Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
ISSN-L : 0026-1165
The Double-Jet and Semi-AnnualOscillations in the Southern Hemisphere Simulated by the Meteorological Research Institute General Circulation Model
Akio KitohKoji YamazakiTatsushi Tokioka
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1990 Volume 68 Issue 2 Pages 251-264

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Abstract

The tropospheric circulation in the Southern Hemisphere has some remarkable features such as a deep circumpolar trough all the year round, a double-jet in winter months and large semi-annual components in the fields of sea-level pressure and zonal wind. Performance of a 12-year integration with the Japan Meteorological Research Institute general circulation model is presented and compared with the observation for the period 1979-1987.
The simulated meridional temperature gradient in July has two distinctive maxima, one at 30°S in the upper troposphere and the other at 60°S in the lower troposphere, and the baroclinity is not zonally uniform. The simulated zonal winds at 500mb show two jets in the Pacific sector, one at 30°S and the other at 60°S, and only one jet in the Atlantic/Indian sector, in accordance with observation. The double-jet structure is somewhat obscured when taking the zonal mean. Quasi-stationary eddy fields with zonal wavenumber 1 at 50°-70°S are associated with this zonal asymmetry.
Even in the Pacific sector two strong baroclinic zones occur only from May to October, while there is one during the rest of the year. Between 50°S and 60°S, baroclinity becomes strong during spring and fall, and semi-annual oscillations are found in zonal wind and sea-level pressure. A good simulation of the seasonal cycle of the Antarctic temperature field such as a rapid cooling in autumn of the Antarctic lower troposphere, lack of a well defined temperature minimum (the coreless winter) and coldest atmosphere in early spring, is crucial to a successful simulation of the semi-annual oscillations and the winter double-jet structure.

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