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Toby R. Ault, Scott St. George, Jason E. Smerdon, Sloan Coats, Justin S. Mankin, Carlos M. Carrillo, Benjamin I. Cook, and Samantha Stevenson

then computed from this output. Although output from the LIM is available at monthly resolution, the PDSI field was averaged over June–August (JJA) for comparison with paleoclimate reconstructions of this same variable over the same season (see next section). c. Reconstructed PDSI data We evaluated the synthetic droughts produced by the LIM by comparing them against the North American Drought Atlas (NADA), which is a gridded set of mean JJA PDSI reconstructions derived from tree rings ( Cook et al

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Kevin J. Anchukaitis, Rosanne D. D’Arrigo, Laia Andreu-Hayles, David Frank, Anne Verstege, Ashley Curtis, Brendan M. Buckley, Gordon C. Jacoby, and Edward R. Cook

1. Introduction Instrumental climate observations indicate that surface air temperatures in northwestern North America are rising faster than nearly anywhere else in the world ( ACIA 2004 ). Hartmann and Wendler (2005) found mean summer (June–August, JJA) temperature increases in Arctic Alaska of 1.4°C between 1951 and 2001, with a 1°C increase between 1977 and 2001 alone. However, efforts to use tree-ring proxy reconstructions to place these recent warming rates within the long-term context

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Ryan Lagerquist, John T. Allen, and Amy McGovern

1. Introduction Frontal boundaries between air masses are an important trigger for precipitation and often serve as foci for severe thunderstorms ( Sanders and Doswell 1995 ; Sanders 1999 ; Garner 2013 ; Catto et al. 2015 ; Dowdy and Catto 2017 ). The significance of fronts has been recognized since observation networks were first established over North America (e.g., Holzman 1937 ). Many climatologies have explored the regional and global occurrence of fronts and their links to

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Anthony Arendt, John Walsh, and William Harrison

maximum and minimum daily temperatures, and were grouped into annual, winter (October–April), and summer (May–September) categories. These averaging periods roughly coincide with glacier accumulation and ablation seasons, respectively, in northwestern North America. Total precipitation measurements include the sum of all liquid and solid precipitation. Changes in temperature (Δ T ) and precipitation (Δ P ) were quantified as the change from the beginning to the end of a linear trend line calculated

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Stefan Sobolowski, Gavin Gong, and Mingfang Ting

. Because of its considerable spatiotemporal variation, and the striking geographical differences between the Northern Hemisphere (NH) landmasses, the influence of snow on atmospheric circulation is an area of ongoing research. In particular, the nature of the large-scale stationary wave response to anomalous North American (NA) snow cover and its associated mechanisms remains largely unexplored. Physically based snow–climate teleconnections have been described more often for Eurasia than for NA. This

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

1. Introduction The nature and cause of North American hydroclimate variability is a subject of heightened concern because of the recent droughts 1 in the American West 2 and in northern Mexico ( Seager 2007 , 2009 ) and because projections of anthropogenic influence on the climate of the twenty-first century indicate a turn toward increasing aridity there ( Seager et al. 2007 ). The latter finding, based on output from climate models that participated in the Intergovernmental Panel on

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M. N. Khaliq and P. Gachon

1. Introduction It is important to explore the linkages between large-scale climate variability and regional-scale hydroclimatic processes because interannual and decadal-scale climate variability is instrumental in planning regional water resources, which are of significant ecological, cultural, and economic value. In some of the previous studies, the variability of winter precipitation and temperature in northwestern North America (NWNA) was associated with the Pacific decadal oscillation

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Paul C. Loikith and Anthony J. Broccoli

change regionally, a stronger understanding of the mechanisms associated with extreme temperatures is essential for accurate projection and interpretation of future extreme events. Several recurrent modes of natural climate variability are associated with characteristic teleconnections that impact regional mean temperature over North America, especially in winter. The Pacific–North American (PNA) pattern is associated with geopotential height anomaly centers southwest of the Aleutian Islands, inland

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James D. Means

1. Introduction In recent decades it has become apparent that the North American monsoon system is one of the largest seasonal weather systems to affect the continent. It brings widespread thunderstorms to the southwestern United States and northwestern Mexico, typically during the months of July, August, and early September. It is an important rain producer, bringing a significant portion of the annual precipitation to the region, albeit with significant year-to-year variability. The scope of

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Stephen J. Vavrus, Fuyao Wang, Jonathan E. Martin, Jennifer A. Francis, Yannick Peings, and Julien Cattiaux

-to-noise ratio, which is relatively weak in observational studies that span only the recent short period of enhanced Arctic warming (since the mid-to-late 1990s). We focus on the projected late twenty-first-century climate change over greater North America (20°–90°N, 50°–160°W) using 40 realizations from the Community Earth System Model Large Ensemble (LENS; Kay et al. 2015 ). For comparison, we also analyze observed trends using the NCEP–NCAR Reanalysis-1 (NNR; Kalnay et al. 1996 ) from 1948 to 2014. A

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