Computer Science
Scientific paper
Jun 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........91c&link_type=abstract
Thesis (PhD). UNIVERSITY OF CALIFORNIA, LOS ANGELES, Source DAI-B 58/12, p. 6627, Jun 1998, 168 pages.
Computer Science
Scientific paper
This investigation aims to gain insight into the reasons for seasonal and inter-annual variations in the circulation of the extratropical stratosphere. Generally, the possible explanations for these variabilities can be divided into external and internal factors. Many previous studies have attributed observed or modeled variations to external factors. For example, the quasi-biennial oscillation in equatorial zonal winds or variations in the tropospheric planetary wave activity. The overall goal of the research described herein is to better understand the mechanisms at work for the seasonal and interannual variability of the stratospheric circulation with emphasis on the role played by internal factors. The primary tool used is an extended version of the quasi- geostrophic, β-plane middle atmosphere model of Holton and Mass (1976), which is developed particularly for this research. The simulated stratospheric circulation that results when a time-independent wave is imposed at the model's lower boundary is examined from the point of view of the steady solutions as well as through time integrations. The steady solutions obtained for winter conditions are explored and their sensitivity to model truncation is analyzed in detail. For a certain range of the amplitude of planetary waves imposed at the lower boundary, there are three branches of steady solutions. The range of wave 2 amplitude for which two stable solutions co-exist narrows as wave 1 amplitude increases. Some oscillatory solutions show sudden transitions from westerlies to easterlies, similar to those observed during stratospheric major warmings. The effect of adding a meridional wavenumber 1 component on the character of both steady and time-dependent solutions is more profound: (1) the number of steady solution branches increases significantly and their structures become more complex, and (2) the routes of transitions between solution branches become more complex. Seasonal variations are introduced by varying the radiative heating with a purely periodic annual cycle. Interannual variations during the late winter through spring seasons are found when the meridional wavenumber 1 component of the flow is included and a realistic value for the Newtonian heating coefficient in the lower stratosphere is used. The amplitude of these variations is comparable to those observed during late winter and spring, and very similar to that found by Farrara and Mechoso (1992). Overall these results strongly suggest that a rich variety of intraseasonal and interannual variability in the stratospheric circulation can exist even in the absence of any such variations in 'external' factors.
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