'South Asian monsoon variations hard to fathom'
[NEW DELHI] Understanding how the South Asian monsoon will change in response to global warming and resolving the uncertainties in projected changes are ‘demanding tasks’ for climate science, a review says.
Current state-of-the-art general circulation models have difficulty simulating the regional distribution of monsoon rainfall, the 24 June review in Nature Climate Change says.
The vagaries of the monsoon on short- and long-term timescales impact the lives of more than a billion people in South Asia who depend on rainfall for agriculture, power generation, industrial development and basic human needs.
Authors, Andrew Turner, National Centre for Atmospheric Science-Climate, University of Reading, and Harisubramaniam Annamalai, International Pacific Research Centre, School of Ocean and Earth Science, University of Hawaii, say variations in the amount of monsoon rains in different years are low (10 per cent of the total summer rainfall).
But, variations within each season, over timescales of a few days or weeks, often have large impacts on agriculture or water supply.
"Perhaps the single biggest scientific challenge is in understanding monsoon variability at intra-seasonal timescales (several weeks), the so-called active and break events in the monsoon, and how they will change in the future," Turner told SciDev.Net.
They "are poorly understood and difficult to predict," he said.
The review also describes observed changes to monsoon rainfall over the second half of the 20th century such as an unprecedented rise in greenhouse gases and aerosols (tiny suspended particles in the air, such as sulphates from industry or soot from cook stoves).
Models linking monsoon responses to global warming suggest a rise in monsoon rainfall, but there is a high degree of uncertainty in these projections. Observations from data sets from most areas indicate a declining trend or no change in monsoons, contrary to the projected rise.
Turner explained that there could be reasons for the mismatch between observed and projected monsoon rainfall trends.
Increases in aerosols from pollution since the 1950s may be absorbing the sun’s radiation reaching the earth, impacting temperature differences over land and sea and, in turn, the monsoon.
Other possible factors include the effects of land-use change (the impact of the green revolution and massive expansion in irrigation in northern India, for example), or natural decadal variations in the rainfall, he said.
The review also highlights the need for reprocessing data, consistent data sets and a better understanding of the physics and complex dynamics behind monsoon circulation.
Co-author Annamalai told SciDev.Net that "as far as the Indian monsoon rainfall data goes, it is mostly about data processing techniques."
When data from observing stations is projected or mapped into regular latitude and longitude grid points, different research groups employ different techniques, he explained.
"It is unclear how many 'true observed station rainfall data' go into each and every grid," Annamalai said.