself, that are of greatest importance. With the population increasing rapidly and the country getting no bigger, the proportion of the population living in 'at risk' areas may increase, and as a result the extent of our influence could become evermore significant. 

IPCC...A Swift Update

To understand both the current and projected changes of the South Asian monsoon and put them into context, it is helpful to get to grips with the whole picture. 

For those haven't found the time to trawl through the IPCC AR5 Summary For Policymakers, let alone all 2216 pages of the Full Report for the Working Group 1 contribution, the IPCC released a summary video a few days ago. Enjoy... 

GHGs, Aerosols & Cookfires

In "What's the story?" we explored the basic science of anthropogenic influence upon the South Asian monsoon, but the devil is very much in the detail, and so I'll be delving into the many whys and hows over the next few posts. So without much further ado...

Figure 1 - Aerosol-Cloud Interactions in (a) clean
 air and (b) polluted air (IPCC AR5:Chapter 7)

The concentration of Carbon Dioxide (CO_₂) has increased since ~1850 (Keeling Curve) and along with other greenhouse gases (GHGs) has driven surface warming and intensification of rainfall associated with the South Asian summer monsoon (Ueda et al., 2006). This relationship is not as simple as it seems due to aerosols, predominantly sulphate and black carbon borne from the continued industrialization of South Asia (Turner and Annamalai, 2012). Aerosols interact with the climate in two main ways; interaction with clouds (Figure 1) and interaction with sunlight (Figure 2). The total radiative forcing effect of aerosols is calculated to be -0.35 (-0.85 to +0.15) W m⁻² (IPCC AR5) and Ramanathan et al (2005) have suggested that aerosols may have masked up to 50% of surface warming from the increasing levels of GHGs. Accounting for up to 60% of black carbon emissions in Asia (UNEP, 2012) through the burning of biomass, the humble cookfire has stepped into the limelight and has recently undergone a technological make-over (BBC video). The reduction of black carbon emissions has been dubbed a saviour of glaciers (which will appear in a later post) in addition to saving lives (Ramanathan, 2013).

Figure 2 - Aerosol-Radiation Interactions. The left panels are
instantaneous & the right, overall effects (IPCC AR5:Chapter 7) 

The effects of aerosols upon the monsoon are hotly debated (Turner and Annamalai, 2012) with many opposing voices. Turner and Annamalai. (2012) suggest solar radiation could be limited by both the direct scattering effect of aerosols and by their increasing the albedo of clouds. A potential result of this is a reduction of the meridional temperature gradient, leading to a lower increase in rainfall than is expected. Ramanathan et al (2005) projected a decrease in Indian Ocean SSTs due to reduced solar radiation, resulting in decreased evaporation and thus rainfall; however Annamalai et al. (2012) note that SST over the Indo-Pacific warm pool have risen, enhancing moisture content, and making reduction in rainfall unlikely. Ueda et al. (2006) calculate the overall effect as increased monsoonal precipitation, due in the most part to the enhanced moisture transport, despite the influence of aerosols. Indeed, many coupled ocean-atmosphere models simulate this result with increases in GHG concentrations (AR5). 

Furthermore there is a suggestion that increased SSTs (Annamalai et al., 2012) along with aerosols (Bollasina et al., 2011) could cause geographic redistribution of monsoon rainfall, most notably a drying of Central India (Krishnamurphy et al., 2009 in AR5). Bollasina et al. (2011) demonstrated that along with the effect upon rainfall, aerosols have driven a weakening of monsoonal circulation. Further effects of aerosols include an increase of cloud burn-off due to increased cloud lifetime and potential aerosol driven tropospheric warming (Koch and Genio, 2010), though increases of cloud cover in some areas and an increase of extreme precipitation events (Goswami et al., 2006) suggest this effect is not a main driver. Levermann et al. (2009) noted that the South Asian monsoon has two stable states; a 'wet' state and a 'dry' state. It is the moisture-advection feedback that predominantly drives the monsoon circulation, and as such a change in radiative forcing that weakens the pressure gradient (Whats, Whys, Wheres & Hows), could prompt an abrupt transition from the current monsoon regime to one characterized by reduced precipitation. On the other hand, the effects of aerosols are somewhat constrained by those of increasing GHGs, meaning a switch in monsoon regime during the 21st century and beyond is unlikely (AR5). 

CMIP models simulate the annual precipitation and temperature cycles quite well for South Asia, but although they are improving (Sperber et al., 2012 in AR5), they are still not brilliant at simulating rainfall variability on regional and local-scales (Turner and Annamalai, 2012). One cause for this uncertainty within the models is driven by the gaps in our knowledge regarding the effects of aerosols, particularly regarding aerosol-cloud interactions, and this is a major stumbling block to our understanding the monsoon. Finding the point at which GHGs overcome the effects of aerosols will also further our understanding of the monsoon, and thus our ability to model it (Turner and Annamalai, 2012). Mankind has been running an unintentional experiment on one of the largest hydrological systems on the planet and so, if having read this you have more questions than when you started, you have likely understood the point of this post...uncertainty reigns for now.  

Procrastination on a global scale...

Procrastinate - verb - delay or postpone action; put off doing something: the temptation will be to procrastinate until the power struggle plays itself out. 
Origin: Late 16th century: From Latin procrastinat - 'deferred till the morning', from the verb procrastinare, from pro 'forward' and crastinus 'belonging to tomorrow' (Oxford English Dictionary)

...At the start of this blog I did mention that I liked the odd tangent or two, but I failed to tell you that these tangents are often the result of procrastination, as perfectly demonstrated above. This blog is largely focussed upon the South Asian monsoon, but it is impossible whilst researching not to be distracted by all of the other sub-systems within the global monsoon system. I've pulled together some key points from the AR5 for monsoons around the globe. As always feel free to ask if you have any burning questions...enjoy! 

NB: Click the 'YouTube Logo' on the bottom right of the video to see it in all its HD glory!

Video References: Image accessed here. Music - Am I Not Human? - Two Steps From Hell (accessed Nov 2013) 

Aside from the monsoon...

A quick aside...I came across this today and although unrelated to the monsoon, I thought it worthy of an appearance here...

Sea level rise - a pressing concern resulting from climate change, and now a visual reality thanks to an interactive map from the National Geographic...take a look! 

Rising Seas - Interactive: If All The Ice Melted

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... 

Monsoon fashion bonanza...

STEP RIGHT UP….STEP RIGHT UP….DON’T BE SHY!! Monsoon fashion must-haves thanks to Vogue India…

Brighten up your monsoon with some banana earrings from Prada or a pair of colourful flip flops. How about a Morinne cotton-twill cape from Diane Von Furstenburg and put down that drab black umbrella…try a nautical stripe bow umbrella from Accessorize! Don’t bother with a dress – you’ll just get muddy! Instead buy these CK shorts and why not throw in some crazy printed wellington boots by Burberry?! For the full experience feel free to find out more here…

Whilst researching for this week’s edition of the blog I stumbled upon this. For those of you who have ever met me you are probably aware of this already, but for those that haven’t, my idea of fashion stretches as far as what was (a) clean, and (b) closest to me at the time I was getting ready. Therefore finding this article amongst the doom and gloom of the many impacts of the South Asian monsoon offered some light relief I thought I ought to share. Unfortunately gentlemen, I'm yet to find any monsoon fashion advice for you, but to the women, you're welcome...

Pictures accessed here