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Siegfried D. Schubert, Ronald E. Stewart, Hailan Wang, Mathew Barlow, Ernesto H. Berbery, Wenju Cai, Martin P. Hoerling, Krishna K. Kanikicharla, Randal D. Koster, Bradfield Lyon, Annarita Mariotti, Carlos R. Mechoso, Omar V. Müller, Belen Rodriguez-Fonseca, Richard Seager, Sonia I. Seneviratne, Lixia Zhang, and Tianjun Zhou

discussion and synthesis, we include a model-based assessment of the dominant large-scale forcing of meteorological drought on seasonal and longer time scales—the response of the atmosphere to SST anomalies (e.g., Hoerling and Kumar 2003 ; Schubert et al. 2004 ; Seager et al. 2005 ). This assessment is based on AMIP-style simulations using prescribed SSTs (see appendix A ), with five different state-of-the-art global climate models; results are presented as combined (rather than individual model

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Mathew Barlow, Benjamin Zaitchik, Shlomit Paz, Emily Black, Jason Evans, and Andrew Hoell

during severe droughts, as noted above. Moreover, given the long-standing sociopolitical tensions in several areas of the region in an environment of limited resources, drought variability may raise the risk of regional conflicts ( El Kharraz et al. 2012 ; Selby and Hoffmann 2012 ; Fröhlich 2013 ; Gleick 2014 ; Kelley et al. 2015 ). In terms of the impacts of drought, therefore, the region is particularly important because of the intersection of population, vulnerability, drought severity, and

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Belen Rodríguez-Fonseca, Elsa Mohino, Carlos R. Mechoso, Cyril Caminade, Michela Biasutti, Marco Gaetani, J. Garcia-Serrano, Edward K. Vizy, Kerry Cook, Yongkang Xue, Irene Polo, Teresa Losada, Leonard Druyan, Bernard Fontaine, Juergen Bader, Francisco J. Doblas-Reyes, Lisa Goddard, Serge Janicot, Alberto Arribas, William Lau, Andrew Colman, M. Vellinga, David P. Rowell, Fred Kucharski, and Aurore Voldoire

Intergovernmental Panel on Climate Change (IPCC) Fourth and Fifth Assessment Reports (AR4 and AR5) process, improving understanding of climate and providing estimates of future climate change that will be useful to those considering its possible consequences. The text is organized as follows. We start in section 2 and 3 by surveying the state-of-the-art knowledge of the SST influence on Sahel rainfall at interannual to decadal time scales, at which the variability of the ocean is the main driver of that in

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Richard Seager and Martin Hoerling

—more so than the observed patterns of soil moisture anomalies for these events. The evidence from these simulations is nonetheless strong that particular conditions of ocean states and/or external radiative forcing during those years significantly increased probabilities for severe drought to occur over the areas that indeed experienced severe drought. Several lines of evidence indicate that the forced signal of dryness and the associated increase in severe drought risk in these three years was mostly

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Siegfried D. Schubert, Hailan Wang, Randal D. Koster, Max J. Suarez, and Pavel Ya. Groisman

feedbacks on temperature variability in the context of model simulations in section 3 . Of course, as mentioned above, cloud cover can also affect temperature variability. In fact, in an assessment of the role of cloud cover and rainfall on the daytime temperature ( T max ), Tang et al. (2012) showed that for the western half of northern Eurasia (where the major agricultural regions reside), summer cloud cover is negatively correlated with T max and that these correlations are much stronger than

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Omar V. Müller, Ernesto Hugo Berbery, Domingo Alcaraz-Segura, and Michael B. Ek

interannual variability in precipitation, which increases the risk not only of flood events but also of droughts with their consequent negative impacts ( Penalba et al. 2010 ). Extreme events in southern South America recognize as the main driver the Pacific Ocean SST anomalies, especially owing to the ENSO phenomenon ( Ropelewski and Halpert 1987 ; Aceituno 1988 ; Mechoso and Perez-Iribarren 1992 ). Positive SST anomalies over the tropical Pacific (El Niño) are known to induce wet spells over southern

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Wenju Cai, Ariaan Purich, Tim Cowan, Peter van Rensch, and Evan Weller

with prolonged dry periods across southeast Australia ( Ummenhofer et al. 2011 ); these IOD events not only weaken heavy rainfall over southeast Australia during winter and spring but also increase the risk of hotter spring extreme temperatures ( Min et al. 2013 ). Positive IOD events also increase the risk of major bushfire occurrences across the southern Australian states, as was witnessed in February 2009 ( Cai et al. 2009a ; Bureau of Meteorology 2010b ). The southeast Queensland summer

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Bradfield Lyon

deleterious consequences it is not surprising that across the Greater Horn, where rain-fed agriculture is the mainstay, where food security is often threatened ( Funk et al. 2008 ; Funk and Brown 2009 ), and where the largest contribution to electricity generation is hydropower ( Kaunda et al. 2012 ), that drought information is especially valued. Nor is the interest in drought information limited to assessments of current conditions or the provision of seasonal forecasts. Seemingly contradictory signals

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