Recent Climate Report Northern Hemisphere summer monsoon intensified by mega-El Niño/southern oscillation and Atlantic multidecadal oscillation by Bin Wang, Jian Liu, Hyung-Jin Kim, Peter J. Webster, So-Young Yim and Baoqiang Xiang (2013) Abstract Prediction of monsoon changes in the coming decades is important for infrastructure planning and sustainable economic development. The decadal prediction involves both natural decadal variability and anthropogenic forcing. Hitherto, the causes of the decadal variability of Northern Hemisphere summer monsoon (NHSM) are largely unknown because the monsoons over Asia, West Africa, and North America have been studied primarily on a regional basis, which is unable to identify coherent decadal changes and the overriding controls on planetary scales. Here, we show that, during the recent global warming of about 0.4 °C since the late 1970s, a coherent decadal change of precipitation and circulation emerges in the entirety of the NHSM system. Surprisingly, the NHSM as well as the Hadley and Walker circulations have all shown substantial intensification, with a striking increase of NHSM rainfall by 9.5% per degree of global warming. This is unexpected from recent theoretical prediction and model projections of the 21st century. The intensification is primarily attributed to a mega-El Niño/Southern Oscillation (a leading mode of interannual-to-interdecadal variation of global sea surface temperature) and the Atlantic Multidecadal Oscillation, and further influenced by hemispherical asymmetric global warming. These factors driving the present changes of the NHSM system are instrumental for understanding and predicting future decadal changes and determining the proportions of climate change that are attributable to anthropogenic effects and long-term internal variability in the complex climate system. Discussion Although the mega-ENSO and AMO are primary sources of the interdecadal variations of the NHSM, one cannot rule out the influence of the global warming. Fig. 3 shows that the NH 2-m air temperature has warmed more than the Southern Hemisphere (SH) counterpart by 0.36 °C over the past 32 y. The NHSM intensity is significantly linked to the hemispheric thermal contrast (HTC) defined by the 2-m air temperature difference between the NH (0°–60°N) and Southern Hemisphere (60°S–0°) (r = 0.63; Fig. 4C). Dynamically, the enhanced hemispheric thermal contrast can generate meridional pressure gradients that drive low-level cross-equatorial flows from the SH to the NH (Fig. 2C) and converge into the NHSM trough regions. We note that the “NH warming faster than the SH” or “warm NH–cold SH” pattern is a characteristic of the projected warming under increasing greenhouse gases forcing (5). There is an increasing demand for decadal climate prediction, which is necessary for infrastructure planning, energy policy, business development, and issues related to sustainability. The results presented here have important ramifications for the prediction of decadal monsoon changes. On this timescale, monsoon climate could be substantially influenced by internal natural variability in the climate system such as mega-ENSO and AMO. The knowledge gained here is also instrumental in determining the proportions of climate change that are attributable to anthropogenic effects and long-term natural variability in the complex climate system. Source: Proc Natl Acad Sci U S A. 2013 April 2; 110(14): 347–5352. Published online 2013 March 18. doi: 10.1073/pnas.1219405110 Earth, Atmospheric, and Planetary Sciences
Posted on: Mon, 26 Aug 2013 13:21:33 +0000
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