Modeling the Monsoons in a Changing Climate

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Over half of the World’s population resides in regions influenced by the Asian Summer Monsoon (ASM). The seasonal mean (June through September) rainfall associated with the ASM is the primary source of water for the agrarian society. To a great extent, the livelihood and socio-economic issues of the people are influenced by the vagaries of the monsoon rainfall. Given their anticipated population rise, the countries experiencing the monsoon will likely face increased stress in issues such as drinking water supply, among other factors. It is therefore important to know how the ASM and its variability would respond in a warmer climate. Climate change refers to various facets of climate, but the effect receiving the widest attention is referred to as global warming. Today, the principal concern is to understand the physical processes that determine the mean temperature of the Earth’s surface, and interpret how this may change in response to changes in atmospheric composition. Since the temperature and atmospheric moisture content are inherently related the obvious question is how the global hydrological cycle will respond to changes in temperature. Observations over the last 1000 years indicate that carbon dioxide (CO2) amount has increased by 25% since the industrial revolution, and continues to increase at a rate of 0.5 per annum. In other words, by releasing considerable amount of CO2 into the atmosphere, humans have applied a stronger forcing with the potential for irreversible effects. To address and assess the impact of this additional forcing, scientists around the world turn to mathematical models of the Earth’s climate system. These climate models are constructed based on the physical principles and executed on the most powerful computers. Various hypotheses are tested to understand the present climatic condition. A classic example is that the observed increase in global mean surface temperature since 1950s is reproducible only if the greenhouse gases are included in the model simulations (IPCC 2001). This surface warming is projected to increase into the 21st century (IPCC 2007). These models also suggest that under increased CO2 forcing, global hydrological cycle will intensify (Held and Soden 2006). However, regional precipitation projections from these models have large uncertainties. Recognizing the strengths and weaknesses of the models, appropriate tools are employed to interpret the future projections. This is the “need of the hour” as policy and decision-makers seek to mitigate and adapt to the undesirable impact of climate change caused by human intervention. Over half of the World’s population lives in regions influenced by the Asian monsoon (Webster et al. 1998). The seasonal mean rainfall, both summer (June through September) and winter (October through December), associated with the monsoons