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Katrina Grantz, Balaji Rajagopalan, Martyn Clark, and Edith Zagona


Analysis is performed on the spatiotemporal attributes of North American monsoon system (NAMS) rainfall in the southwestern United States. Trends in the timing and amount of monsoon rainfall for the period 1948–2004 are examined. The timing of the monsoon cycle is tracked by identifying the Julian day when the 10th, 25th, 50th, 75th, and 90th percentiles of the seasonal rainfall total have accumulated. Trends are assessed using the robust Spearman rank correlation analysis and the Kendall–Theil slope estimator. Principal component analysis is used to extract the dominant spatial patterns and these are correlated with antecedent land–ocean–atmosphere variables. Results show a significant delay in the beginning, peak, and closing stages of the monsoon in recent decades. The results also show a decrease in rainfall during July and a corresponding increase in rainfall during August and September. Relating these attributes of the summer rainfall to antecedent winter–spring land and ocean conditions leads to the proposal of the following hypothesis: warmer tropical Pacific sea surface temperatures (SSTs) and cooler northern Pacific SSTs in the antecedent winter–spring leads to wetter than normal conditions over the desert Southwest (and drier than normal conditions over the Pacific Northwest). This enhanced antecedent wetness delays the seasonal heating of the North American continent that is necessary to establish the monsoonal land–ocean temperature gradient. The delay in seasonal warming in turn delays the monsoon initiation, thus reducing rainfall during the typical early monsoon period (July) and increasing rainfall during the later months of the monsoon season (August and September). While the rainfall during the early monsoon appears to be most modulated by antecedent winter–spring Pacific SST patterns, the rainfall in the later part of the monsoon seems to be driven largely by the near-term SST conditions surrounding the monsoon region along the coast of California and the Gulf of California. The role of antecedent land and ocean conditions in modulating the following summer monsoon appears to be quite significant. This enhances the prospects for long-lead forecasts of monsoon rainfall over the southwestern United States, which could have significant implications for water resources planning and management in this water-scarce region.

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Andrea J. Ray, Gregg M. Garfin, Margaret Wilder, Marcela Vásquez-León, Melanie Lenart, and Andrew C. Comrie

Merideth 2002 ), and conducts user-oriented experiments ( NRC 2001 ), which are a specific form of these interactions. 4. Overview of the applications context The NAME science plan defines the monsoon region in process-based tiers including the core monsoon area dominated by frequent, diurnal convective processes (tier I); an area associated with intraseasonal, transient variability of the monsoon (tier II); and the area in which continental-scale, warm-season circulation and precipitation patterns

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Wayne Higgins and David Gochis

completed, a process that will take a number of years. This issue is organized to report upon several areas, including precipitation characteristics, circulation and transient observations/analyses, ocean observations/analyses, land–atmosphere interactions and hydrology, modeling studies, and societal applications; a brief synthesis of some of the key findings is given in section 4 . The set of papers is necessarily diverse to illustrate the scope of the program. 2. NAME timeline NAME planning began in

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Michelle Hallack-Alegria and David W. Watkins Jr.

. 1990 ). While accurate forecasting of when a drought will begin or end is extremely difficult, better understanding of the expected intensity, duration, and recurrence frequency of droughts is essential to drought mitigation planning. This is particularly true for the agriculture sector, in which appropriate planning, monitoring, and prediction can greatly reduce economic hardships in the event of drought ( Wilhite 2002 ; Rossi 2003 ). In this study we provide an overview of seasonal, annual, and

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Enrique R. Vivoni, Hugo A. Gutiérrez-Jurado, Carlos A. Aragón, Luis A. Méndez-Barroso, Alex J. Rinehart, Robert L. Wyckoff, Julio C. Rodríguez, Christopher J. Watts, John D. Bolten, Venkataraman Lakshmi, and Thomas J. Jackson

conclusions, and recommend fruitful avenues for future work. 2. Observations and sampling methods In the following, we describe the study site, data collection, instrumentation, and analysis used to investigate the hydrometeorological conditions along the topographic transect. The field experiment was designed to assess the influence of land surface properties on the variability of monsoon precipitation and its hydrologic response. Our experimental plan is based on similar soil moisture field campaigns

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Wanqiu Wang and Pingping Xie

for the recent period from June 2002 when all individual inputs are accessable. In contrast, the weekly OI analysis is available back to November 1981 ( Reynolds et al. 2002 ). In addition, the multichannel AVHRR SST data extend back to 1981 at spatial resolutions of 9, 18, and 54 km and temporal resolutions of daily and weekly (information online at ). A new improved daily version of the OI analysis is planned to be produced for the last two decades on a 0.25° × 0

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X. Gao, J. Li, and S. Sorooshian

temperature anomalies in the Gulf of California on North American monsoon rainfall. J. Geophys. Res. , 108 . 4112, doi:10.1029/2002JD002403 . NAME Project Science Team , 2004 : North American Monsoon Experiment (NAME): Science and implementation plan. NOAA/NCEP/CPC, 96 pp. [Available online at .] . Negri , A. J. , R. F. Adler , R. A. Maddox , K. W. Howard , and P. R. Keehn , 1993 : A regional rainfall climatology over Mexico and

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Christopher R. Williams, Allen B. White, Kenneth S. Gage, and F. Martin Ralph

. Meneghini , J. Awaka , and K. Okamoto , 2000 : Rain-profiling algorithm for the TRMM Precipitation Radar. J. Appl. Meteor. , 39 , 2038 – 2052 . Lang , T. J. , D. Ahijevych , S. Nesbitt , R. Carbone , S. Rutledge , and R. Cifelli , 2007 : Radar-observed characteristics of precipitating systems during NAME 2004. J. Climate , 20 , 1713 – 1733 . NAME Project Science Team , cited . 2006 : North American Monsoon Experiment (NAME): Science and implementation plan. NOAA

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David J. Gochis, Christopher J. Watts, Jaime Garatuza-Payan, and Julio Cesar-Rodriguez

patterns of increased precipitation intensity in climate model simulations. Geophys. Res. Lett. , 32 . L18719, doi:10.1029/2005GL023680 . NAME Project Science Team , cited . 2005 : North American Monsoon Experiment (NAME): Science and implementation plan. NOAA/NCEP/CPC, 96 pp. [Available online at .] . Quintas , I. , 2000 : ERIC II: Documentación de la base de datos climatológica y del programa extractor (ERIC II: Documentation of the

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