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On the structure of climate variability near the tropopause and its relationship to equatorial planetary waves




Grise, Kevin M., author
Thompson, David W. J., advisor
Birner, Thomas, committee member
Eykholt, Richard E., committee member
Randall, David A., committee member
Randel, William J., committee member

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The tropopause is an important interface in the climate system, separating the unique dynamical, chemical, and radiative regimes of the troposphere and stratosphere. Previous studies have demonstrated that the long-term mean structure and variability of the tropopause results from a complex interaction of stratospheric and tropospheric processes. This project provides new insight into the processes involved in the global tropopause region through two perspectives: 1) a high vertical resolution climatology of static stability and 2) an observational analysis of equatorial planetary waves. High vertical resolution global positioning system radio occultation profiles are used to document fine-scale features of the global static stability field near the tropopause. Consistent with previous studies, a region of enhanced static stability, known as the tropopause inversion layer (TIL), exists in a narrow layer above the extratropical tropopause and is strongest over polar regions during summer. However, in the tropics, the TIL possesses a unique horizontally and vertically varying structure with maxima located at ~17 and ~19 km. The upper feature peaks during boreal winter and has its largest magnitude between 10° and 15° latitude in both hemispheres; the lower feature exhibits a weaker seasonal cycle and is centered at the Equator. The spatial structure of both features resembles the equatorial planetary wave response to the climatological distribution of deep convection. Equatorial planetary waves not only dominate the climatological-mean general circulation near the tropical tropopause but also play an important role in its intraseasonal and interannual variability. The structure of the equatorial planetary waves emerges as the leading pattern of variability of the zonally asymmetric tropical atmospheric circulation. Regressions on an index of the equatorial planetary waves reveal that they are associated with a distinct pattern of equatorially symmetric climate variability characterized by variations in: 1) the distribution of convection in the deep tropics; 2) the eddy momentum flux convergence and the zonal-mean zonal wind in the tropical upper troposphere; 3) the mean meridional circulation of the tropical and subtropical troposphere; 4) temperatures in the tropical upper troposphere, the tropical lower stratosphere, and the subtropical troposphere of both hemispheres; and 5) the amplitude of the upper tropospheric anticyclones that straddle the Equator over the western tropical Pacific Ocean. The pulsation of the equatorial planetary waves in time provides a framework for interpreting a broad range of climate phenomena. Variability in the equatorial planetary waves is associated with variability in the tropical TIL and is linked to both the El Niño-Southern Oscillation and the Madden-Julian Oscillation (MJO). Evidence is presented that suggests that the MJO can be viewed as the linear superposition of: 1) the pulsation of the equatorial planetary waves at a fixed location and 2) a propagating component. Variability in the equatorial planetary waves may also contribute to variability in troposphere/stratosphere exchange and the width of the tropical belt.


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equatorial planetary waves


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