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Alfredo Ruiz-Barradas and Sumant Nigam

, of annual mean global SSTs for the period 1901–2004. These modes of SST variability are referred to as Trend , Pacific , and Atlantic forcing patterns ( Figs. 1a–c ). The associated rotated principal components (RPCs) are also shown here ( Fig. 1d ) but they have been standardized for the 1902–99 period. The Trend pattern shows a nonlinear long-term time evolution (thin continuous line). The Pacific pattern varies on both interannual (ENSO) and decadal time scales (thick continuous line). The

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Yochanan Kushnir, Richard Seager, Mingfang Ting, Naomi Naik, and Jennifer Nakamura

such events is important for future planning and predictions. The present work is concerned with these aspects as they affect interannual-to-decadal time-scale variability. Recent modeling studies indicate that the main regulating mechanism of interannual and multiyear variability of precipitation in the American West and northern Mexico, are low-frequency fluctuations in tropical Pacific sea surface temperatures (SSTs). It is well known that tropical Pacific SSTs exhibit large fluctuations with a

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Alfredo Ruiz-Barradas and Sumant Nigam

-only precipitation index is extracted from the smoothed seasonal precipitation index derived from the 1–2–1 binomial filter applied 12 times. Significant SST correlations from observations show a coherent basin-scale structure over the Pacific basin with both tropical and extratropical imprints, similar to the Pacific decadal variability pattern; equally important are correlations over the subtropical and extratropical Atlantic basin. The features in both basins, with maximum correlations of 0.3, have been shown

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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

precipitation variations over the United States. GCM experiments have also been used to diagnose the role of tropical SST forcing on central U.S. warm season precipitation. Bates et al. (2001) show that the 1993 pluvial over the Great Plains was related to tropical Pacific SST anomalies; however, no such conclusion was drawn for the 1988 drought. Decadal SST variability has also been implicated in forcing drought and pluvial over the United States ( Schubert et al. 2004 , 2008 ; Seager et al. 2005

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Matías Méndez and Víctor Magaña

pattern in precipitation anomalies. Such contrast in persistent climatic anomalies is a characteristic of the decadal climate variability over Mexico and Central America. Even, tree-ring reconstructions of precipitation in North America ( Cook et al. 2004 ) show that a prolonged drought in northern Mexico, as the one in the sixteenth century, corresponds with wet conditions in the south ( Fig. 1 ). On the other hand, a drought in southern Mexico in the 1630s corresponds to wet conditions in the north

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Antonietta Capotondi and Michael A. Alexander

leading mode of Pacific decadal SST variability in the model, which is, in turn, in very good agreement with the spatial pattern of the interdecadal Pacific oscillation (IPO) as described by Power et al. (1999) and Arblaster et al. (2002) . This pattern is characterized by anomalies of the same sign in a broad triangular-shaped region centered along the equatorial Pacific, and anomalies of opposite sign in the central-western midlatitudes of both hemispheres, and is similar to the Pacific decadal

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Philip J. Pegion and Arun Kumar

include the Pacific decadal oscillation (PDO) for the northern Pacific ( Mantua et al. 1997 ), the Interdecadal Pacific Oscillation (IPO) the basinwide pattern ( Power et. 1999 ), the North Atlantic multidecadal oscillation (AMO) ( Enfield et al. 2001 ), and the Indian Ocean dipole (IOD) ( Saji et al. 1999 ). In addition to the quasiperiodic modes of SST variability, trends in the SSTs, for example, an upward trend in SSTs in all tropical ocean basins, have also been identified. Modes of SST

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Siegfried Schubert, David Gutzler, Hailan Wang, Aiguo Dai, Tom Delworth, Clara Deser, Kirsten Findell, Rong Fu, Wayne Higgins, Martin Hoerling, Ben Kirtman, Randal Koster, Arun Kumar, David Legler, Dennis Lettenmaier, Bradfield Lyon, Victor Magana, Kingtse Mo, Sumant Nigam, Philip Pegion, Adam Phillips, Roger Pulwarty, David Rind, Alfredo Ruiz-Barradas, Jae Schemm, Richard Seager, Ronald Stewart, Max Suarez, Jozef Syktus, Mingfang Ting, Chunzai Wang, Scott Weaver, and Ning Zeng

next? What is the role of the different ocean basins, including the impact of El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), the Atlantic multidecadal oscillation (AMO), and warming trends in the global oceans? What is the role of the land? To what extent can droughts develop independently of oceanic variability resulting from year-to-year memory that may be inherent to the land? To address these questions, the working group proposed that a number of mechanistic

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Kingtse C. Mo, Jae-Kyung E. Schemm, and Soo-Hyun Yoo

difficult to separate decadal trends and ENSO. Trends are not linear and results may depend on the method used to isolate trends. One possibility is to rely on AGCM experiments to confirm observational findings. The AGCM experiments designed by the U.S. Climate Variability and Predictability (CLIVAR) drought working group ( Schubert et al. 2009 ) are well suited for this purpose. The drawback of model experiments is that models have errors. Not all model errors can be reduced by correcting the model

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M. Biasutti, A. H. Sobel, and Suzana J. Camargo

21st centuries. Proc. Natl. Acad. Sci. USA , 102 , 17891 – 17896 . Herceg , D. , A. Sobel , and L. Sun , 2007 : Regional modeling of decadal rainfall variability over the Sahel. Climate Dyn. , 29 , 89 – 99 . Hoerling , M. , J. Hurrell , J. Eischeid , and A. Phillips , 2006 : Detection and attribution of twentieth-century northern and southern African rainfall change. J. Climate , 19 , 3989 – 4008 . Houghton , J. T. , L. G. Meira Filho , B. A. Callander , N

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