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Cary J. Mock

Abstract

The western United States is characterized by complex seasonal precipitation regimes due to the hierarchy of climatic controls that operate at different spatial scales. A climatology of month to month changes in precipitation, using data from 4027 stations, illustrates how different climatic controls govern the annual cycle of precipitation response and explains the spatial distribution of the seasonal precipitation maximum. These results particularly indicate that smaller-scale climatic controls must be considered along with larger-scale ones in order to explain patterns of spatial climate heterogeneity over mountainous areas. The examination of seasonal precipitation maxima during years characterized by abnormally low winter Pacific-North American teleconnection (PNA) patterns and abnormally strong summer monsoons reveal few changes spatially as compared to normal. The lack of changes illustrate that spatial heterogeneity of precipitation seasonally is the rule rather than the exception for much of the mountainous West. The results of this study offer important implications for scholars interested in assessing spatial climatic variations of the western United States at timescales ranging from interannual to the late-quaternary.

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Cary J. Mock and Karl W. Birkeland

The snow avalanche climate of the western United States has long been suspected to consist of three main climate zones that relate with different avalanche characteristics: coastal, intermountain, and continental. The coastal zone of the Pacific mountain ranges is characterized by abundant snowfall, higher snow densities, and higher temperatures. The continental zone of the Colorado Rockies is characterized by lower temperatures, lower snowfall, lower snow densities, higher snow temperature gradients, and a more persistently unstable snowpack resulting from depth hoar. The intermountain zone of Utah, Montana, and Idaho is intermediate between the other two zones. A quantitative analysis of snow avalanche climate of the region was conducted based on Westwide Avalanche Network data from 1969 to 1995. A binary avalanche climate classification, based on well-known thresholds and ranges of snowpack and climatic variables, illustrates the broadscale climatology of the three major zones, some spatially heterogeneous patterns, and variations with elevation. Widespread spatial shifts toward more coastal conditions occurred during 1985/86 and 1991/92, and shifts toward more continental conditions occurred during 1976/77 and 1987/88. Height anomalies at 500 mb explain many of these shifts, but daily plots of climate and avalanche variables during seasonal extremes for sites in northern Utah also illustrate the importance of understanding snowpack and weather variations that occur at daily to weekly timescales. Data from several central Rocky Mountain sites indicate some relationships with the Pacific–North American teleconnection pattern and the Pacific decadal oscillation, illustrating the importance of applying long-term records in an avalanche hazard assessment.

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Dorian J. Burnette, David W. Stahle, and Cary J. Mock

Abstract

A continuous record of 65 987 daily-mean temperature observations since 1828 has been developed for Manhattan, Kansas, by screening and correcting original station records of the U.S. Army Surgeon General, the Smithsonian Institution, and the Signal Service. Hourly, minimum, and maximum temperature observations from seven discontinuous historical stations and four modern stations in Kansas, Missouri, and Oklahoma were used to compile this unbroken record of daily-mean temperature. The historical temperature data were linked with the modern temperature record for Manhattan after these data were adjusted for time of observation differences, station movements, and changes in the environment around the station. The new daily-mean temperature reconstruction for Manhattan now extends with confidence back to 1 July 1855 and with more uncertainty back to 1 July 1828. The uncertainty prior to 1855 is due to instrumentation changes in 1843 and changes in observation practices in 1855 that occurred at many stations. The error estimates reported in this paper do not reflect these potential inhomogeneities and should be considered lower limits. Nonetheless, this new daily record indicates significant warming in all seasons; in heating and cooling degree-days; in the warmest and coldest days of the year; in extremes above the 90th percentile and below the 10th percentile; in the frequency of winter cold waves and summer heat waves; and in the overall annual-mean temperature, which has warmed by 1.57° ± 0.23°C since 1855 (1.27° ± 0.23°C since 1829). The warm Dust Bowl event in the summer of the 1930s and cold winters of the 1870s and 1880s dominate the reconstruction and included some of the warmest and coldest daily extremes, respectively, of the last 154–180 yr. This new reconstruction is currently the longest unbroken daily corrected record in the Americas. These data indicate that the nineteenth century was fundamentally cooler than the twentieth and early twenty-first century.

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Cary J. Mock, Michael Chenoweth, Isabel Altamirano, Matthew D. Rodgers, and Ricardo García-Herrera

Major hurricanes are prominent meteorological hazards of the U.S. Atlantic and Gulf coasts. However, the official modern record of Atlantic basin tropical cyclones starts at 1851, and it does not provide a comprehensive measure of the frequency and magnitude of major hurricanes. Vast amounts of documentary weather data extend back several centuries, but many of these have not yet been fully utilized for hurricane reconstruction. These sources include weather diaries, ship logbooks, ship protests, and newspapers from American, British, and Spanish archives. A coordinated effort, utilizing these historical sources, has reconstructed a major hurricane in August 1812, which is the closest to ever pass by New Orleans, Louisiana, including Hurricane Katrina. The storm became a tropical depression in the Caribbean Sea, passed south of Jamaica as a tropical storm, and then strengthened to hurricane strength in the Gulf of Mexico. It made landfall about 65 km southeast of New Orleans and passed just to the west of the city. Historical storm surge and damage reports indicate it as a major hurricane at landfall. Given that conditions during 1812 include having lower sea level, higher land elevation prior to human-induced subsidence, and more extensive wetlands, a recurrence of such a major hurricane would likely have a greater detrimental societal impact than that of Hurricane Katrina. The 1812 hurricane study provides an example of how historical data can be utilized to reconstruct past hurricanes in a manner that renders them directly comparable with those within our modern record.

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