Nice Weather For Ducks: The Asian monsoon (Part II)

In my previous post, I discussed some of the key factors controlling the distribution and extent of the Asian monsoon. A review of the literature demonstrated substantial variability in the monsoon depending on surface and atmospheric temperatures, snow cover and other meso-scale factors. Overall, studies noted a weakening in Asian monsoon from the 1920s and Kripalini et al., (2003) observed a de-linking of the winter snow-monsoon relationship from a negative to positive correlation. This post will focus on how a weakening of the monsoon may impact the sensitivity of Himalayan glaciers to climate change and suggests, like many other factors, there is substantial complexity affecting general trends.

SNOWFALL, GLACIAL FORMATION AND CHANGING CLIMATE.

Snowfall is fundamental to the formation and preservation of glaciers. As snow accumulates it is compressed and compacted by the overlying layers. Over time, the density of the snow increases as air bubbles are removed forming glacial ice at 850kg cubic metres (Figure 1).

Figure 1: Video describing how glaciers are formed. 

Therefore changes in snowfall will have a substantial impact on the glacial mass balance of glaciers in the Himalayas. The high altitude location of most glacial catchments means that the rising summer temperatures are unlikely to significantly alter the proportion of precipitation that falls as snow (Benn and Owen, 1998). Thus, in this post, changes in snowfall are assumed to be due to declines in precipitation.

In addition to monsoon advecting snow moisture-rich air masses to the Himalayas, generally, more snow is expected to fall in a warmer atmosphere due to increases evaporation (Shekhar et al., 2010). This is due to greater cloud formation and hence more snowfall at high altitudes. However, a recent study by Dimri and Kumar (2008) shows a reduction in snowfall over the western Himalayas despite the warming climate. Conducting research over four different mountain ranges, overall snowfall declined by 280cm between 1988/89 and 2007/08 over the region (Dimri and Kumar, 2008). This trend was not uniform, but suggests that a weakening monsoon may be counteracting rising evaporation rates, as the air masses are contain relatively less moisture and are not extending as far into the Himalayan mountain range.

Shekhar et al., (2010) observed a decreasing number of snowfall days in the western Himalayas during the peak snowfall months (June-March) although no significant trends were observed in changes to the annual frequency of western disturbances. This supports the idea that changes to snowfall may be partly attributed to a decline in precipitation due to the weakening monsoon.

SPATIAL COMPLEXITY IN GLACIAL RESPONSE

Orographic and other mesoscale influences cause regional variability in the distribution of monsoon precipitation across the Himalayas. Thus a weakening monsoon is likely to cause different responses in different spatial regions (Benn and Owen, 1998). Summer precipitation falls sharply from south to north across the Himalayas and is much higher in Nepal than Karakoram and the western Tibetan Plateau (Benn and Owen, 1998). Between 1901 and 1995, summer precipitation has decreased by 19%, 9% and 6% in Nepal, Bangladesh and North-east India respectively, indicating a decline in precipitation due to the Asian monsoon (Duan et al., 2006). While glaciers in these regions have generally been decreasing during the 20^th century, glaciers in the Karakoram have remained relatively static or have advanced (Scherler et al., 2011). Relating this regional disparity to the observation by Benn and Owen (1998) the decrease in the influence of the Asian monsoon may not be as significant in Karakoram which is dominated by western midlatitute systems (Figure 2).

Figure 2. Map of the Himalayan mountain range, showing the location of the advancing or static glaciers in the Karakoram region in the north and retreating glaciers in Nepal and the southern Himalayan chain.

OTHER FACTORS

An additional forcing complicating the impact of the Asian monsoon on glacial mass balance is the affect of a weakening monsoon on the duration that ABCs are present over Asia (Bonsani et al., 2010). The onset of the summer monsoon removes aerosols from the atmosphere. As discussed in an earlier post, aerosols can have two counteracting impacts on glacial mass balance causing global dimming when aerosols are present in the atmosphere and surface darkening once black carbon has been added to the surface layer (Flanner et al., 2009). As the South Asian monsoon cycle determines atmospheric, a delay and weakening of the monsoon season may increase the time that atmospheric aerosols contribute to global dimming, counteracting global warming.

CONCLUSIONS

Similar to other impacts of climate change in the Himalayan region, the influence of the Asian monsoon on glacial mass balance is highly complex with substantial regional variability. Studies such as Duan et al., (2006) and Benn and Owen (1998) have suggested that a weakening monsoon has contributed to a decrease in precipitation in the southern regions of the Himalayan chain in Nepal and North-east India. However, the contribution of the summer monsoon to annual precipitation is less significant in northern regions (due to orographic effect), resulting in a less marked impact in these areas.

Additionally a delay in the onset of the monsoon may allow regional ABCs to persist for longer, counteracting global warming and the retreat of glaciers to rising temperatures.

Overall, the current weakening of the Asian monsoon is likely to accelerate the decline in glacial mass in the southern Himalayas due to decreases in precipitation. In northern regions, the relationship is less clear. The monsoon controls fluctuations in the Himalayan glaciers on millennial, decadal and inter-annual timescales (Benn and Owen, 1998). Therefore, in order to fully understand the response to the weakening monsoon and the influences on associated feedbacks (including the snow-monsoon forcing), substantially more research needs to be carried out.

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

Bonsani, P., P. Laj, A. Marinoni, M. Sprenger, F. Angerlini, J. Arduini, U. Bonafe, F. Calzolani, T. Colombo, S. Decessari, C. Di. Biagio, A. G. di Sarra, F. Evangelisti, R. Duchi, M. C. Facchini, S. Fuzzi, G. P. Gobbi, M. Maione, A. Panday, F. Roccato, K. Sellagri, H. Venza, G. P. Verza, P. Villani, E. Vuillermoz and P. Cristofanelli (2010) ‘Atmospheric brown clouds in the Himalayas: first two years of continuous observations at the Nepal Climate Obsrervatory Pyramid (5079m)’, Atmospheric Chemistry and Physics, 1- 7515-7531.

Dimri, A.P. and A. Kumar (2008) ‘Climatic variability of weather parameters over the western Himalayas: a ase study’, in P. K. Satyawali and A. Ganju (Eds). Proceedings of the National Snow Science Workshop, 11–12 January 2008, Chandigarh, Snow and Avalanche Study Establishment: Chandigarh, 167–173.

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

Flanner, M.G., C.S. Zender, P.G. Hess, N.M. Mahowald, J.H. Painter, V. Ramanathan and P.J. Rasch (2009) ‘Springtime warming and reduced snow cover from carbonaceous particles’, Atmospheric Chemistry and Physics, 9: 2481-2497.

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.

Scherler, D., B. Bookhagen and M. R. Stecker (2011) 'Spatially variable response of Himalayan glaciers to climate change affected by debris cover', Nature Geoscience, 4,3: 156-159

Shekhar, M. S., H. Chang, S. Kumar, K. Srinivasari, and A. Ganju (2010) ‘Climate-change studies in the western Himalayas’, Annals of Glaciology, 51, 54: 105-112.