Nice Weather For Ducks: The Asian Monsoon (Part 1)

Looking back over some of my previous posts, it became apparent that global warming and changes to the atmosphere composition are affecting the strength and circulation of the south Asian monsoon system. According to Oerlemans and Fortuin (1992), precipitation, radiation and air temperature are the most important factors determining glacier mass balance. Thus, the next two posts will focus on recent changes that have occurred to the Asian monsoon system variability and the possible consequences for Himalayan glaciers. Due to the complexity of this subject, I will split the topic into two posts, with this post focusing on recent changes to the Asian monsoon.

ASIAN MONSOON: THE BASICS

The word ‘monsoon’ is derived from an Arabic word meaning seasons (Goswani, 2005). The South Asian monsoon is part of an annually reversing wind system, characterised by the contrast between the Asian sub-continent and the Indian Ocean.

 As the temperature gradient increases in the summer (June-August), variation in land and sea heat capacities cause the atmospheric circulation to reverse advecting moisture-rich air masses northward from the Indian Ocean over India to the Himalayas (Figure 1). This causes south Asia’s wet season that provides most of the precipitation for snow accumulation to the Himalayan Mountains.

 Figure 1: Video describing seasonal changes to south Asia's monsoon.

SPATIAL AND TEMPORAL VARIABILITY

The Asian monsoon varies spatially and temporally on inter-annual, decadal and centennial timescales (Mayewski et al., 1980; Duan et al., 2006) Spatial variability occurs within a region - with a sharp decrease from south to north across the Himalayan region- and between monsoons depending on the strength of the mid-latitude westerlies and oscillations in the sub-tropical jet stream (SJS) which control the upper-most extent of the Asian monsoon (Benn and Owen, 1998).

Temporal variability is highly complex. It changes depending on the dynamic feedbacks that influence the land-ocean thermal gradient including insolation, snow albedo and atmospheric and surface temperature. Generally, the greater the temperature difference between the land and the ocean, the more intense the Asian monsoon, thus advecting more precipitation to higher latitudes which falls as snow in high Himalayan regions (Benn and Owen, 1998).

COMPLEXITIES

Kripalani et al., (2003) investigated the correlation between winter/spring snow estimates over the western Himalayas using satellite data between 1986-2000. The satellite images indicated a decrease in spring snow area and an increase snow melt rate from winter to spring (Feb-May) after 1993. More importantly, Kripalani et al., (2003) proposed that there were correlations between the snow cover, area and spring snow melt and the subsequent Indian Monsoon Rainfall (IMR) for that year. May snow cover was negatively related with subsequent summer monsoon rainfall, while February to May snow melt was found to have a positive correlation. This suggests a smaller snow area and faster snow melt is conducive for a strong monsoon across India (Kripalani et al., 2003). These findings were also supported by the conclusions of Prasad et al., (2009) from their general circulation model (GCM). The GCM indicated an inverse snow-monsoon relationship observed between monsoon rainfall and pre-monsoon (April) snowfall over the Tibetan Plateau (TP).  The weakening of the monsoon suggested by these studies is due to high snow-albedo, with a greater proportion of the incoming insolation being reflected back out into the atmosphere and hence less is absorbed to heat the land surface. Thus there is less of a temperature gradient between the land and the ocean inducing a weaker monsoon.

However, this contrasts with several studies that have observed a decrease in the strength of the monsoon in the latter part of the 20^th century coinciding with a decrease in snow extent in the Himalayas (Duan et al., 2006; Li et al., 2011). Duan et al., (2006) study of ice cores on the Dasuopi glacier in the Central Himalayas highlighted an inverse relationship between Northern Hemisphere temperature and the strength of the Asian monsoon (Table 1). Overall, the reconstructed correlation from the Dasoupi ice cores indicated that an average 0.1°C change in Northern Hemisphere temperature is associated with about 100± 10mm change in net snow balance. Thus, the de-linking of the historic snow-monsoon relationship may be attributed to rising global and regional temperatures due to global warming.

Table 1: Relative changes in Northern Hemisphere temperature and the Asian monsoon summer precipitation between 1770-1995 from a reconstructed ice core.

Year	Change in relative Northern Hemisphere temperature	Relative changes to Asian monsoon precipitation
1700-1770	Slight increase	Slight decrease
1770-1850	Decrease -0.3°C	Increase +300mm
1850-1920	Lowest in the past 300 years	Generally highest
1920-1995	Rapid rise +0.5°C	Gradual decline from 900mm in the 1920s to 400mm in the 1990s.

 (Source: Duan et al., (2006)

The IPCC (2007) support this claim, reporting that global warming will cause an increase in Asian summer monsoon variability and strength. Kripalini et al., (2003) also noted a reversal of the negative relationship between winter snow and summer rainfall in the most recent satellite data. This may be attributed to changes in winter snow cover extent and/or depth due to global warming. However the negative spring Himalayan summer-snow monsoon relationship was still maintained highlighting the complexity of the feedback forcing between the land and atmosphere.

ATMOSPHERIC BROWN CLOUDS

Another cause for the weakening of the monsoon may be attributed to anthropogenic climate change due to atmospheric brown clouds (ABC). Ramanathan et al., (2008) argue that regional dimming is a major cause for the weakening of the Indian summer monsoon. The formation of ABCs over northern India is suggested to introduce north-south asymmetries that disrupt the temperature gradient. This statement is supported by the findings of Duan et al., (2006: 1) who observed a spatial decrease in monsoon precipitation in Nepal, Bangladesh and NE India by around 19%, 9% and 6% respectively.  However, the feedbacks that occur between the land and the atmosphere are highly complex with direct and indirect feedbacks. As Kripalini et al., (2003) paper demonstrated, global warming has resulted in changes to some relationships whilst others are currently unaffected. Further work is needed to evaluate the affect global warming and other anthropogenic-induced climate change may have on monsoon systems, which is an important factor determining glacier mass balance.

CONCLUSIONS

The strength of the Asian monsoon varies temporally responding to differences in the land-ocean temperature gradient between the Asian subcontinent and the Indian Ocean. Research has shown that intense monsoons coincide with greater temperature contrasts between the land and ocean, and in the past was inversely related to winter snow cover in the Himalayas. However, although the association between monsoons and land-sea temperature contrasts has remained constant, there has been a weakening in the monsoon system during the mid-20^th century coinciding with a decline in snow cover. Historically, the decline in snow cover would have caused a strengthening of the monsoon due to changes in surface albedo. Deviation from this trend suggests a de-linking of the snow-monsoon relationship, with another factor becoming dominant in influencing the temperature gradient in the region. Several studies have suggested global warming may be this forcing factor with additional secondary forcing due to ABCs.

Precipitation is widely accepted as one of the three most important factors influencing glacial mass balance. Thus the weakening of the monsoon will undoubtedly affect the distribution and coverage of Himalayan glaciers. Tele-connections to the El-Nino Southern Oscillation (ENSO) and other atmospheric circulations across the globe mean substantially more research is needed to understand the dynamics influencing the monsoon system. However, it is apparent that at a regional scale, a dynamic snow-monsoon feedback exists with changes in Himalayan snow cover affecting the monsoon, which, in turn, causes changes to snow extent. This is a fundamental point to note in my subsequent post as it suggests that changes to the glacial extent may also affect the future strength of the monsoon. Overall, observations indicate the Asian monsoon has weakened during the 20^th century leading to a decline in precipitation over northern India and the Himalayas. Previous posts have highlighted the importance of Himalayan glaciers as a source for freshwater for Asian communities, and it seems, if this decrease continues, this dependence and hence the importance of the Himalayan glaciers is likely to increase in the future.

Reference:

Benn, D. I. and L. A. Owen (1998) ‘The role of the Indian summer monsoon and the mid-latitude westerlies in Himalayan glaciation: review and speculative discussion’, Journal of the Geological Society’ 155: 353-363.

Duan, K., T. Yao and L.G. Thompson (2006) ‘Response of monsoon precipitation in the Himalayas to global warming’, Journal of Geophysical Research, 111, D19110: 1-8.

Goswani, B.N. (2005) ‘South Asian Monsoon’,  in W.K.M. Lau and D.E. Waliser (eds) Intraseasonal Variability in the Atmosphere Ocean Climate System, Springer: London, 19-61.

IPCC (2007) Climate Change 2007: The Physical Science Basis, in S. Solomon, D. Quin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Millers (eds) Contributions of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press: Cambridge.

Kripalani, R.H., A. Kulkami and S.S. Sabada (2003) ‘Western Himalayan snow cover and Indian monsoon rainfall: a re-examination with INSAT and NCEP/NCAR data’, Theoretical and Applied Climatology, 74, 1: 1-18.

Oerlemans, J. and J.P.F Fortuin (1992) 'Sensitivity of glaciers and small ice caps to greenhouse warming', Science, 258, 5079: 115-117.

Prasad, A.K., K.-H.S. Yang, H.M. El-Askary and M. Kafatos (2009) 'Melting of major glaciers in the western Himalayas: evidence of climatic changes from long ter, MSU derived tropospheric temperature trend (1978-2008)', Annuals of Geophysics, 27: 4505-4519.

Mayewski, P.A., G.P. Pregent, P. A. Jeschke and N. Ahmad (1980) ‘Himalayan and Trans-Himalayan glacier fluctuations and the South Asian monsoon record’, Arctic and Alpine Research, 12, 2: 171-182.

Ramanathan, V. M. Agrawal, H. Akimoto, M. Aufhammer, S. Devotta, L. Emberson, S. I. Hasnain, M. Iyngarasan, A. Jayaraman, M. Lawrence, T. Nakajima, T. Oki, H. Rodhue, M. Muchirawat, S.K. Tan, J. Vincent, J.Y. Wang, D. Yang, Y.H. Yang, H. Autrup, L. Barregard, P. Bonasoni, M. Brauer, B. Brunekreef, G. Carmichael, C.E. Chung, J. Dahe, Y. Feng, S. Fuzzi, T. Gordon, A. K. Gosain, N. Htun, J. Kim, S. Mourato, L. Naeher, P. Navasumrit, B. Ostro, T. Panwar, M. R. Rahman, M. V. Ramana, M. Rupakheti, D. Settachan, A. K. Signh, G. Sat. Helen, P. V. Tun, P.H. Viet, J. Yinlong, S.C. Yoon, W. C. Chang, X. Chang, J. Zelikoff and A. Zhu (2008) Atmospheric Brown Cloud: Regional Assessment Report with Focus on Asia, UNEP: Kenya.