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Matthew H. Hitchman, John C. Gille, Clive D. Rodgers, and Guy Brasseur

Abstract

An examination of satellite-derived temperatures reveals that the winter polar stratopause is usually elevated and warmer than the adjacent midlatitude stratopause. This “separated stratopause” occurs in both hemispheres, but is more pronounced and persistent in the southern winter. It descends with time towards spring and exhibits week to week variability. Observational diagnostics and results from a two dimensional (2-D) model suggest that gravity wave driving can account for this separated polar stratopause by driving a meridional circulation with downwelling over the winter pole. In the model, the solar heating pattern induces stronger winter westerlies than summer easterlies, which leads to a stronger gravity wave driven circulation in the winter hemisphere. Spherical geometry and the high latitude location of the winter westerly jet combine to yield a concentrated region of downwelling. Model results suggest that descent of the temperature maximum with time is probably caused by wave–mean flow interaction.

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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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