Rainfall Extremes Over India, 1951-2003

Several papers have been published in recent years on Indian monsoon rainfall extremes and their possible linkage to climate change. In this latest paper, Ghosh et al. (2012) conduct a statistical analysis of daily rainfall extremes obtained from a high-resolution monsoon dataset on a 1° grid, using over 1800 rain-gauge stations located over various regions in India. A generalized extreme value distribution is used to examine trends in extremes over the past half century (1951-2003), while the total summer monsoon rainfall is examined using available data for about 200 years (1813-2006). The study obtains several interesting results, as summarized below.

First, the total summer monsoon rainfall reveals a low-frequency cyclical pattern, with increasing/decreasing rainfall over approximately a 40-60 year period. Since about 1950, the summer monsoon rainfall shows a steadily declining trend. Second, the spatial average of annual maxima (of daily rainfall extreme) calculated on the basis of all grids shows a statistically significant increasing trend: Temporal means of annual maxima shows geographical heterogeneity; also daily rainfall extremes show considerable spatial variability over the past half century. There is no statistically significant uniform trend in the spatial variability of extreme rainfall. Also the annual rainfall maxima (of daily extremes) over each grid show no discernible increasing or decreasing trend anywhere in India, other than a few isolated grid points. Third, the spatial variance of annual maximum (mm² day^-2) shows an increasing trend since 1950; also spatial variances of 30-year and 100-year return levels show statistically significant increasing trends.

It is of interest to analyze the above findings in the context of present debate on global warming, climate change and possible future impact on rainfall variability, rainfall extremes and trends. Most climate models simulate an intensification of the Indian/Asian monsoon circulation in a warming climate. However the reality is quite different so far, as evidenced by a declining total monsoon rainfall since about 1950. This declining trend in monsoon rains over India has also been identified over other tropical monsoon regions (Chase et al. 2003). The daily rainfall extremes show an increasing trend over the past half century, however there is considerable spatial variability in this trend and there are no statistically significant, spatially uniform, trends anywhere. Furthermore, the trends in spatial variances of return levels are quite large indicating rapid increase in geographical heterogeneity of rainfall extremes over the past half century. Finally the increasing trends in annual maxima of daily rainfall extremes can be interpreted as due primarily due to impact of rapid urbanization and uneven population growth of many cities and towns in India (a recent paper by Kishtawal et al., 2010, identified an urbanization signature in heavy rainfall climatology over India).

In summary, many of the findings of this paper appear to be at odds with the model-based projections of the IPCC with respect to future climate change impacts on Indian/Asian monsoon and rainfall extremes, etc.

Additional References
Chase, T.N., Knaff, J.A. Pielke, R.A., Sr. and Kalnay, E. 2003. Changes in global monsoon circulations since 1950. Natural Hazards 29: 229-254.

Kishtawal, C.M., Niyogi, D., Tewari, M., Pielke, R.A., Sr. and Sheppard, J.M. 2010. Urbanization signature in the observed heavy rainfall climatology over India. International Journal of Climatology 30: 1908-1916