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

1. Introduction Australia is one of the driest inhabited continents in the world, with a climate that is highly variable and which experiences seasonal-scale droughts with large interdecadal variability ( Gallant et al. 2007 ). Australia is also influenced by several modes of seasonal-scale variability such as the Indian Ocean dipole (IOD), the southern annular mode (SAM), and El Niño–Southern Oscillation (ENSO; e.g., Nicholls et al. 1996 ; Ashok et al. 2003 ; Hendon et al. 2007 ; Risbey et

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Richard Seager and Martin Hoerling

observed and idealized sea surface temperatures (SSTs). These were used to test hypotheses of oceanic forcing of drought-inducing atmospheric circulation anomalies. Links between North American precipitation variability and the El Niño–Southern Oscillation (ENSO), with, in its El Niño phase, a tendency to increased winter precipitation across southern North America, had begun to be noticed in the 1970s and early 1980s (see Rasmusson and Wallace 1983 ) and explained in terms of Rossby wave propagation

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Bradfield Lyon

maximum in southernmost areas ( Ogallo 1988 ). This is largely associated with the southern migration of the ITCZ across the western Indian Ocean into that region. While precipitation during the MAM and OND seasons depends fundamentally on the meridional translation of the ITCZ across the equator, topography also plays a role in affecting the annual cycle of precipitation. The long rains, for example, typically undergo a substantial meridional “jump” of several degrees latitude during MAM as low

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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

. Anomalous warming of the southern tropical Atlantic enhances ascent over the Gulf of Guinea and descent over the Sahel. A warming in the Pacific and Indian Oceans generates equatorial Rossby waves that contribute to subsidence over the Sahel and thus to reduce regional precipitation. In addition, Mediterranean warm events are linked to increased moisture flux convergence over the Sahel. Decadal SST variability and global warming are also relevant to Sahelian drought. In recent decades the Sahel has been

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

include the entire domain; more restrictive definitions focus on the coastal countries. The region as a whole is also sometimes referred to as western Asia. As discussed further in the next section, the climate of the region is generally semiarid to very arid, including several deserts, but annual precipitation does exceed 60 cm along the eastern Mediterranean region and on many of the mountain slopes in the region and ranges as high as 180 cm in a small region on the southern coast of the Caspian Sea

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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

, Pacific, and Indian Oceans and the Mediterranean Sea) on the WAM precipitation at interannual time scales. a. Influence of the tropical Atlantic Ocean Since the early papers by Hastenrath and Lamb (1977) , Lamb (1978) , and Hastenrath (1984) , many others have documented the tropical Atlantic influence on West African rainfall. This influence unfolds at different time scales: the variability in the equatorial and southern sectors affects that in interannual time scales, while that in the northern

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

variability in summer moisture over Europe that was tied to SST variability. In addition to a drying trend over Europe associated with warming SST over all oceans, they found a link between previous winter La Niña and negative Pacific decadal oscillation (PDO) events and summer dry conditions over southern Europe extending into western Russia, and wet conditions over the Scandinavian peninsula, with the atmospheric anomalies resembling aspects of the Pacific–North American pattern (PNA) and (the positive

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Lixia Zhang and Tianjun Zhou

–Pacific teleconnection pattern. The precipitation anomaly features a meridional tripolar or sandwich pattern, with excessive precipitation in central-eastern China along the Yangtze River valley and Japan but drier or even drought conditions in southern and northern China. It is forced by an anomalous anticyclone that appears over the WNP, which is forced by the SST anomalies over the WNP and the Indian Ocean during El Niño decaying summer. The other teleconnection is the Silk Road teleconnection, which is forced by

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