The capricious monsoon?

Capricious – adjective – changing according to no discernible rules; unpredictable: a capricious climate (Oxford English Dictionary)...or in other words, the South Asian monsoon...or is it? 

Several observational datasets exhibit a reduction in South Asian monsoonal rainfall since the 1950s, with this drying tendency particularly evident over central India (Annamalai et al., 2013). Although this drying trend could be attributable to the factors discussed in GHGs, Aerosols & Cookfires and Land-use & the monsoon, another cause in the form of anthropogenically driven SST warming has been proposed (Annamalai et al., 2013). This will be the focus of today's post and I'll be focussing on a couple of studies along the way...

Figure 1 - Running mean (31 year), using two datasets over June-
September (JJAS) or July-August (JA) (Annamalai et al., 2013) 

Annamalai et al. (2013) show a succession of ~30 year wet and dry periods that can be seen throughout the 20th century (Figure 1), and as such suggest that South Asia should have entered a wet phase in ~1990 -this has not been the case. They point towards the western Pacific SST warming, as the culprit for the reduced South Asian monsoon rainfall (this effect can be seen here - NOAA). They also note that despite Indian Ocean SST rising, sea level pressure (SLP) has increased, decreasing the monsoonal circulation and thus evaporative potential. The western Pacific exhibits low SLP and increased rainfall, and Annamalai et al. (2013) suggest that this will incite a Rossby wave that forces descending air westwards, drying South Asia. Additionally they note the eastward shifting trend of ENSO, which could further explain the pattern of reduced rainfall over the South Asian monsoon region. Turner and Annamalai (2012) note that by the end of the 21st century the South Asian summer monsoon will experience more rainfall, and as such Annamalai et al. (2013) propose that the monsoon is currently in a transient phase. 

El Niño–Southern Oscillation (ENSO) is entwined with the South Asian monsoon. The ENSO-monsoon relationship generally brings drought conditions during El Niño and flood conditions during La Niña; for example recent moderate El Niño events in 2002 and 2004 led to All India Rainfall (AIR) deficit of 19% and 13% respectively (Annamalai et al., 2007). Krishnamurthy and Krishnamurthy (2013) studied the Pacific Decadal Oscillation (PDO) and suggested that potentially it could influence ENSO (Figure 2) through:

Figure 2 - Scatter plot showing Niño 3.4 index vs PDO index. The col-
ours relate to the IMR (Indian Monsoon Rainfall) index. Higher 
IMR=higher rainfall (Krishnamurthy & Krishnamurthy, 2013)   

• Warm phase PDO + El Niño = Enhanced drought as they complement each other.
• Warm phase PDO + La Niña = No strong signature as they counteract each other.
• Cold phase PDO + El Niño = No strong signature as they counteract each other.
• Cold phase PDO + La Niña = Enhanced flooding as they complement each other.   

Krishnamurthy and Krishnamurthy (2013) also separated ENSO from PDO and found that the effects of ENSO cover the whole of India, whilst in isolation the effects of PDO are confined to north of 18°N. They propose that ENSO/PDO affect the monsoon through the Walker and Hadley Circulation, whereby low SLP anomalies in the North Pacific during winter, create an SST footprint in the subtropics that persists through to the following summer (Vimont et al., 2001). This causes the eastward propagation of the ascending limb of the Walker Cell, due to the enhanced westerly trade winds. A descending and ascending limb of the Walker Cell will then form over the Maritime Continent and equatorial Indian Ocean respectively. The Hadley Cell linked to the Indian Ocean, will thus ascend much further south in the Indian Ocean, rain out and then descend upon India causing a rainfall deficit (Krishnamurthy and Krishnamurthy, 2013). Annamalai et al. (2007) note that rather than being related to climate change, much of ENSO appears to be spontaneous. However, there may be predictability of the mean monsoon and its interannual variability due to the slowly varying boundary conditions associated with ENSO (Annamalai et al., 2007). 

The effect of the oceans on the variability of the South Asian monsoon is clear from the evidence, and with no sign of anthropogenic emissions slowing, their influence upon the South Asian monsoon looks set to grow. Over the last few posts we've looked through the effects of aerosols, land-use and now the ocean, and hopefully you can now appreciate the difficulty in (a) identifying the cause for change in the monsoon, (b) attributing that change to anthropogenic influence and (c) creating models to form accurate projections. Further complicating this task are other variables interacting on intraseasonal and interseasonal time-scales; factors such as Madden-Julian Oscillation, Indian Ocean Dipole and Tropical Biennial Oscillation. Despite this, there are green shoots regarding projection but for now the monsoon remains somewhat of a mystery, albeit a little less capricious than before.  

Heavy science over...next up...the effects!