Whats, Whys, Wheres & Hows…

Before delving any further into this blog, I thought it might be worth a quick review of the whats, whys, wheres and hows of the Asian monsoon...

The Asian monsoon system, sometimes referred to as the Asia-Australian monsoon, affects almost two-thirds of the world’s population and demonstrates the influence of Mother Nature on mankind (Clift and Plumb, 2008: vii). The South Asian monsoon is a sub-system of the Asian monsoon, affecting the Indian subcontinent and its surroundings, and is formed of two distinct periods; the South-west monsoon and North-east monsoon (retreating monsoon).

In its simplest form, the South-west monsoon is driven by seasonal cycles of solar heating whereby the land undergoes much more rapid warming than the adjoining seas, therefore developing a large scale temperature gradient (Turner and Annamalai, 2012). In the case of the South Asian monsoon, temperatures in the interior of the subcontinent regularly exceed 40°C, creating a north-south pressure gradient as shown in Figure 1.

Figure 1 - Formation of the Indian monsoon circulation with strong heating over Tibet in Summer, causing the rising of air that flows equator-wards aloft, which then causes moist air to be drawn in from the equator (Clift and Plumb, 2008: 17)

Coupled with this, the northern Indian Ocean, especially the Arabian Sea and the Bay of Bengal, SSTs (sea surface temperature) have warmed and can provide the moisture-laden winds required to drive the monsoon. Deep convection still does not occur though as the MSE (moist static energy) over the land surface is still less than that over the oceans. Once overcome, deep convection and rainfall occurs fairly rapidly (Figure 2) up to approximately 20° N and the South-West monsoon circulation is established.

Figure 2 - Evolution of the wind speed over the Arabian Sea throughout the year (day 1 is January 1st). The vertical lines represent the rapid onset of circulation between day number 142 and 160 (Clift and Plumb, 2008: 17) .

The temperatures in the subcontinent decrease somewhat and as a result cooler air, that is full of moisture, moves inland and the monsoon rain belt moves northwards (Figure 3).  This process contributes 80% of annual rainfall to South Asia between the four months of June-September (Turner and Annamalai, 2012). The North-west, or retreating, monsoon occurs around October when the winds reverse direction and is a much more gradual process than the onset of the monsoon and lasts to the end of November.

Figure 3 - Wind currents and ITCZ in the time frames for the onset of the Indian monsoon (Burroughs, 1999:138)

Orography of the Himalayas and Tibetan Plateau have a profound impact upon the monsoon and arguably have done so since the Indian and Asian plates collided around 45-55 Ma, forming the landscape we recognise today. Prior to this the monsoon would have existed but it is believed that the intensity would have been much lower, as Asia was both smaller and less elevated (Clift and Plumb, 2008: 29). Prell and Kutzbach (1992) note from General Circulation Modelling experiments, that  the elevation of Tibet-Himalaya has to be at least half of today's value to have a strong monsoon that is driven by solar forcing. The shear scale of the Himalayas means that they are able to disrupt atmospheric circulation and in the case of the South Asian monsoon, act as a barrier causing the uplift of moisture-laden air over a large depth of the troposphere and "anchoring the monsoon onset and intensity" (Turner and Annamalai, 2012). 

The take home message from all of the above...the Asian monsoon really is a "fully coupled ocean-land-atmosphere system" (Turner and Annamalai, 2012), and it is the interactions between processes on a global scale that make it one the the most dramatic phenomena on the planet. It's a highly sensitive system, both to short term variations such as ENSO/snow cover and by orbital forcing on even longer timescales (something I'll come back to another time!). Humanity has become a force perhaps as great as nature and I think that given that the phrase 'low confidence' appeared in the IPCC AR5 a total of 295 times, there is still much that is not fully understood and thus the consequences of our actions could be catastrophic. 

Today I'll finish off with this...

"For millennia, humans have behaved as rebels against a superpower we call "Nature." In the 20th century, however, new technologies, fossil fuels, and a fast-growing population resulted in a "Great Acceleration" of our own powers. Albeit clumsily, we are taking control of Nature's realm, from climate to DNA. We humans are becoming the dominant force for change on Earth. A long-held religious and philosophical idea - humans as the masters of the planet Earth - has turned into a stark reality. What we do now already affects the planet of the year 3000 or even 50,000." (Crutzen and Schwägerl, 2011).