Abstract
Characteristics of the intraseasonal oscillations (ISO) in the mesosphere and lower thermosphere (MLT) are investigated using meteor radar wind observations from a South American, equatorial station, Sao Joao do Cariri (7.4°S, 36.5°W) during 2008. A prominent ISO signature is observed during January-May in the period band approximately 40 to 70 days in the MLT zonal wind. In the lower atmosphere, a dominant ISO is observed in the period band approximately 30 to 60 days in the outgoing longwave radiation (OLR) (a proxy for convection), total columnar water vapor (a proxy for tides), and zonal wind. Considerable high correlation of the ISO between the MLT and lower atmosphere indicates significant dynamical coupling between the lower and middle atmosphere during the observational period. The MLT-ISO shows conspicuous downward propagation of the peak amplitude indicating the role of the dissipating upward propagating waves for its generation. The amplitudes in the zonal wind of the dominant tidal components in the MLT exhibit conspicuous ISO modulation. The eastward propagating waves and tides are surmised to be responsible for communicating the ISO signature from the lower atmosphere to the MLT. The origin of the MLT-ISO is believed to take place in the lower troposphere, below 4 km. The MLT-ISO is believed to be the imprint of the lower atmospheric Madden-Julian oscillation which travels eastward from the Indian Ocean-western Pacific Ocean to the present location.
Highlights
The mean dynamical state of the equatorial middle atmosphere is significantly controlled by large-scale long period oscillations, i.e., semiannual oscillations (SAO) and quasibiennial oscillations (QBO) (Burrage et al 1996; Baldwin et al 2001; Guharay et al 2014a)
In order to find out the instraseasonal oscillations (ISO) signature in the mesosphere and lower thermosphere (MLT) winds, we have estimated the Lomb-Scargle periodogram at each altitude bin of the meteor radar-derived daily mean zonal and meridional winds in the range 81 to 99 km in 2008 which are plotted in Figure 1a, b
The present investigation from an equatorial South American station, Cariri reveals a prominent feature of the ISO in the MLT zonal wind during January-May of 2008 as well as a conspicuous relationship with the lower atmospheric Madden-Julian oscillation (MJO) signature indicating lower atmospheric origin
Summary
The mean dynamical state of the equatorial middle atmosphere is significantly controlled by large-scale long period oscillations, i.e., semiannual oscillations (SAO) and quasibiennial oscillations (QBO) (Burrage et al 1996; Baldwin et al 2001; Guharay et al 2014a). These oscillations show downward propagation of their phases with altitude since they are driven by deposition of energy from upward propagating gravity waves, Rossby waves, Kelvin waves, etc. Eckermann et al (1997) furthered the work with nearly 5 years of long-term data and reported dominant ISO in the MLT zonal wind at Christmas Island with periods approximately 60, 40, and 20 days They found strong ISO modulation of the gravity waves and diurnal tide
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