Vertical structure and characteristics of 23–60 day (zonal) oscillations over the tropical latitudes during the winter months of 1986 — Results of equatorial wave campaign—II |
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| Authors: | R Raghavarao R Suhasini R Sridharan B V Krishnamurthy O P Nagpal |
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| Institution: | (1) Present address: Code 614, Planetary Atmospheres Br., Goddard Space Flight Centre (GSFC), NASA, 20771 Greenbelt, MD, USA;(2) Physical Research Laboratory, 380009 Navarangpura, Ahmedabad, India;(3) Physical Research Laboratory, 380009 Navarangpura, Ahmedabad, India;(4) Space Physics Laboratory, Vikram Sarabhai Space Centre, 695022 Trivandrum, India;(5) Delhi University, 110001 Delhi, India |
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| Abstract: | The equatorial wave campaign-II which formed a part of the Indian Middle Atmosphere Programme (IMAP), was conducted from SHAR
(13.7°N, 80.2°E) from 15 January to 28 February 1986. Winds were measured from ground to 60 km by means of high altitude balloon
and a meteorological rocket (RH-200), once everyday, for 45 days. The frequencies of the oscillations in the deviations of
the east-west component of the winds from its mean at each height with one kilometer interval were obtained by the maximum
entropy (ME) method and phases/amplitudes of these frequencies were determined by the least squares technique on the wind
variation time series. The ME method has the inherent advantage of providing periodicities up to 1.5 times the data length.
The height structure of the long period waves of > 23 day periodicities that have larger amplitudes nearly by a factor of
2 as compared to the medium (9 to 22 day) or shorter period (4 to 8 day) ones, reveal two height regions of enhanced amplitudes,
one in the troposphere and another in the upper stratosphere/lower mesosphere, that too, mostly in the regions of positive
(westerly increasing or easterly decreasing with height) wind shears. The waves are seen to be inhibited in the negative wind
shear regions. From the abrupt changes in the altitude variation of phase, the possible source region has been identified.
The vertical wavelengths have been estimated to be 34 km and 19 km in the troposphere and lower stratosphere respectively
and 8 km in the upper stratosphere and lower mesosphere. Around 56 km the wave amplitude is reduced to 1/4 of its value below,
while the vertical shear strength in the mean wind doubled up. The tropospheric waves are suggested to be Rossby waves of
extratropical origin penetrating to tropical latitudes. The stratospheric/mesospheric waves however appear to emanate from
a source around the stratopause. |
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| Keywords: | Equatorial waves vertical structure 23-60 day period zonal-oscillations Indian Middle Atmosphere Programme source regions |
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