Signature of quasi–biennial oscillation and long-period equatorial disturbances in the lower atmosphere over Thiruvananthapuram |
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| Authors: | A K Das U C Mohanty S K Roy Bhowmik |
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| Institution: | (1) India Meteorological Department, Lodi Road, New Delhi, India;(2) Centre for Atmospheric Sciences, Indian Institute of Technology, Hauz Khas, New Delhi, India |
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| Abstract: | Summary An attempt has been made in this paper to examine different modes of oscillation in the wind field during different seasons
over Thiruvananthapuram (lat. 8.29° N, long. 76.59° E, located at the extreme southwest coast of India) based on daily upper
air observations for the period from January 1997 to December 1999. A power spectral analysis is carried out with the upper
air data of the station. The study shows that one and half cycle of Quasi–Biennial Oscillation (QBO) and the power spectra
of the meridional wind component exhibit peaks between the period of four days and seven days (corresponding frequency range
between 0.25 day−1 and 0.15 day−1) during all seasons. The seasonal variation of these large-scale oscillations over the station depends upon the background
mean zonal flow, which in turn closely related to the QBO structure.
The time sequence of power spectra shows that the disturbances with periods between four days and seven days dominantly prevail
in the upper troposphere and lower stratosphere throughout the year. The regimes of high power spectral intensity in this
period range are maintained in the levels where the mean zonal flow (westerly or easterly) weakens and changes with height.
The study establishes the fact that disturbances (mixed Rossby-gravity waves) acquire maximum power in the winter season whereas
the south-west monsoon exhibits minimum spectral intensity when spreading of energy over a frequency range takes place. |
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